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The electron-metallography and crystallography of copper-aluminum martensites
THE
ELECTRON-METALLOGRAPHY COPPER-ALUMINUM
AND CRYSTALLOGRAPHY MARTENSITES
OF
and H. WARLIMONTt
P. R. SWANNf
The microstructures, cryst81 structures and mechanic81 properties of the copper-81uminum mertensites, forming from the j3 solid solution, have been investigated by electron microscopy, electron diffmcThree types of m8rtensite have been observed; internally faulted tion and hardness measurements. f.c.c. mertensite, B’ internally faulted tetr8gonal (ordered) martensite, &‘, and internally twinned orthorhombic (ordered) martensite, y’. The presence of the superlctttice ,!?I has been shown to exert 8 considerable restriction on the number of operative slip systems 8veilable during the transformation to B1’. It ~8s determined that short range diffusion oocurs during the initial stages of the /? to 8’ transformation. Calculations using the phenomenological theory of mertensite transformation were found to agree with the results of the structural observations. The mechanical properties are rel8ted to the ordering phenomena occurring during the trrtnsformation rather th8n to the internal faulting or twinning. The composition ranges of the metastable equilibrie 8re discussed with reference to the composition dependence of the stacking fault energy of the primary solid solution 8nd to internal stresses. MICROGRAPHIE
ELECTRONIQUE
ET
CRISTALLOGRAPHIE
DES
MARTENSITES
CUIVRE-ALUMINIUM Les auteurs ont Btudib 18 microstructure, la structure cristalline et les propri&% mbcaniques des martensites cuivre-aluminium form&es B partir de 18 solution solide p, en utilisant la microscopic Blectronique, la diffraction des Electrons, et des mesures de duret& Trois type de martensite ont &t6 observ&: une martensite c.c.f. 8vec dbfauts internes fi’, une martensite t&ragonale (ordonnbe) 8vec dbfauts interries, B1’ et une m8rtensite orthorhombique (ordonnbe) avec macles internes, y’. Les auteurs ont observe que 18 presence du superrbseau ,!ll provoque une diminution importante du nombre de systemes de glissement susceptibles d’intervenir dans 18 transformation donnant PI’. 118 QtB montrb qu’il se produit de la diffusion B courte distance pendant les premiers stctdes de 18 transformation de p en B’. Les calculs utilisant 18 thborie ph6nombnologique de la transformation m8rtensitique ont 6t& trouvbs en bon accord 8vec le’s r&sultats des observations structurales. Les propri&& m&aniques sont libes 8ux dbplecements internes se produisant au tours de 18 tiansformation, plut6t qu’8ux dbfauts ou 8ux macles. Les auteurs discutent les limites de composition des gquilibres m&astables en consid&+& 18 maniere dont 1’Qnergie des fautes d’empilement de 18 solution solide primaire varie 8vec la composition, et en consid&ant Bgalement les contraintes internes. STRUKTURUNTERSUCHUNGEN
VON
MARTENSITISCHEN
KUPFER-ALUMINIUM-LEGIERUNGEN Mikrostruktur, Kristrtllstruktur und mechanische Eigensch8ften von martensitischen Kupfer-Aluminium-Phasen, die su8 dam p-Mischkristall entstehen, wurden mit Hilfe von Elektronenmikroskopie, Drei Martensitarten wurden beobachtet: ein Elektronenbeugung und HBrtemessungen untersucht. fehlerhafter kubisch fl&chenzentrierter Marten&t (B’), ein fehlerhafter tetragonaler (geordneter) Martensit (PI’) und ein verzwillingter orthorhombischer (geordneter) Martensit (y’). Wie sich gezeigt hat, wird die Anzahl der Gleitsysteme, die wiihrend der Umw8ndlung in die &‘-Phase betiitigt werden kannen, durch die Anwesenheit der p,-U‘berstruktur wesentlich eingeschriinkt. Am Anf8ng der B + ,Y-Umwandlung wurde eine suf kurze Entfernungen beschrgnkte Diffusion festgestellt. Die Ergenbisse, die die phlinomenologische Theorie der Martensitumwandlung liefert, stehen in Einklang mit Die mechanischen Eigenschaften sind wohl mehr auf Ordnungsdenjenigen der Strukturuntersuchungen. vorg&nge wiihrend der Umwandlung aIs auf Fehlerhaftigkeit oder Zwillingsbildung zuriickzufiihren. Eine Diskussion der Konzentrstionsbereiche, in denen metastabile Gleichgewichte mdglich sind, beriicksichtigt innere Sp8nnungen und die Abhiingigkeit der Stapelfehlerenergie von der Zusammensetzung im prim&m Mischkristall.
INTRODUCTION
Recent
observations
transmission
electron
microscopy
presence of a fine twin structure plates.(1*2) these
martensities have revealed
internal
twins
act
by the
within the martensite
It was suggestedc2) that the boundaries
motion of dislocations dominant contribution martensites.
of martensite
of ferrous
as strong
barriers
of
to the
plates.t2)
the increase
METALLURGICA,
VOL.
11, JUNE
in strength
1963
In
contrast
to
iron-carbon
of twinned martensites,
accounted for by differences in microstructure. The part of the copper-aluminum equilibrium diagramt4) which is pertinent to this study is shown in Fig. 1 (full lines). Upon quenching from the j3 range, alloys containing more than 11 wt.% Al first become ordered to form & and then transform martensiti-
* Received August 3, 1962. 7 Edgar C. Bein, Laboratory for Fundamental Research, United States Steel Corporation Research Center, Monroeville, Pennsylvania. ACTA
in iron was related
copper-aluminum martensites (/3’ and y’) show no extraordinary strengthc3) and in the present study we have tried to determine whether this could be
and in this way provide the to the strength of ferrous
Furthermore,
with carbon content
to the observed increase in the proportion
tally 511
to
either
,!S1’ or
y’.
The
temperature
and
ACTA
512 ATOMIC
METALLURGICA,
VOL.
11,
a phase.(g-ll)
PER CENT ALUMINUM
diffuseness
1963
This uncertainty
of many
The /?-&’
lattice relationship
by Greninger,(‘)
...
I
0
’ B
3.0-
b g
2.5-
\
\
A900
The crystal structure of y’ has been determined
M, \
$
LO-
a + y2 ___....’ -500
!
‘Y’
i
z z
and
diffraction
2.0-
z
3 = i?
&
B(I, k...
x
Iv)
I 5
CU WEIGHT
--%--....
I _
-
they found that y’ is basically
Greninger”)
200
that fi alloys containing distinction However, symbol
I )fy;5,,,
8’.
the ordered
in this paper it is convenient
contains
two
twin-related
For
the
structure
in this study
of 8’ and PI’.
often been described recently
as distorted
as monoclinic
p-y’
orientations lattice
of the same
relationship,
Kurd-
Wl)Y’
11Wl)B
wwY’
11WllB
It has also been reported
by Kurdjumov,(5’
Grenin-
ger,(‘) and Nakanishi(*) that pl’ may be converted into y’ by deformation.
no
and dis8’.
to restrict the
/3’ to describe only the disordered
martensite.
are described
Table 1. There has been some uncertainty crystal
Previously,
which were both designated
The phases involved
symmetry.
jumov(15) gives
less than 11 wt.% Al trans-
was made between
in rhombic
of these metastable
martensite,
ordered martensites,
h.c.p. with
that each y’ plate usually
lattice.
EXPERIMENTAL
phased5) are also shown in Fig. 1. It will be noticed dis0dered
has oberved
IO PER CENT ALUMINUM
ranges of formation
by
X-ray
but since the martensite
larger and may be described
Pm. 1. Copper rich end of the copper aluminum equi),‘*I also showing M, temperlibrium diagram (--atures of /I’, PI’ and y’(- * -. -) us) temperatures (- * * - . *)(15) and the ~tack~~f<~$$~ of c( phase alloys, (- . . . -).(as)
composition
From
inherits the order from /& the resulting unit cell of y’ is
-400
I jj\Y’ I Bl , I
‘\
co-workers.(12-14)
a = 2.60 and c = 4.22 kX,
: ;
, ‘\ 4 - 300
I.\
iI
0.5-
form to a
who found that the pole of the basal
be about 2” from {133jfi. Kurdjumov
1.5-
lines.
has been investigated
deviates by 4” from { 110}s. The habit plane is given to
a4
fz 5
by the
diffraction
plane of the martensite (assuming hexagonal structure)
a
E
was caused
of the X-ray
in
melts prepared
were homogenized atmosphere
and hot-rolled
The compositions Table 2.
as to the exact
The structure
has
but it has also
been suggested that B1 is closely related to the f.c.c. TABLE 1. Stable and metastable
mixture cont.
containing
HCl and
electrothinning
phases in copper-rich
of 1 mm.
are listed in
were all solution
martensite.
treated at
Thin foils suitable
were prepared
the martensite
electrolyte
stock
between 850°C and 1000°C and quenched
into brine to produce thinning
to a thickness
of the alloys studied
The specimens
temperatures
from high purity
for 24 hr at 900°C in a hydrogen
for electron microscopy
h.c.p.(6*7) and more
or triclinic?)
Levitation
PROCEDURE
specimens
by chemically
to 0.05 mm in a
four parts cont. HNO,,
five parts using
the
cone.
H,PO,,
window
used for electrothinning copper-aluminum
one part and
technique. consisted
then The
of one
alloys
Unit cell dimensions Phase 5z
Structure
type
f.c.c., Al
WXI
@Xl
3.61-3.66
-
-
-
@Xl
PI, metastable
cubic, ordered,
2.917 2.95 5.84
,Y, metastable B I’, metastable
“0, f.c.c., Al tetragonal,
3.676
B
Yz y’, metastable
b.c.c.,
A2
ordered
rhombohedral orthorhombic ordered
-
(9) 4.51
5.20
7.352
4.22
Remarks
Reference
0 to 9.0 wt.% Al, room
36
temperature. 12.5 wt.% Al. 11.3 wt.% Al, 672°C. 11.9 wt.% Al, 350°C.
3: 3
Martensite, heavily faulted. 11.86 wt. y0 Al; martensite, heavily faulted; lattice parameters derived from published d-spacings. Essentially y-brass type. 13.6 wt.% Al; martensite, finely twinned.
This work. This work.
36 7, 14, this work.
8
SWANN
AND
WARLIMONT:
COPPER-ALUMINUM
MARTENSITES
513
TABLE 2. Chemical composition and phases present after quenching from the B-range to room temperature No. --____
Al [wt.%] 9.1 10.0 10.8 11.0 11.1 11.2 11.4 11.6 11.7 11.9 12.0 12.1 12.2 13.0 13.7 14.1
B 3 4 5 6 7 : 10 11 12 13 14 15 16
Al [at.%]
Phase
19.1 20.7 22.2 22.5 22.7 22.9 23.2 23.6 23.8 24.1 24.3 24.5 24.7_R 26.027.2 27.9-
a + B’ B’
part HNO, and two parts CH,OH
A
t%’ +
Y’
Y’
solution,*
operated
at 15 to 20 volts and -20°C. (b) EXPERIMENTAL
Metallography
RESULTS
of /I’ and PI’
The electron microstructure
of specimens containing
between 10.0 per cent Al and 13.0 per cent Al quenched from
900°C consists
of acicular
plates
(see Fig.
Within every ,6’ and ,$I/ plate there are numerous striations
(see Fig.
3(a))
which
appear
similar scale to the fine twins observed and by Kelly
2). fine
to be on a by Pitsch,(l)
and Nuttingt2) in ferrous martensites.
In Fig. 2 it appears that the spacing of the striations is measurable, varies
but such a spacing is misleading
considerably
in the
same
area
with
since it slight
changes of diffraction that the striations
contrast. It will be shown later are the result of a high density of
Quenched FIG. 3. /11’ martensite. 12.2 wt.% Al.
from
1000°C.
Cu-
(a) Showing the internal striations across t)wo martensite plates. Transmission electron micrograph. (b) A quadrant of the selected area diffraction of a region A in Fig. 3(a). Zone [IlO].
pattern
(c) The diffraction pattern, Fig. 3(b), indexed according to the f.c.c. structure. Note that reflections with a non-zero phase shift $ are displaced from f.c.c. positions. Reflections due to (i) primary diffraction l (ii) double diffraction 0 (iii) threefold diffraction n (iv) fourfold diffraction 0.
FIG. 2. B1’ martensite, consisting of thin parallel-sided plates with internal striations. Cu-12.1 wt.% Al. Transmission electron micrograph. * This electrolyte should be mixed slowly during mixing to avoid an explosive reaction.
and kept
cool
FIG. 4. /&’ martensite showing stacking fault contrast terminated by partial dislocations. Cu--12.2 wt.% Al. Transmission electron micrograph.
514
ACTA
METALLURGICA,
stacking faults extending across the martensite plates. In fact, under certain contrast conditions, partial dislo~tions ~rminating the chara~~ristic fringe contrast of stacking faults could be observed (see Fig. 4). The existence of a superlattice in quenched B alloys containing more than about 10.5 per cent aluminum could be detected by selected area diffraction, and also by dark field microscopy. In the latter case the antiphase domain boundaries of the superlattice were revealed in good contrast (see Fig. 5) by allowing only one low index superlattioe reflection to pass through the objective aperture and form the image. The antiphase domain boundaries seen in Fig. 6 show no preferred orientation and also extend indiscriminately across the boundaries of martensite plates.
FIG. 5. fil’ martensite showing antiphase domain boundaries extending indiscriminately across the interfaces of martensitc plates. Cu-12.2 wt.% Al. Transmission electron micrograph, dark field illumination.
The domain size of the superlattice is decreased by increasing the quenching rate. It is also dependent on composition and increases with increasing aluminum concentration. Fig. 6(a)-6(f) show a series of micrographs illustrating this observation and in Fig. 7 measurements of domain diameter of specimens quenched into boiling water versus the aluminum concentration have been plotted. The character of the domains also changes with composition. The domains of those alloys near the composition Cu,Al are in contact, whereas at lower aluminum contents they are separated by regions of lower order or disorder. This latter observation is best i~ustra~d in Fig. 6(a) where the regions between the superlattice domains are dark throughout, indicating an absence of the @r’ superlattice there. With decreasing aluminum content the intensity of superlattice reflections diminishes and
VOL.
11,
1963
vanishes in an alloy containing 10.0 wt.% aluminum, even after a relatively slow quench into boiling water. The large martensite plates in this latter specimen are unique among the alloys examined, through the presence of a band of preferential dissolution extending along their length (see Fig. 8(a)). It is interesting to note in Fig. S(a) that these bands are not observed in the smaller plates. It is also charac~risti~ of this alloy that dislocation loops are found to be strung out along the stacking fault traces in a [11218’ direction from the edges of the thin regions to the interfaces of the martensite plates. This is shown in Fig. S(b) where the stacking faults are parallel to the electron beam and in weak contrast A center band free from dislocation loops is not observed in alloys of higher aluminum content. Some 0’ and /&’ specimens were cold rolled to determine their mode of deformation. In these specimens bands of high dislocation density appear parallel to the stacking fault striations of each martensite plate. There are also indications in the micrographs of a considerable amount of dislocation movement on planes intersecting the striations, which was confirmed by the diffuseness of selected area diEraction patterns. In addition, a large number of plates contain numerous grains shaped like deformation twins (see Fig. 9). However, the twin relationship could not be confirmed by selected area diffraction because of the pronounced relaxation of Laue conditions resulting from deformation. The observations of Kurdjumov,(5) Greninger”) and Nakanishi(*) that are Pi’ may be converted into y’ by defo~ation incorrect. This is demonstrated dy comparing Fig. 9 showing deformed &’ and Fig. 14(a) showing y’. Nature of the internal striations of #2’ and /?I
The st~cture of /Y and pl’ and the nature and separation of the internal striations were determined by selected area electron diffraction of plates greater than 2,~ in width. A considerable difficulty is encountered in the interpretation of the diffraction data because some of the diffraction maxima are extended in certain directions from one reciprocal lattice reflection to another. This results in an apparent continuous range in d-spacings and angles between certain crystal planes, (see Fig. 10). In order to obtain unambiguous patterns the specimens had to be oriented such that the extension of reciprocal lattice points (streaks) lay in the plane of the diffraction pattern (see Fig. 3(b)). Specimens were placed in these special orientations using a double tilting specimen holder.(l”) The positions of the intensity maxima in planes containing the streaks are then
SWANN
AND
WARLIMONT:
COPPER-ALUMINUM
MARTENSITES
FIG. 6. Illustrating the dependence of the superlattice domain size of B1’ on the aluminum concentration (wt.%) after quenching from 1000°C into boiling water. Transmission electron micrographs, dark field illumination.
515
516
ACTA
METALLURGICA,
VOL.
tions
11, 1963
correspond
reciprocal
are displaced
VALUE OBTAINED FROM A SPECIMEN HEAT TREATED SEPARATELY.
El kl a 0
I500
exactly
lattice
points.
to
certain
However,
by a constant
f.c.c.
0: phase
other reflections
amount from the remain-
ing f.c.c. reflection positions, e.g. lil or 3i3. These reflections are all shown as filled circles in Fig. 3(c). The remaining correspond
reflections
found
on Fig. 3(b) do not
to the a or the twin a reciprocal
lattice,
but may be explained in terms of multiple diffraction. Extra reflections resulting from multiple diffraction are not observed in perfect f.c.c. crystals because the re-diffracted beams coincide with the positions of primary ALUMINUM
CONCENTRATION
beams, altering their intensity.
FIG. 7. Dependence of the average superlattice domain size of /II’ on the Al concentration in martensitic G-Al alloys quenched into boiling water.
determined,
production
of streaked patterns
if
are displaced from f.c.c. reciprocal as in the present case, the repositions,
lattice
diffracted beams produce a set of additional reflections. The positions
accurately
However,
some reflections
since it was found
that the
of the type shown in
double
of these reflections
diffraction
geometrically
produced
by the
of any beam hkl may be obtained
by displacing the origin of the reciprocal
hkl, and since the beam hkl is
Fig. 3(b) is very sensitive to orientation.
lattice
The diffraction streaks were found to be always perpendicular to the striations observed on the
inclined to the incident beam, the sphere of reflection
micrographs.
on hkl.
This direction
of the streaks indicates perpendicular
In contrast fine
scale
streaking &’ plates.
in a
to the striations.
be eliminated
since twin symmetry
any selected
of the
every few _&gstroms
to ferrous martensites,
can
continuity
that the periodicity
lattice must be interrupted direction
and complete
area diffraction
for
on a the
was never observed in patterns
of single ,8’ or
It was observed that in diffraction
patterns
of p’ and pi’ plates (see Fig. 3(b)), some of the reflec-
FIG. 8. B martensite.
should also be tilted through a small angle depending may
However,
be neglected
for low order reflections because
the small
this tilt
divergence
of
the electron beam and the buckling and thinness of the specimen relax the Laue conditions
twinning
as a cause
to the point
re-diffracted diffracted
beams to be observed.
a system of reflections is produced
is represented
by the lenticular
remaining
diffraction
If all the primary
beams shown in Fig. 3(c) (black circles) are
re-diffracted The
sufficiently for the
may
reflections only
be
not
which
shapes (see Fig. 3(c)). produced
explained
by
Quenched from 1000°C into water at 100°C. Cu-10.0 wt.% Al. Transmission electron micrographs. (a) Showing the presence of thin bands of preferential dissolution within large /3’ martensite plates. (b) The high density of elongated dislocation loops across a large ,!I’ martensite plate.
by double three-fold
SWANN
WARLIMONT:
AND
COPPER-ALUMINUM
MARTENSITES
517 9 0
048
0 025
000
FIG. 11(a). A quadrant of the diffraction pattern of planes with a [421] j3r’ zone axis. This pattern contains no superlattice reflections and only reflections with a zero phase shift.
FIG. 9. B’ martensite, deformed 50 per cent by rolling, showing profuse twinning. Cu-10.0 wt.% Al. Transmission electron micrograph.
(triangles)
and four-fold
intensity
of reflections
the order of diffraction reflections tion.
increases will
subsequent
diffraction
high density
order of diffracdue to multiple
from
drawings
of
of streaked reflections
from the
may be explained
by the presence of a
of stacking faults.
This effect has been
for the case of X-ray and Hirsch,os)
and that of Whelan
for the case of electron
These workers show that the
I DIRECTION ELECTRON
theory of Patterson(17)
diffraction
and Howie(ls)
in thin foils.
I
FIG. 11(b). Showing a quadrant of the diffraction pattern of planes with a [ilO] /?r’ zone axis. The extra reflections due to multiple diffraction are not included in the drawing.
OF BEAM I----7
II DIRECTION OF STREAKS
the
patterns.
analyzed using the kinematical
diffraction
The
of reflections
be omitted
The displacement f.c.c. positions
diffraction.
with increasing
An indication
diffraction
(squares)
correspondingly decreases as increases. Also, the breadth of
POLE OF DIFFRACTING RECIPROCAL LATTICE PLANE
Ii
Fm. 10. A two-dimensional illustration showing the origin of displaced diffraction maxima due to relaxation of a Laue condition. Crystals having a true do* spacing may appear to have a spacing d,*. *--true reciprocal lattice points. O-apparent diffraction maxima.
I FIG. 11(c). As Fig. 11(b) but from planes with a [311] /3r’ zone axis. Note that superlattice reflections 033 and 14i are not displaced and hence the faulting shear on (112) must be t[lli], rather than &[2;11] or h[421].
ACTA
518
METALLURGICA,
VOL.
11,
Cz7.3521
it was inferred
I
dominantly faults.
above
intrinsic
This
Howieog)
1963
is
that /?’ and pi’
supported
by
the
who showed that extrinsic
in small concentrations opposite
direction
contain
rather than extrinsic
calculation stacking
of
faults
cause displacements
to those observed
pre-
or growth
in the
in Fig. 3(c) and
although JohnsonczO) has shown that higher densities of extrinsic
stacking
faults yield a more complicated
diffraction
pattern,
our
compatible Further, served
-a
faults
of reflections is determined
waves due to the faulting and by
the density of faults. corresponding produced
to
The phase change for a reflection the
reciprocal
lattice
vector
g
by a shear R is given by @=2ng.R
(I)
For an intrinsic stacking fault on (111) in the f.c.c. lattice the possible shears, R, are $2111
; [El],
or
seen that
the magnitude
of the phase
shift for a
particular
f.c.c.
(hkl) is the same for all
values of R, since h, k and 1 are either all even or all odd.
Hence,
for faulting
chosen as (n-/3)(h where h faulting
the phase
on (111).
may
will be unaffected
that
of the high fault density,
lattice.og)
intrinsic
in ,d’ and
the ob-
symmetrical stacking
&‘.
However,
the presence
of a
of extrinsic and growth faults cannot
In contrast to Obinata’a) and Isaitschew
et aZ.(zl) we
find that diffraction
patterns of /I’ and pi may not be
indexed
to the
according
hexagonal
system,
since
reflections like (0110) which would not be displaced by faulting
are absent.
The present
indicate that disordered structure. order
to
diffraction
@’ is a heavily
interpret
diffraction
results
faulted f.c.c.
results
of
the
be by
was deduced reported
from the DO,
by 0binatat6)
structure
assumed that the transformation b.c.c. to f.c.c., positions,
of &, already
and Wassermannol)
It was
involved
is basically
and that a correspondence
of atomic
typical
of martensitic
during this transformation.
reactions,
persists
The unit cell deduced in
this way is shown in Fig. 12. It consists of two f.c.c.
This is verified in the diffraction
pattern Fig. 3(b), (c) for reflections 232, 333, etc.
shift
2k + 1) and thus those reflections
2k + 1 = 6n07@)
be
ordered /?r’ the probable unit cell of the ,13r’superlattice
Inserting these values of R into equation (1) it may be reflection
would
not
faulting.
Structure of /3’ and pi
In
$1121.
are still
extrinsic
of the hexagonal
concluded
small admixture be excluded.
by the phase
of
faults were present
maxima
are predominant
because
observations
degree
if only growth diffraction
It was therefore
05576 B FIG. 12. The proposed unit cell of B,‘(Cu,Al) mertensite. The structure is of the type DO,, (like Al,Ti) and is shown prior to faulting.
change of diffracted
any
about reflection positions
0
displacement
with
111, 222, etc. and
The phase shifts @ for each row of
reflections are indicated
in Fig. 3(c) and in subsequent diffraction patterns of pr’. It has also been shown(17~ls) that reflections undergoing a negative phase change as a result of intrinsic faulting are displaced in the [ill] direction; in the [ill]
and those having a positive direction.
phase change,
This is found to be the case for
the diffraction pattern Fig. 3(c) and for all the diffraction patterns of /?’ and pl’ which have been analyzed (see Figs. 11(a), (b), and (c)). Fig. 11(a) is the particular case of a diffraction pattern from a zone of reflecting planes which are not disturbed by faulting and hence there is no displacement or broadening of diffraction maxima. From the observed displacements of f.c.c. reflections
FIG
13. The positions of Cu and Al etoms in two
adjacent (112) leyers of &’ showing the three faulting shears, only one of which, $[lli], does not disturb the superlattice. Cu atoms: @--first layer, O-second layer. Al atoms: m-first layer, n-second layer.
SWANN
cells forming atoms
AND
a tetragonal
at positions
WARLIMONT:
COPPER-ALUMINUM
unit cell with aluminum
of the type
440, 004 (shown
filled circles in Fig. 12), and copper atoms at 000,
as
O&t,
the kinematical those reflections affected
(hk2) of planes in the disordered
analysis
become
(hk2Z) in the ordered
similarly,
B1’; become
indices
[uv(w/2)]
the proposed
giving
martensite,
tetragonal
lattice,
directions
even, and h and k are mixed, patterns
studied,
and their displacements agree
with
11(a), (b), and (c). The lower symmetry
if 1 is odd.
if 1 is
In all the
superlattice
structure,
reflections
i.e. Figs.
of the ordered
by comparing
3(b),
structure
is
Fig. 3(c) with Fig.
11(b), both of which show basically
the same pattern
(IlO)@G however only the pattern Fig. 1 l(b) whose zone axis, [ilO], is 120” away from that of the pattern in Fig. 3(c), contains superlattice of the lower symmetry possible
reflections.
between
shears which may be expected (112),1,
plane.
plane,
I+J421].
i.e.
Hence
Because
of the ordered structure,
to differentiate
close-packed
it is
the three faulting
to occur on any one
In pi’, the faulting shears on the (111)8’,
the
become
phase
1LZ[241], &[lli],
changes
(a)
due
to
faulting may be either ;[2h
-
f[h + k -
$[-4h It will be noticed
etc., and in Fig. 11(c) the ii4,006, are not affected by faulting, the Q[lli]
stacking
14ij
118, etc. reflections
which indicates that only
fault shear operates
on the (112)
plane, since this is the only shear on that plane which produces
no phase shift for these reflections.
of the types
1+[241]
and
,$[421]
(Shears
would
give
rise
to phase shifts of 7~12for 033 and 14i, etc. and 7r for ii4,006, and 118, etc.) The above reasoning holds for all the close packed planes of &‘. faulting
shears
Furthermore,
are restricted
it may
+(lll)
on
(112).
that the operating
the order
of
the
&’
(see Fig. 13).
The density of stacking faults in /3’ and br’
this
analysis
stacking
Paterson
fault extends
tion volume;
and N is any integer.
assumed
completely
that
(a) each
across the diffrac-
(b) only one set of close packed planes
is faulted,
and
(c)
the
distribution
of
faults
is
random. The ,Y and pi’ specimens are such that they comply with the first two assumptions. Patterson
analysis
distribution
However,
in using the
we have had to assume that the
of faults in /j’ and j3i’ is random.
In Table3 the measuredvalues ties u have
been
compositions
listed
for
h, and the probabili-
specimens
and heat treatments.
of different
It may be seen
from Table 3 that within the accuracy of our measurements the density of stacking faults across pi’ plates is independent
of composition
and of the time of iso-
thermal ordering prior to transformation. deformed
of selected area diffraction showed
&’
approaches After
that
specimens
by rolling,
attain
hexagonal
positions,
because
of faulting
system.
Although
the
patterns
stacking
fault
0.5 with increasing amounts of
diffuse
the
were reduced the diffraction
50 per maxima
but these are extremely on more
structure
than
of
one slip
deformed
pi’
becomes hexagonal in nature the microstructure is very different from the hexagonal y’, compare Figs. 9 and 15, and it is therefore
a misinterpretation
of
TABLE 3. Diffraction peak-position parameter, h,, and stacking fault probability parameter, a, of /?’ and pl’ mctrtensites at various compositions and after different heat treatments
Composition Al [wt.%] 10.8 11.2 + 1.1 wt.% Sn 12.1
The spacing of the stacking faults in p’ and /J’ may not be determined by direct measurement since on the
12.1
micrographs they are too closely spaced to be clearly resolvable. However, Patersono7) has shown, using
12.2 13.0
3
. . . is
of a reflection measured in
In general, therefore, to
be shown
shears $(l 11) do not disturb superlattice
In
cent in thickness the 033,
Paterson fault will
1
the direction of its displacement
deformation.
that in Fig. 1 l(b)
the
l-Ltanf:(h,-3N+$) 1/3
probability
+ 2k + 11.
to
u that a stacking
occur at any given layer in a sequence ABCABC
of
I]
According
where h, is the co-ordinate
of
patterns which are
given by
Measurements
4k + Z],
or
faulting.
that for
due to faulting were found to
the proposed
easily recognized
by
that the density
from the displacement
in the diffraction
the probability
(Fig. 12) all (McZ),~, reflec-
tions are allowed for which h and k are unmixed diffraction
i.e.
[UWUJ]in B’,
It was calculated
in Bi’.
superlattice
i.e. B’,
theory of diffraction,
of faults may be calculated
tOi, O!&, &Op, &$a (not shown in Fig. 12). The indices
519
MARTENSITES
Heat treatment
Peak position”” h, * 0.02
Stacking fault probability bl + 0.01
850”C,quenched 85O”C, quenched
1.42 1.43
0.45 0.45
85O”C, 5’ 525”C, quenched 85O”C, 2’ 525”C, quenched to 100°C 85O”C, quenched 85O”C, quenched
1.38
0.44
1.37
0.44
1.38 1.37
0.44 0.44
520
ACTA
METALLURGICA,
(a) Showing the internally twinned acicular plates. The plate in the center of the micrograph has its twinning plane parallel to the surface of the specimen.
(c) Showing the fine cross striations within the twins c)f & y’ plate.
VOL.
11,
1963
(b) Part of a plate showing internal twinning on two different systems.
Showing the superlattice domain boundaries in dark field illumination.
FIG. 14. Microstructures of y’-martensite Cu-14.1 wt.% Al.
Transmission electron micrographs.
SWANN
AND
WARLIMONT:
COPPER-ALUMINUM
521
MARTENSITES
structures of the type shown in Fig. 15. The deformation of y’ proceeds partially heavily
faulted
twins.
transformation
twins
by the formation
of new
The matrix
and the internal
of
become
heavily
patterns
of y’ is
y’
also
faulted.
Electron diffraction of y’ The interpretation complicated
of diffraction
by the relaxation
in contrast to the case of/l’,
of Laue conditions,
occurs in at least two different directions plate.
and
the streaking of reflections in any one
However, diffraction patterns could be analyzed
by orienting the specimens carefully such that one set of diffraction streaks lay in the plane of the pattern. The electron diffraction results confirm the structure of y’ derived from X-ray 15. y’ martensite deformed by rolling to 40 per cent reduction in thickness. Cu-13.7 wt.% Al. Transmission electron microgreph.
et a1.(14) Disregarding
FIG.
of y’ is h.c.p. orthorhombic
X-ray
diffraction
converted
measurements
that
/J’
may
be
to y’ by deformation.(5p7>8)
Quenched
and
alloys
with
more
than
13 wt.% Al
y’, (see Fig. 14(a)), which is from ,!?r’. It is noteworthy that
,!lr’ are indistinguishable under the optical Each y’ plate contains parallel bands
microscope.
which are about 100-500 field illumination
that
(121) orthorhombic
readily distinguishable y’
diffraction
A thick, and by using dark
and electron diffraction
analysis.
({liol),
h.c.p.).
analysis
striations
Examples
of the
In the twin patterns studied
were found which were attributable (see Fig.
matrix and twins of the martensite trace
are internally
twin patterns are shown in
the very fine cross striations by
is used
The diffraction
plane was found to be (2011,
observed
Figs. 17(a), (b), and (c). no reflections
the structure
of y’ is, however,
the y’ plates
The twinning
most commonly
the martensite
by Kurdjumov
(see Fig. 16), and this description
indicate
twinned.
Metallography of y’ contain
The superlattice
in the subsequent patterns
observations
the superlattice,
it
are parallel
was
to
14(c)), in the
plates.
determined
However, that
these
to either the twinning
plane
it was found
{202}, (122) or the faulting plane (001) in addition to
that the bands are twin-related. The thickness ratios between the twins and the matrix vary from 1.3 +
the normal y’ twinning plane {201}, (121). The diffraction patterns in Fig. 17 show that diffraction
3.3 : 1. Most of the internal twins are on one system
streaks
and extend
plane, and must therefore be the result of a reduction
completely
but occasionally
across the martensite
plates
plates contain two sets of twins (see
Fig. 14(b)).
G4.228
The matrix and twins of the martensite further
subdivided
by fine striations
several sets of traces (see Fig. 14(c)). the determined
trace
plates
that
revealed
directions these
domain
belonging
to of
fine striations
structure
plates are
An analysis
on ten
twins of higher order and stacking The antiphase
are always
martensite represent
faults. due to the super-
lattice of y’ was revealed by dark field illumination (see Fig. 14(d)). It will be noted that the domain boundaries are discontinuous across the twins, indicating that the martensite has formed subsequent to the ordering reaction. It is also evident that the twinning of y’ disturbs the order to a greater extent than the faulting of ,&‘. Deformation of y’ martensite by cold rolling to 40 per cent reduction in thickness results in micro-
perpendicular
to
twinning
b=5.20%
A
0
the
F-l=! I t
a=4.51X
l-
ALUMINUM
ATOMS 0
-COPPER
ATOM
FIG. 16. The orthorhombic unit cell of y’ martensite projected on the (001) plane. (After Kurdjumov et ,Z.“*))
522
ACTA
in size of the coherently internal twinning, It
has
been
scattering
calculated
that
METALLURGICA,
k = 2n’
the
and
patterns
forbidden
contain
according
I=
Metallography
et a1.(14)
2n” + 1 Some of the
reflections
which
are
to this rule, e.g. the retlections
{021), (321) in Fig. 17(c). may be explained diffraction.
1963
orthorhombic
where n, n’ and n” are any integers. diffraction
11,
crystal due to the
superlattice of y’ proposed by Kurdjumov requires reflections hkl to be forbidden if h = 3n,
VOL.
However,
satisfactorily,
these reflections
as the result of double
of mixtures of /I; and y’
Two alloys with 13 per cent and 13.7 per cent Al were observed
to
Unfortunately,
these compositions
contain
mixtures
lower and upper concentration
of &’
and
y’.
were just at the
limits of the two phase
region, and it was difficult to find the areas showing both phases. In
both
alloys
distinguishable
the
and
two
may
martensites occur
plates or both may be contained both ,!ll’ and y’ are contained
are readily
either
as separate
within one plate.
If
in one plate the transi-
tion is abrupt (see Fig. 18). The faulting frequency y’ is clearly less than it is in PI’. indicates
in
The microstructure
that many plates have begun transforming
to ,Q1’and have then continued
to grow lengthwise
by
Although the type of forming y’, or vice versa. martensite may change during the lengthwise growth of a plate no further change is observed in the sidewise growth.
In a single two-phase
plate the rate of side-
wise growth is the same for either phase and the habit plane of the plate is only slightly deviated. Hardness measurements
of /II’ and y’
The metallographic
and crystallographic
tions were supplemented over the composition was realized
that
by hardness
range lo-13 the hardness
domain size depend sensitively
wt.% and
observa-
measurements Al.
Since it
the antiphase
on the quenching
rate,
all specimens were cut to the same size (2 x 2 x 0.2 cm3) and quenched at two different rates. The specimens of each set were treated
simultaneously.
The
results have been plotted in Fig. 19 and it is shown that hardness decreases with increasing quenching rate and with aluminum contents approaching the The composition dependence composition Cu,Al. agrees with that found by previous investigators.‘3p22) DISCUSSION
Martensite crystallography Experimental high-temperature
observations b.c.c.
have
shown
phase /? decomposes
that
the
during
FIG. 1’7. Quadrants of diffraction patterns of y’ martensite with zone axes parallel to the twinning plane. Note the streaking in all patterns normal to the twinning -plane (121). l e-main and superlattice reflections of twin I. C? o-main and sunerlattice reflections of twin II. n n 0 O-reflections due to double diffraction.
(4 The [234] zone. (b) The [274] zone.
Note the splitting of higher order reflections, i.e. 281, due to streaking inclined to the normal of the pattern resulting from second order twinning or faulting.
(0) The [012] zone. Showing forbidden reflections due to multiple diffraction.
SWANN
quenching
WARLIMONT:
AND
to form martensites
COPPER-ALUMINUM
which have basically
either f.c.c. (p’ and p,‘) or h.c.p. (y’) structures.
MARTENSITES
their superstructures.
The relevant
Fig. 20. Using the data shown in Table 1 the principal
an extent
strains of the Bain distortion
placed
almost
certain
diffraction
to hexagonal
maxima
positions.
are dis-
is expected content,(23)
to
decrease
independent
with
of composition.
that a constant
amount
increasing
of faults
vi = aJ2/g,
aluminum
was found
to be
From this it is inferred
of faulting is inherent in the
an undistorted pi’.
i.e. the faulting
deformation Also,
occurred
interface
intrinsic
dominate
necessary
the
between
rather
and f.c.c. structures. From Q and q2 it was calculated shear necessary
of
growth
faults
plane strain is
and y’ Al alloy.
The density
of stacking
faults
is then
given by
gd(111) ‘%h
pre-
=
~
b
= 0.355
that the faulting has to the face
In the case of the h.c.p. y’ invariant deformation is
provided by twinning. It is possible to calculate
g = 0.251.
of the b.c.C.
that the total slip
to attain an invariant
/3 and @’ or ,di and
than
and thus it is probable
production
after the lattice transformation
centered cubic structure. martensite the lattice
twinning
is the lattice-invar-
for
= 0.8910
where ab and af are the lattice parameters
18. Showing the junction between pl’ portions of a martensite plate in a Cu-13.0 wt.% Transmission electron micrograph.
iant
are
q2 = aJa, = 1.2600
FIG.
transformation,
in
and
the energy of stacking faults in #?’ and ,!lr’ the density
are illustrated
This has led
previous X-ray investigators to interpret the structure of p’ and pi’ as distorted h.c.p.(%‘), monoclinic or triclinic.(*) Although
of the transformation
crystallographic
The B’ and pi’ martensites were found to be faulted to such that
relationships
523
the extent of faulting
in /l’ and &’ martensites
or
using the theories
is the where4111,
spacing between slip planes and b is
experimentally.
The discrepancy
and
stacking
the Burgers vector of a partial dislocation (a,/6)[112]. This value is to be compared with sex = 0.44 found theoretical
arises from
fault
the assumption
between densities
observed probably
in the calculation
that
of Wechsler et a1.(24)or of Mackenzie and Bowles.(25) In the present case the former analysis was used. It was assumed that stacking faults extend completely across martensite plates and that the b.c.c. matrix and
faults extend completely across martensite plates. This assumption is not fully supported by the experimental observations since partial dislocations are observed within the martensite plates (see Fig. 4), and
the f.c.c. martensites are related through the Bain correspondence. In the case of & + pl’ this latter
thus more faults are required to achieve the necessary
assumption
was verified by the relationship
between
shear. In y’
martensite
the
lattice
invariant
shear
is
ACTA
524
METALLURGICA,
VOL.
11,
1963
260 240
22
I II
23 I
ALUMINUM
or.-%
24
I 12
25
I 26 13
wt.-% CONdENTRATlON
FIG. 19. Hardness of martensitic Cu-Al alloys after quenching at different rates. C--Quenched in water at 100°C. @-Quenched in brine at 25°C.
accomplished mainly by twinning on {201}, (121) of the orthorhombic lattice and with small amounts of twinning and slip on {202}, (122) and (OOl}, respectively. Assuming the twinning plane to be (121) or {201}, Mackenzie and Bowles(25) have calculated the ratio of the thickness of the two twin orientations in a y’ plate to be 3.5 : 1. The experimentally observed ratios are between 1.3 : 1 and 3.3 : 1. This discrepancy between the theoretical and the lower experimental values may again be explained by the fact that some of the twins do not extend completely across the martensite plates. The habit, planes of both /J’ and y’ have been determined experimentally by Greninger .(7) Mackenzie and Bowles(25) have calculated the habit plane of y’ and have obtained agreement with the experimental results. The habit plane of /II’, however, was not determined theoretically because of the uncertainty
h
.%Ob_ -
COlOlf COlOlb 4 I
I
I
IO
20
FIG. 21. Stereographic representation of the habit plane of p’ martensite showing the extent of the experimentally determined ( 1m )“1 and calculated (0) poles in relation to the @ lattice.
In the present study the WechslerLieberman-Read theory has been used to determine the habit plane of PI’ on the basis that the transformation is b.c.c. -+ f.c.c. Using the values given above for qI and qZ the direction cosines of the habit plane normal are found to be in its structure.
-0.1791 0.7252 0.6648 [ IB The position of this pole agrees well with Greninger’s results which are plotted in Fig. 21. The diffraction results show that only four variants of a possible twelve partial dislocations may create stacking faults during the /I1 to PI’ transformation. The preference for certain partial dislocations may also be deduced from consideration of the types of bonds changed during the motion of each of the twelve possible partial dislocations. For example, from Fig. 13 it may be seen that the partial dislocation &[lli] does not disturb nearest or next nearest neighbor relations, whereas the partials ,%[&.21], &2&l] do; see also Table 4. Since the interaction energies of next nearest neighbors in close packed structures are small,(26127)the motion of the parrtials &[321], &2&l] would create high energy antiphase boundaries and is therefore avoided. This restriction TABLE 4. Change in bonding per atom across the faulting plane (112) resulting from the three possible faulting shears in PI’ martensite Faulting shear f[ llT1
OlREGTlON
Neighbor relation
Al-Al
cu-cu
nearest
0
next nearest
0
~___
Cu-Al
0
0
0
0
OF SHEAR
FIG. 20. Crystallographic relationships of the &B distortion. Drawing is projected on to the (fOl)f or (001)b plane.
h[421]
nearest
+a
++
-4
$.J%l]
next nearest
-4
-4
+1
SWANN
WARLIMONT:
AND
COPPER-ALUMINUM
in the shearing process limits the number of possible habit plane variants
generated
during the pi to &
The ordering
of ,3
The superlattice
in &’ martensites is always formed
transformation from twenty four to eight. However, since the selected-area diffraction patterns in this
in the high temperature
investigation
appearance
were taken from martensite
plates with
transformation
a width of at least two microns, it is possible that the
lattice
remaining
boundaries
16 habit plane variants
are found
among
formation
tion of the high energy stacking faults.
temperature
plane variants (excluding
the number
is 24 since three partial
those
producing
extrinsic
were shown to be absent) operating close packed
planes
of habit
dislocations faults,
which
on four different
may each generate
two undis-
torted planes.
It was found 13.0-13.7
that the transition
occurs
from
/3r’ to y’
in the concentration range from It is interesting to consider this
per cent Al.
result in relation to the composition
dependence
stacking
copper-aluminum
alloys,
fault which
boundaries
of the
indiscriminately plates.
super-
across
The Cu,Al
the
super-
lattice is analogous to Fe,Si, and the low temperature of Fe,Al,
which
have
DO,
of a superlattice form
structures.
analogous
of Fe,Al
The
to the high
(B2 structure)
was not
observed. The grown-in
antiphase
show no tendency crystallographic
domain
boundaries
in /3i’
to be parallel with any specific and form a continuous plane,
network (above 11 .O per cent Al) with three-boundary junctions predominating. These observations are in agreement with calculations concerning antiphase
The relative stability of /?i and y’ martensite
extend
of martensite
driving force may be high enough- to allow the formamartensite
b.c.0. phase p prior to the This is evidenced by the
pi’.
of antiphase
form
In the disordered
to
which
the smaller plates. This might occur since the smaller plates form at lower temperatures where the chemical
525
MARTENSITES
energy
of
is shown
f.c.c.
in Fig.
1.
There
of the
is some
boundaries of the DO, structures by Marcinkowski Brown.(2g)
and
It has been shown that the ordering of ,!l occurs by the growth of isolated domains of #J into a disordered matrix.(30)
Alloys nearer the stoichiometric
tion
a larger
have
domain
ordering temperature is faster.
size, since
composi-
the
critical
is higher and, therefore, growth
The domain
size may also be increased
by
justification for considering the stacking fault energy of the f.c.c. structure to represent the difference in free
lowering
energy between f.c.c. and h.c.p. phases,(23s28) i.e. when
proceed further with less undercooling.
the stacking fault energy is zero, the f.c.c. and h.c.p. phases are in equilibrium. By extrapolation into the
num concentration
metastable
and are enclosed in a disordered or short range ordered
f.c.c.
region
the
stacking
fault
energy
the
domains
quenching
rate,
is reduced
of the superlattice
because
growth
may
If the alumi-
below
11.0 wt.%
the
are no longer in contact
(Fig. 1) is found to be zero at 13.5 -& 0.5 per cent Al where &’ and y’ do in fact coexist. The fact that ,&’ is
matrix.
ordered thus appears to have little effect on the free
(M,) is only a few degrees for the 11 per cent Al alloy,
energy of the f.c.c. structure.
and presumably the ordering reaction is interrupted
Since the transformation expect
the pi’-y’
composition 13.0-13.7 the
is diffusionless,
boundary
to occur
we would
at a unique
rather than over the composition
per cent Al.
stacking
fault
However,
energy
range
it may be seen that
is rather
insensitive
to
The difference between the ordering temper-
ature (To) and the martensite
the martensitic transformation. range
order
quenched
should
In an alloy containing
tant
this /3’ martensite
martensites periphery
are formed. of a growing
growth to be continued compositions is supported
whether
the
f.c.c.
or
h.c.p.
Thus the strain field at the martensite
plate may favor
by the alternate
structure
where it is chemically less stable. by the experimental observations
at
This that
,!?i’ and y’ may both be contained in one plate. At compositions outside the range 13.0-13.7 per cent Al the nonchemical factors such as strain energy are presumably too weak to cause the chemically stable martensite to form.
less
not
by
With a slight extrapo-
is above
in the
10 per cent aluminum the M, however,
contains
it is believed
regions
of short
This may be deduced by considering
which some of the larger martensite mid-region
slowly
less than 10.5 per cent Al.
To (see Fig. 1) and the super-
lattice is not detectable; order.
be present
alloys containing
temperature
determining
temperature
lation of the results in Fig. 7 it may be seen that long
composition in alloys containing more than ~6 per cent Al and thus nonchemical factors may be imporin
formation
that range
Fig. 8, in
plates contain
a
which appears lighter than the rest of the
plate. By examining the edges of the thin foils it was shown that these regions are thinner and must therefore dissolve faster during the electropolishing process. This difference in rate of electrochemical attack in the larger plates is associated with a varying degree of short-range order across them. The occurrence of such a variation may be explained by supposing that the
526
ACTA
METALLURGICA,
VOL.
11, 1963
long light bands are the first martensite plates to form
structurally
from disordered ,!I, i.e. at about 430°C.
hardness is predominantly
identical
martensite,
pi’, it is seen that
The lengthwise growth of these initial plates continues until collision
superlattice.
with
and stacking fault density do not change measurably
other
similar
plates;
however,
their
sidewise
growth occurs more slowly and at temperatures short range ordering smaller martensite temperatures mid-regions
is taking
plates which are nucleated at lower
should
therefore
The large martensite have
contrast
contain
thinner
strongly
This is shown to be the
plates in the 10 per cent Al
curved
boundaries.
which are constrained It is suggested
that
This is in
with higher Al-
to have planar interthe relaxation
of this
constraint in the 10 per cent Al martensite is connected with the short range diffusion
process which we have
shown to occur during the formation More specifically, development loops
of the curved boundary
behind
CS!.(~~))accounts
8(b)).
of vacancies
or ahead
Such a mechanism dislocation martensite
of these plates.
the shape change accompanying
by the condensation
(originally high
the
proposed
interface.
by Baker et
density
of elongated
In the 10 per cent Al alloy the smaller marten-
expected plates
have
straight
by the above are
diffusion
formed
interfaces reasoning,
at
is more difficult.
be inferred not
lower
which
is to be
since the smaller
temperatures
where
The results also show that
the sidewise growth of p’ plates can proceed
at rates
which are much lower than those measured for ferrous
contribute
composition
studies
concerned
martensites
effect of solid-solution of carbon,
microstructure
does
in hardness
with
to the variations plotted
in Fig.
19.
metric composition,
Cu,Al,
superlattice.(34) the variation
increase
in
is similar to the behavior of the Fe,Al
(DO,)
This behavior may be associated with of the antiphase
superlattice.
The
from the stoichio-
By comparison
domain
size of the
of Figs. 7 and 19 it is
seen that upon lowering the aluminum
concentration
from 25 at.% the hardness increases whereas the domain size decreases. A corresponding relationship between the resistance the “antiphase Flinn(35)
to dislocation
boundary
space,”
r=-
motion, t, was
7, and
given
by
(r
t where
(r is the energy
boundary
of the additional
antiphase
area created by the passage of dislocations.
There is a complication hardness
and
in this interpretation
domain
size are both
that the
increased
by
decreasing the quenching rate. an increase quenching
in antiphase
This may be related to boundary energy at lower
rates which overrides the weakening
due to an increase in the domain size. the short range order component at lower quenching
effect
An increase in
of the superlattice
rates may, also, contribute
to an
increase in strength. ACKNOWLEDGMENTS We
Thestrength of p’ and PI’ martensites Recent
range from 10-13 wt.% Al it will
hardness with increasing deviation
martensites.(32$33)
ferrous
since the microstructure
that the martensitic
is accommodated
loops found near the interface of the plates in the 10 per cent Al alloys (see Fig.
site plates
in the concentration
to form dislocation
of the advancing
for the
Furthermore,
found in flow stress measurements
to the plates of martensite
content faces.
not
and should not be in contact with the thin
regions of the larger plates. case (see Fig. 8(a)). alloy
where
place in the j3. The
due to the presence of the
with
the
have emphasized hardening
strength
either,
of
(1) the
due to the presence
or (2) the presence of a fine internal twin
structure which presents a strong barrier to dislocation motion. In the case of copper-aluminum martensites, solid solution hardening is unimportant since the hardness decreases with increasing
helpful
The correlation of the strength of the /?’ and /!li martensites with their micro- and crystal-structures depends on several factors. It may be supposed that some work hardening has occurred due to the passage of the partial dislocations during the transformation. However, by comparing the hardness of the disordered martensite, @‘, with that of the ordered but otherwise
suggestions,
performed
members
of this
and to K. D. Fike who kindly
the hardness measurements.
to thank S. Amelinckx,
Centre d’Etude
Also we wish de 1’Energie
Nucleaire, Mol-Donk, Belgium; and D. W. Pashley, Tube Investments Laboratory, Hinxton Hall, U.K. REFERENCES
solute concentra-
tion (see Fig. 19).
wish to thank the following
Laboratory: We are particularly grateful to C. A. Johnson for his discussion M. J. Marcinkowski for
i: 3. 4. 5. 6. 7.
W. PITSCH, J. Inst. Met. 87, 444 (1959). P. M. KELLY and J. NUTTING, J. Iron St. Inst. 197, 199 (1960). I. TARORA, Nippon. Kink. Gakk. 13, 16 (1949). M. HANSEN. Constitution of Binaw Allow, McGraw-Hill, New York (‘1958). ” _ G. V. KURDXXvfOV, G. Bull. Acad. Sci. U.S.S.R., Chew Ser. 2, 271 (1936). J. OBINATA, Mem. Ryoj. Coil. Engng. 2, 205 (1929); 3, 87 (1930); 3, 285 and 295 (1931). A. B. GRENINGER, Trans. Amer. Inst. Min. (Metall.) Engrs. 133, 204 (1939).
SWANN
AND
WARLIMONT:
COPPER-ALUMINUM
8. N. NAPANISHI, Trans. Jap. Inst. Met. 2, 85 (1961). 9. N. A~EEV and G. V. KURDJUMOV, Phye. J. Soviet Union 2, 146 (1932). 10. A. J: BRADLEY and P. JONES, J. Inst. Met. 51, 131 (1933). 11. G. WASSERMANN, Metallwirtschaft 8, 133 (1934). 12. G. V. KURDJUMOV and T. STELLETZKY, Metallwirtschaft
18, 304 (1934).
13. V. GAVRANEK, E. KAMINSKY and G. V. KURDJUMOV, Metallwirtschaft 15. 370 f 1936). 14. G. V. KURDJUMOV, V. MIRE&II, and T. STELLETZKAYA, J. Tech. Phys. U.S.S.R. 8, 1959 (1938). 15. G. V. KURDJUMOV, Zhumal Tech. Fiz. 18, 999 (1948). 16. G. V. PATSER and P. R. SWANN, J. Sci. Instrum. 39, 58 (1962). 17. M. S. PATERSON, J. AppZ. Phys. 23, 805 (1952). 18. M. J. WHELAN and P. B. HIRSCH, Phil. Mug. 2, 1121; 1303 (1957). 19. A. HOWIE, Ph.D. Dissertation, Universitv of Cambridge (1960). 20. C. A. J~~~~~~,submittedtoActaCryst. 21. I. ISAITSCREW,E. KAMINSKY and G. V. KURDJUMOV, Trans.Amer. Inst. Min. (Metall.) Engrs. 128, 361 (1938). 22. M. J. BOULDOIRES,Rev. MLt. 24, 463 (1927). 23. A. HOWIE and P. R. SWANN, PhiZ. Mag. 6, 1215 (1961).
MARTENSITES
527
24. M. 25.
S. WECHSLER, D. S. LIEBERMAN and T. A. READ, Trans.Amer. Inst. Min. (Metall.) Engrs. 197, 1503 (1953). J. K. MACKENZIE and J. S. BOWES. Acta Met. 5. 137 -7
(1957).
26. J. M. COWLEY, Phye. Rev. 77, 669 (1950). 27. C. H. SUTCLIFFEand F. E. JAUMOT, Acta Met.
1. 725 (1953). 28. C. S. BARRETT, Trans.188, 123 (1950). 29. M. J. MARCINKOWSKI and N. BROWN, J. AppZ. Phys. 33, 537 (1962). 30. P. R. SWANN and H. WARLIMONT, Acta Met. 10, 903 (1962). 31. R.G. BAKER,D.G.BRANDON and J. NUTTING,PhiZ.Mag. 4, 1339 (1959). F. F~RSTER and E. SCHEIL,2. MetaZZk. 82, 165 (1940). ii: R.F. BuNsRAR~~~R.F.MERL, Trans.Amer. Inst.Min. (Metall.) Engrs. 197, 1251 (1953). communication. 34. P. X. KAYSER, private 35. P. A. FLINN,T&s. Amer. Inst. Min. (Meta,ZZ.)Engrs.218, 145 (1960). 36. W. B. PEARSON, A Handbook of Lattice Spacings and
StructureofMetaZs
(1958).
and Alloys, Pergamon Press,NewYork
ELECTRON-METALLOGRAPHY COPPER-ALUMINUM
AND CRYSTALLOGRAPHY MARTENSITES
OF
and H. WARLIMONTt
P. R. SWANNf
The microstructures, cryst81 structures and mechanic81 properties of the copper-81uminum mertensites, forming from the j3 solid solution, have been investigated by electron microscopy, electron diffmcThree types of m8rtensite have been observed; internally faulted tion and hardness measurements. f.c.c. mertensite, B’ internally faulted tetr8gonal (ordered) martensite, &‘, and internally twinned orthorhombic (ordered) martensite, y’. The presence of the superlctttice ,!?I has been shown to exert 8 considerable restriction on the number of operative slip systems 8veilable during the transformation to B1’. It ~8s determined that short range diffusion oocurs during the initial stages of the /? to 8’ transformation. Calculations using the phenomenological theory of mertensite transformation were found to agree with the results of the structural observations. The mechanical properties are rel8ted to the ordering phenomena occurring during the trrtnsformation rather th8n to the internal faulting or twinning. The composition ranges of the metastable equilibrie 8re discussed with reference to the composition dependence of the stacking fault energy of the primary solid solution 8nd to internal stresses. MICROGRAPHIE
ELECTRONIQUE
ET
CRISTALLOGRAPHIE
DES
MARTENSITES
CUIVRE-ALUMINIUM Les auteurs ont Btudib 18 microstructure, la structure cristalline et les propri&% mbcaniques des martensites cuivre-aluminium form&es B partir de 18 solution solide p, en utilisant la microscopic Blectronique, la diffraction des Electrons, et des mesures de duret& Trois type de martensite ont &t6 observ&: une martensite c.c.f. 8vec dbfauts internes fi’, une martensite t&ragonale (ordonnbe) 8vec dbfauts interries, B1’ et une m8rtensite orthorhombique (ordonnbe) avec macles internes, y’. Les auteurs ont observe que 18 presence du superrbseau ,!ll provoque une diminution importante du nombre de systemes de glissement susceptibles d’intervenir dans 18 transformation donnant PI’. 118 QtB montrb qu’il se produit de la diffusion B courte distance pendant les premiers stctdes de 18 transformation de p en B’. Les calculs utilisant 18 thborie ph6nombnologique de la transformation m8rtensitique ont 6t& trouvbs en bon accord 8vec le’s r&sultats des observations structurales. Les propri&& m&aniques sont libes 8ux dbplecements internes se produisant au tours de 18 tiansformation, plut6t qu’8ux dbfauts ou 8ux macles. Les auteurs discutent les limites de composition des gquilibres m&astables en consid&+& 18 maniere dont 1’Qnergie des fautes d’empilement de 18 solution solide primaire varie 8vec la composition, et en consid&ant Bgalement les contraintes internes. STRUKTURUNTERSUCHUNGEN
VON
MARTENSITISCHEN
KUPFER-ALUMINIUM-LEGIERUNGEN Mikrostruktur, Kristrtllstruktur und mechanische Eigensch8ften von martensitischen Kupfer-Aluminium-Phasen, die su8 dam p-Mischkristall entstehen, wurden mit Hilfe von Elektronenmikroskopie, Drei Martensitarten wurden beobachtet: ein Elektronenbeugung und HBrtemessungen untersucht. fehlerhafter kubisch fl&chenzentrierter Marten&t (B’), ein fehlerhafter tetragonaler (geordneter) Martensit (PI’) und ein verzwillingter orthorhombischer (geordneter) Martensit (y’). Wie sich gezeigt hat, wird die Anzahl der Gleitsysteme, die wiihrend der Umw8ndlung in die &‘-Phase betiitigt werden kannen, durch die Anwesenheit der p,-U‘berstruktur wesentlich eingeschriinkt. Am Anf8ng der B + ,Y-Umwandlung wurde eine suf kurze Entfernungen beschrgnkte Diffusion festgestellt. Die Ergenbisse, die die phlinomenologische Theorie der Martensitumwandlung liefert, stehen in Einklang mit Die mechanischen Eigenschaften sind wohl mehr auf Ordnungsdenjenigen der Strukturuntersuchungen. vorg&nge wiihrend der Umwandlung aIs auf Fehlerhaftigkeit oder Zwillingsbildung zuriickzufiihren. Eine Diskussion der Konzentrstionsbereiche, in denen metastabile Gleichgewichte mdglich sind, beriicksichtigt innere Sp8nnungen und die Abhiingigkeit der Stapelfehlerenergie von der Zusammensetzung im prim&m Mischkristall.
INTRODUCTION
Recent
observations
transmission
electron
microscopy
presence of a fine twin structure plates.(1*2) these
martensities have revealed
internal
twins
act
by the
within the martensite
It was suggestedc2) that the boundaries
motion of dislocations dominant contribution martensites.
of martensite
of ferrous
as strong
barriers
of
to the
plates.t2)
the increase
METALLURGICA,
VOL.
11, JUNE
in strength
1963
In
contrast
to
iron-carbon
of twinned martensites,
accounted for by differences in microstructure. The part of the copper-aluminum equilibrium diagramt4) which is pertinent to this study is shown in Fig. 1 (full lines). Upon quenching from the j3 range, alloys containing more than 11 wt.% Al first become ordered to form & and then transform martensiti-
* Received August 3, 1962. 7 Edgar C. Bein, Laboratory for Fundamental Research, United States Steel Corporation Research Center, Monroeville, Pennsylvania. ACTA
in iron was related
copper-aluminum martensites (/3’ and y’) show no extraordinary strengthc3) and in the present study we have tried to determine whether this could be
and in this way provide the to the strength of ferrous
Furthermore,
with carbon content
to the observed increase in the proportion
tally 511
to
either
,!S1’ or
y’.
The
temperature
and
ACTA
512 ATOMIC
METALLURGICA,
VOL.
11,
a phase.(g-ll)
PER CENT ALUMINUM
diffuseness
1963
This uncertainty
of many
The /?-&’
lattice relationship
by Greninger,(‘)
...
I
0
’ B
3.0-
b g
2.5-
\
\
A900
The crystal structure of y’ has been determined
M, \
$
LO-
a + y2 ___....’ -500
!
‘Y’
i
z z
and
diffraction
2.0-
z
3 = i?
&
B(I, k...
x
Iv)
I 5
CU WEIGHT
--%--....
I _
-
they found that y’ is basically
Greninger”)
200
that fi alloys containing distinction However, symbol
I )fy;5,,,
8’.
the ordered
in this paper it is convenient
contains
two
twin-related
For
the
structure
in this study
of 8’ and PI’.
often been described recently
as distorted
as monoclinic
p-y’
orientations lattice
of the same
relationship,
Kurd-
Wl)Y’
11Wl)B
wwY’
11WllB
It has also been reported
by Kurdjumov,(5’
Grenin-
ger,(‘) and Nakanishi(*) that pl’ may be converted into y’ by deformation.
no
and dis8’.
to restrict the
/3’ to describe only the disordered
martensite.
are described
Table 1. There has been some uncertainty crystal
Previously,
which were both designated
The phases involved
symmetry.
jumov(15) gives
less than 11 wt.% Al trans-
was made between
in rhombic
of these metastable
martensite,
ordered martensites,
h.c.p. with
that each y’ plate usually
lattice.
EXPERIMENTAL
phased5) are also shown in Fig. 1. It will be noticed dis0dered
has oberved
IO PER CENT ALUMINUM
ranges of formation
by
X-ray
but since the martensite
larger and may be described
Pm. 1. Copper rich end of the copper aluminum equi),‘*I also showing M, temperlibrium diagram (--atures of /I’, PI’ and y’(- * -. -) us) temperatures (- * * - . *)(15) and the ~tack~~f<~$$~ of c( phase alloys, (- . . . -).(as)
composition
From
inherits the order from /& the resulting unit cell of y’ is
-400
I jj\Y’ I Bl , I
‘\
co-workers.(12-14)
a = 2.60 and c = 4.22 kX,
: ;
, ‘\ 4 - 300
I.\
iI
0.5-
form to a
who found that the pole of the basal
be about 2” from {133jfi. Kurdjumov
1.5-
lines.
has been investigated
deviates by 4” from { 110}s. The habit plane is given to
a4
fz 5
by the
diffraction
plane of the martensite (assuming hexagonal structure)
a
E
was caused
of the X-ray
in
melts prepared
were homogenized atmosphere
and hot-rolled
The compositions Table 2.
as to the exact
The structure
has
but it has also
been suggested that B1 is closely related to the f.c.c. TABLE 1. Stable and metastable
mixture cont.
containing
HCl and
electrothinning
phases in copper-rich
of 1 mm.
are listed in
were all solution
martensite.
treated at
Thin foils suitable
were prepared
the martensite
electrolyte
stock
between 850°C and 1000°C and quenched
into brine to produce thinning
to a thickness
of the alloys studied
The specimens
temperatures
from high purity
for 24 hr at 900°C in a hydrogen
for electron microscopy
h.c.p.(6*7) and more
or triclinic?)
Levitation
PROCEDURE
specimens
by chemically
to 0.05 mm in a
four parts cont. HNO,,
five parts using
the
cone.
H,PO,,
window
used for electrothinning copper-aluminum
one part and
technique. consisted
then The
of one
alloys
Unit cell dimensions Phase 5z
Structure
type
f.c.c., Al
WXI
@Xl
3.61-3.66
-
-
-
@Xl
PI, metastable
cubic, ordered,
2.917 2.95 5.84
,Y, metastable B I’, metastable
“0, f.c.c., Al tetragonal,
3.676
B
Yz y’, metastable
b.c.c.,
A2
ordered
rhombohedral orthorhombic ordered
-
(9) 4.51
5.20
7.352
4.22
Remarks
Reference
0 to 9.0 wt.% Al, room
36
temperature. 12.5 wt.% Al. 11.3 wt.% Al, 672°C. 11.9 wt.% Al, 350°C.
3: 3
Martensite, heavily faulted. 11.86 wt. y0 Al; martensite, heavily faulted; lattice parameters derived from published d-spacings. Essentially y-brass type. 13.6 wt.% Al; martensite, finely twinned.
This work. This work.
36 7, 14, this work.
8
SWANN
AND
WARLIMONT:
COPPER-ALUMINUM
MARTENSITES
513
TABLE 2. Chemical composition and phases present after quenching from the B-range to room temperature No. --____
Al [wt.%] 9.1 10.0 10.8 11.0 11.1 11.2 11.4 11.6 11.7 11.9 12.0 12.1 12.2 13.0 13.7 14.1
B 3 4 5 6 7 : 10 11 12 13 14 15 16
Al [at.%]
Phase
19.1 20.7 22.2 22.5 22.7 22.9 23.2 23.6 23.8 24.1 24.3 24.5 24.7_R 26.027.2 27.9-
a + B’ B’
part HNO, and two parts CH,OH
A
t%’ +
Y’
Y’
solution,*
operated
at 15 to 20 volts and -20°C. (b) EXPERIMENTAL
Metallography
RESULTS
of /I’ and PI’
The electron microstructure
of specimens containing
between 10.0 per cent Al and 13.0 per cent Al quenched from
900°C consists
of acicular
plates
(see Fig.
Within every ,6’ and ,$I/ plate there are numerous striations
(see Fig.
3(a))
which
appear
similar scale to the fine twins observed and by Kelly
2). fine
to be on a by Pitsch,(l)
and Nuttingt2) in ferrous martensites.
In Fig. 2 it appears that the spacing of the striations is measurable, varies
but such a spacing is misleading
considerably
in the
same
area
with
since it slight
changes of diffraction that the striations
contrast. It will be shown later are the result of a high density of
Quenched FIG. 3. /11’ martensite. 12.2 wt.% Al.
from
1000°C.
Cu-
(a) Showing the internal striations across t)wo martensite plates. Transmission electron micrograph. (b) A quadrant of the selected area diffraction of a region A in Fig. 3(a). Zone [IlO].
pattern
(c) The diffraction pattern, Fig. 3(b), indexed according to the f.c.c. structure. Note that reflections with a non-zero phase shift $ are displaced from f.c.c. positions. Reflections due to (i) primary diffraction l (ii) double diffraction 0 (iii) threefold diffraction n (iv) fourfold diffraction 0.
FIG. 2. B1’ martensite, consisting of thin parallel-sided plates with internal striations. Cu-12.1 wt.% Al. Transmission electron micrograph. * This electrolyte should be mixed slowly during mixing to avoid an explosive reaction.
and kept
cool
FIG. 4. /&’ martensite showing stacking fault contrast terminated by partial dislocations. Cu--12.2 wt.% Al. Transmission electron micrograph.
514
ACTA
METALLURGICA,
stacking faults extending across the martensite plates. In fact, under certain contrast conditions, partial dislo~tions ~rminating the chara~~ristic fringe contrast of stacking faults could be observed (see Fig. 4). The existence of a superlattice in quenched B alloys containing more than about 10.5 per cent aluminum could be detected by selected area diffraction, and also by dark field microscopy. In the latter case the antiphase domain boundaries of the superlattice were revealed in good contrast (see Fig. 5) by allowing only one low index superlattioe reflection to pass through the objective aperture and form the image. The antiphase domain boundaries seen in Fig. 6 show no preferred orientation and also extend indiscriminately across the boundaries of martensite plates.
FIG. 5. fil’ martensite showing antiphase domain boundaries extending indiscriminately across the interfaces of martensitc plates. Cu-12.2 wt.% Al. Transmission electron micrograph, dark field illumination.
The domain size of the superlattice is decreased by increasing the quenching rate. It is also dependent on composition and increases with increasing aluminum concentration. Fig. 6(a)-6(f) show a series of micrographs illustrating this observation and in Fig. 7 measurements of domain diameter of specimens quenched into boiling water versus the aluminum concentration have been plotted. The character of the domains also changes with composition. The domains of those alloys near the composition Cu,Al are in contact, whereas at lower aluminum contents they are separated by regions of lower order or disorder. This latter observation is best i~ustra~d in Fig. 6(a) where the regions between the superlattice domains are dark throughout, indicating an absence of the @r’ superlattice there. With decreasing aluminum content the intensity of superlattice reflections diminishes and
VOL.
11,
1963
vanishes in an alloy containing 10.0 wt.% aluminum, even after a relatively slow quench into boiling water. The large martensite plates in this latter specimen are unique among the alloys examined, through the presence of a band of preferential dissolution extending along their length (see Fig. 8(a)). It is interesting to note in Fig. S(a) that these bands are not observed in the smaller plates. It is also charac~risti~ of this alloy that dislocation loops are found to be strung out along the stacking fault traces in a [11218’ direction from the edges of the thin regions to the interfaces of the martensite plates. This is shown in Fig. S(b) where the stacking faults are parallel to the electron beam and in weak contrast A center band free from dislocation loops is not observed in alloys of higher aluminum content. Some 0’ and /&’ specimens were cold rolled to determine their mode of deformation. In these specimens bands of high dislocation density appear parallel to the stacking fault striations of each martensite plate. There are also indications in the micrographs of a considerable amount of dislocation movement on planes intersecting the striations, which was confirmed by the diffuseness of selected area diEraction patterns. In addition, a large number of plates contain numerous grains shaped like deformation twins (see Fig. 9). However, the twin relationship could not be confirmed by selected area diffraction because of the pronounced relaxation of Laue conditions resulting from deformation. The observations of Kurdjumov,(5) Greninger”) and Nakanishi(*) that are Pi’ may be converted into y’ by defo~ation incorrect. This is demonstrated dy comparing Fig. 9 showing deformed &’ and Fig. 14(a) showing y’. Nature of the internal striations of #2’ and /?I
The st~cture of /Y and pl’ and the nature and separation of the internal striations were determined by selected area electron diffraction of plates greater than 2,~ in width. A considerable difficulty is encountered in the interpretation of the diffraction data because some of the diffraction maxima are extended in certain directions from one reciprocal lattice reflection to another. This results in an apparent continuous range in d-spacings and angles between certain crystal planes, (see Fig. 10). In order to obtain unambiguous patterns the specimens had to be oriented such that the extension of reciprocal lattice points (streaks) lay in the plane of the diffraction pattern (see Fig. 3(b)). Specimens were placed in these special orientations using a double tilting specimen holder.(l”) The positions of the intensity maxima in planes containing the streaks are then
SWANN
AND
WARLIMONT:
COPPER-ALUMINUM
MARTENSITES
FIG. 6. Illustrating the dependence of the superlattice domain size of B1’ on the aluminum concentration (wt.%) after quenching from 1000°C into boiling water. Transmission electron micrographs, dark field illumination.
515
516
ACTA
METALLURGICA,
VOL.
tions
11, 1963
correspond
reciprocal
are displaced
VALUE OBTAINED FROM A SPECIMEN HEAT TREATED SEPARATELY.
El kl a 0
I500
exactly
lattice
points.
to
certain
However,
by a constant
f.c.c.
0: phase
other reflections
amount from the remain-
ing f.c.c. reflection positions, e.g. lil or 3i3. These reflections are all shown as filled circles in Fig. 3(c). The remaining correspond
reflections
found
on Fig. 3(b) do not
to the a or the twin a reciprocal
lattice,
but may be explained in terms of multiple diffraction. Extra reflections resulting from multiple diffraction are not observed in perfect f.c.c. crystals because the re-diffracted beams coincide with the positions of primary ALUMINUM
CONCENTRATION
beams, altering their intensity.
FIG. 7. Dependence of the average superlattice domain size of /II’ on the Al concentration in martensitic G-Al alloys quenched into boiling water.
determined,
production
of streaked patterns
if
are displaced from f.c.c. reciprocal as in the present case, the repositions,
lattice
diffracted beams produce a set of additional reflections. The positions
accurately
However,
some reflections
since it was found
that the
of the type shown in
double
of these reflections
diffraction
geometrically
produced
by the
of any beam hkl may be obtained
by displacing the origin of the reciprocal
hkl, and since the beam hkl is
Fig. 3(b) is very sensitive to orientation.
lattice
The diffraction streaks were found to be always perpendicular to the striations observed on the
inclined to the incident beam, the sphere of reflection
micrographs.
on hkl.
This direction
of the streaks indicates perpendicular
In contrast fine
scale
streaking &’ plates.
in a
to the striations.
be eliminated
since twin symmetry
any selected
of the
every few _&gstroms
to ferrous martensites,
can
continuity
that the periodicity
lattice must be interrupted direction
and complete
area diffraction
for
on a the
was never observed in patterns
of single ,8’ or
It was observed that in diffraction
patterns
of p’ and pi’ plates (see Fig. 3(b)), some of the reflec-
FIG. 8. B martensite.
should also be tilted through a small angle depending may
However,
be neglected
for low order reflections because
the small
this tilt
divergence
of
the electron beam and the buckling and thinness of the specimen relax the Laue conditions
twinning
as a cause
to the point
re-diffracted diffracted
beams to be observed.
a system of reflections is produced
is represented
by the lenticular
remaining
diffraction
If all the primary
beams shown in Fig. 3(c) (black circles) are
re-diffracted The
sufficiently for the
may
reflections only
be
not
which
shapes (see Fig. 3(c)). produced
explained
by
Quenched from 1000°C into water at 100°C. Cu-10.0 wt.% Al. Transmission electron micrographs. (a) Showing the presence of thin bands of preferential dissolution within large /3’ martensite plates. (b) The high density of elongated dislocation loops across a large ,!I’ martensite plate.
by double three-fold
SWANN
WARLIMONT:
AND
COPPER-ALUMINUM
MARTENSITES
517 9 0
048
0 025
000
FIG. 11(a). A quadrant of the diffraction pattern of planes with a [421] j3r’ zone axis. This pattern contains no superlattice reflections and only reflections with a zero phase shift.
FIG. 9. B’ martensite, deformed 50 per cent by rolling, showing profuse twinning. Cu-10.0 wt.% Al. Transmission electron micrograph.
(triangles)
and four-fold
intensity
of reflections
the order of diffraction reflections tion.
increases will
subsequent
diffraction
high density
order of diffracdue to multiple
from
drawings
of
of streaked reflections
from the
may be explained
by the presence of a
of stacking faults.
This effect has been
for the case of X-ray and Hirsch,os)
and that of Whelan
for the case of electron
These workers show that the
I DIRECTION ELECTRON
theory of Patterson(17)
diffraction
and Howie(ls)
in thin foils.
I
FIG. 11(b). Showing a quadrant of the diffraction pattern of planes with a [ilO] /?r’ zone axis. The extra reflections due to multiple diffraction are not included in the drawing.
OF BEAM I----7
II DIRECTION OF STREAKS
the
patterns.
analyzed using the kinematical
diffraction
The
of reflections
be omitted
The displacement f.c.c. positions
diffraction.
with increasing
An indication
diffraction
(squares)
correspondingly decreases as increases. Also, the breadth of
POLE OF DIFFRACTING RECIPROCAL LATTICE PLANE
Ii
Fm. 10. A two-dimensional illustration showing the origin of displaced diffraction maxima due to relaxation of a Laue condition. Crystals having a true do* spacing may appear to have a spacing d,*. *--true reciprocal lattice points. O-apparent diffraction maxima.
I FIG. 11(c). As Fig. 11(b) but from planes with a [311] /3r’ zone axis. Note that superlattice reflections 033 and 14i are not displaced and hence the faulting shear on (112) must be t[lli], rather than &[2;11] or h[421].
ACTA
518
METALLURGICA,
VOL.
11,
Cz7.3521
it was inferred
I
dominantly faults.
above
intrinsic
This
Howieog)
1963
is
that /?’ and pi’
supported
by
the
who showed that extrinsic
in small concentrations opposite
direction
contain
rather than extrinsic
calculation stacking
of
faults
cause displacements
to those observed
pre-
or growth
in the
in Fig. 3(c) and
although JohnsonczO) has shown that higher densities of extrinsic
stacking
faults yield a more complicated
diffraction
pattern,
our
compatible Further, served
-a
faults
of reflections is determined
waves due to the faulting and by
the density of faults. corresponding produced
to
The phase change for a reflection the
reciprocal
lattice
vector
g
by a shear R is given by @=2ng.R
(I)
For an intrinsic stacking fault on (111) in the f.c.c. lattice the possible shears, R, are $2111
; [El],
or
seen that
the magnitude
of the phase
shift for a
particular
f.c.c.
(hkl) is the same for all
values of R, since h, k and 1 are either all even or all odd.
Hence,
for faulting
chosen as (n-/3)(h where h faulting
the phase
on (111).
may
will be unaffected
that
of the high fault density,
lattice.og)
intrinsic
in ,d’ and
the ob-
symmetrical stacking
&‘.
However,
the presence
of a
of extrinsic and growth faults cannot
In contrast to Obinata’a) and Isaitschew
et aZ.(zl) we
find that diffraction
patterns of /I’ and pi may not be
indexed
to the
according
hexagonal
system,
since
reflections like (0110) which would not be displaced by faulting
are absent.
The present
indicate that disordered structure. order
to
diffraction
@’ is a heavily
interpret
diffraction
results
faulted f.c.c.
results
of
the
be by
was deduced reported
from the DO,
by 0binatat6)
structure
assumed that the transformation b.c.c. to f.c.c., positions,
of &, already
and Wassermannol)
It was
involved
is basically
and that a correspondence
of atomic
typical
of martensitic
during this transformation.
reactions,
persists
The unit cell deduced in
this way is shown in Fig. 12. It consists of two f.c.c.
This is verified in the diffraction
pattern Fig. 3(b), (c) for reflections 232, 333, etc.
shift
2k + 1) and thus those reflections
2k + 1 = 6n07@)
be
ordered /?r’ the probable unit cell of the ,13r’superlattice
Inserting these values of R into equation (1) it may be reflection
would
not
faulting.
Structure of /3’ and pi
In
$1121.
are still
extrinsic
of the hexagonal
concluded
small admixture be excluded.
by the phase
of
faults were present
maxima
are predominant
because
observations
degree
if only growth diffraction
It was therefore
05576 B FIG. 12. The proposed unit cell of B,‘(Cu,Al) mertensite. The structure is of the type DO,, (like Al,Ti) and is shown prior to faulting.
change of diffracted
any
about reflection positions
0
displacement
with
111, 222, etc. and
The phase shifts @ for each row of
reflections are indicated
in Fig. 3(c) and in subsequent diffraction patterns of pr’. It has also been shown(17~ls) that reflections undergoing a negative phase change as a result of intrinsic faulting are displaced in the [ill] direction; in the [ill]
and those having a positive direction.
phase change,
This is found to be the case for
the diffraction pattern Fig. 3(c) and for all the diffraction patterns of /?’ and pl’ which have been analyzed (see Figs. 11(a), (b), and (c)). Fig. 11(a) is the particular case of a diffraction pattern from a zone of reflecting planes which are not disturbed by faulting and hence there is no displacement or broadening of diffraction maxima. From the observed displacements of f.c.c. reflections
FIG
13. The positions of Cu and Al etoms in two
adjacent (112) leyers of &’ showing the three faulting shears, only one of which, $[lli], does not disturb the superlattice. Cu atoms: @--first layer, O-second layer. Al atoms: m-first layer, n-second layer.
SWANN
cells forming atoms
AND
a tetragonal
at positions
WARLIMONT:
COPPER-ALUMINUM
unit cell with aluminum
of the type
440, 004 (shown
filled circles in Fig. 12), and copper atoms at 000,
as
O&t,
the kinematical those reflections affected
(hk2) of planes in the disordered
analysis
become
(hk2Z) in the ordered
similarly,
B1’; become
indices
[uv(w/2)]
the proposed
giving
martensite,
tetragonal
lattice,
directions
even, and h and k are mixed, patterns
studied,
and their displacements agree
with
11(a), (b), and (c). The lower symmetry
if 1 is odd.
if 1 is
In all the
superlattice
structure,
reflections
i.e. Figs.
of the ordered
by comparing
3(b),
structure
is
Fig. 3(c) with Fig.
11(b), both of which show basically
the same pattern
(IlO)@G however only the pattern Fig. 1 l(b) whose zone axis, [ilO], is 120” away from that of the pattern in Fig. 3(c), contains superlattice of the lower symmetry possible
reflections.
between
shears which may be expected (112),1,
plane.
plane,
I+J421].
i.e.
Hence
Because
of the ordered structure,
to differentiate
close-packed
it is
the three faulting
to occur on any one
In pi’, the faulting shears on the (111)8’,
the
become
phase
1LZ[241], &[lli],
changes
(a)
due
to
faulting may be either ;[2h
-
f[h + k -
$[-4h It will be noticed
etc., and in Fig. 11(c) the ii4,006, are not affected by faulting, the Q[lli]
stacking
14ij
118, etc. reflections
which indicates that only
fault shear operates
on the (112)
plane, since this is the only shear on that plane which produces
no phase shift for these reflections.
of the types
1+[241]
and
,$[421]
(Shears
would
give
rise
to phase shifts of 7~12for 033 and 14i, etc. and 7r for ii4,006, and 118, etc.) The above reasoning holds for all the close packed planes of &‘. faulting
shears
Furthermore,
are restricted
it may
+(lll)
on
(112).
that the operating
the order
of
the
&’
(see Fig. 13).
The density of stacking faults in /3’ and br’
this
analysis
stacking
Paterson
fault extends
tion volume;
and N is any integer.
assumed
completely
that
(a) each
across the diffrac-
(b) only one set of close packed planes
is faulted,
and
(c)
the
distribution
of
faults
is
random. The ,Y and pi’ specimens are such that they comply with the first two assumptions. Patterson
analysis
distribution
However,
in using the
we have had to assume that the
of faults in /j’ and j3i’ is random.
In Table3 the measuredvalues ties u have
been
compositions
listed
for
h, and the probabili-
specimens
and heat treatments.
of different
It may be seen
from Table 3 that within the accuracy of our measurements the density of stacking faults across pi’ plates is independent
of composition
and of the time of iso-
thermal ordering prior to transformation. deformed
of selected area diffraction showed
&’
approaches After
that
specimens
by rolling,
attain
hexagonal
positions,
because
of faulting
system.
Although
the
patterns
stacking
fault
0.5 with increasing amounts of
diffuse
the
were reduced the diffraction
50 per maxima
but these are extremely on more
structure
than
of
one slip
deformed
pi’
becomes hexagonal in nature the microstructure is very different from the hexagonal y’, compare Figs. 9 and 15, and it is therefore
a misinterpretation
of
TABLE 3. Diffraction peak-position parameter, h,, and stacking fault probability parameter, a, of /?’ and pl’ mctrtensites at various compositions and after different heat treatments
Composition Al [wt.%] 10.8 11.2 + 1.1 wt.% Sn 12.1
The spacing of the stacking faults in p’ and /J’ may not be determined by direct measurement since on the
12.1
micrographs they are too closely spaced to be clearly resolvable. However, Patersono7) has shown, using
12.2 13.0
3
. . . is
of a reflection measured in
In general, therefore, to
be shown
shears $(l 11) do not disturb superlattice
In
cent in thickness the 033,
Paterson fault will
1
the direction of its displacement
deformation.
that in Fig. 1 l(b)
the
l-Ltanf:(h,-3N+$) 1/3
probability
+ 2k + 11.
to
u that a stacking
occur at any given layer in a sequence ABCABC
of
I]
According
where h, is the co-ordinate
of
patterns which are
given by
Measurements
4k + Z],
or
faulting.
that for
due to faulting were found to
the proposed
easily recognized
by
that the density
from the displacement
in the diffraction
the probability
(Fig. 12) all (McZ),~, reflec-
tions are allowed for which h and k are unmixed diffraction
i.e.
[UWUJ]in B’,
It was calculated
in Bi’.
superlattice
i.e. B’,
theory of diffraction,
of faults may be calculated
tOi, O!&, &Op, &$a (not shown in Fig. 12). The indices
519
MARTENSITES
Heat treatment
Peak position”” h, * 0.02
Stacking fault probability bl + 0.01
850”C,quenched 85O”C, quenched
1.42 1.43
0.45 0.45
85O”C, 5’ 525”C, quenched 85O”C, 2’ 525”C, quenched to 100°C 85O”C, quenched 85O”C, quenched
1.38
0.44
1.37
0.44
1.38 1.37
0.44 0.44
520
ACTA
METALLURGICA,
(a) Showing the internally twinned acicular plates. The plate in the center of the micrograph has its twinning plane parallel to the surface of the specimen.
(c) Showing the fine cross striations within the twins c)f & y’ plate.
VOL.
11,
1963
(b) Part of a plate showing internal twinning on two different systems.
Showing the superlattice domain boundaries in dark field illumination.
FIG. 14. Microstructures of y’-martensite Cu-14.1 wt.% Al.
Transmission electron micrographs.
SWANN
AND
WARLIMONT:
COPPER-ALUMINUM
521
MARTENSITES
structures of the type shown in Fig. 15. The deformation of y’ proceeds partially heavily
faulted
twins.
transformation
twins
by the formation
of new
The matrix
and the internal
of
become
heavily
patterns
of y’ is
y’
also
faulted.
Electron diffraction of y’ The interpretation complicated
of diffraction
by the relaxation
in contrast to the case of/l’,
of Laue conditions,
occurs in at least two different directions plate.
and
the streaking of reflections in any one
However, diffraction patterns could be analyzed
by orienting the specimens carefully such that one set of diffraction streaks lay in the plane of the pattern. The electron diffraction results confirm the structure of y’ derived from X-ray 15. y’ martensite deformed by rolling to 40 per cent reduction in thickness. Cu-13.7 wt.% Al. Transmission electron microgreph.
et a1.(14) Disregarding
FIG.
of y’ is h.c.p. orthorhombic
X-ray
diffraction
converted
measurements
that
/J’
may
be
to y’ by deformation.(5p7>8)
Quenched
and
alloys
with
more
than
13 wt.% Al
y’, (see Fig. 14(a)), which is from ,!?r’. It is noteworthy that
,!lr’ are indistinguishable under the optical Each y’ plate contains parallel bands
microscope.
which are about 100-500 field illumination
that
(121) orthorhombic
readily distinguishable y’
diffraction
A thick, and by using dark
and electron diffraction
analysis.
({liol),
h.c.p.).
analysis
striations
Examples
of the
In the twin patterns studied
were found which were attributable (see Fig.
matrix and twins of the martensite trace
are internally
twin patterns are shown in
the very fine cross striations by
is used
The diffraction
plane was found to be (2011,
observed
Figs. 17(a), (b), and (c). no reflections
the structure
of y’ is, however,
the y’ plates
The twinning
most commonly
the martensite
by Kurdjumov
(see Fig. 16), and this description
indicate
twinned.
Metallography of y’ contain
The superlattice
in the subsequent patterns
observations
the superlattice,
it
are parallel
was
to
14(c)), in the
plates.
determined
However, that
these
to either the twinning
plane
it was found
{202}, (122) or the faulting plane (001) in addition to
that the bands are twin-related. The thickness ratios between the twins and the matrix vary from 1.3 +
the normal y’ twinning plane {201}, (121). The diffraction patterns in Fig. 17 show that diffraction
3.3 : 1. Most of the internal twins are on one system
streaks
and extend
plane, and must therefore be the result of a reduction
completely
but occasionally
across the martensite
plates
plates contain two sets of twins (see
Fig. 14(b)).
G4.228
The matrix and twins of the martensite further
subdivided
by fine striations
several sets of traces (see Fig. 14(c)). the determined
trace
plates
that
revealed
directions these
domain
belonging
to of
fine striations
structure
plates are
An analysis
on ten
twins of higher order and stacking The antiphase
are always
martensite represent
faults. due to the super-
lattice of y’ was revealed by dark field illumination (see Fig. 14(d)). It will be noted that the domain boundaries are discontinuous across the twins, indicating that the martensite has formed subsequent to the ordering reaction. It is also evident that the twinning of y’ disturbs the order to a greater extent than the faulting of ,&‘. Deformation of y’ martensite by cold rolling to 40 per cent reduction in thickness results in micro-
perpendicular
to
twinning
b=5.20%
A
0
the
F-l=! I t
a=4.51X
l-
ALUMINUM
ATOMS 0
-COPPER
ATOM
FIG. 16. The orthorhombic unit cell of y’ martensite projected on the (001) plane. (After Kurdjumov et ,Z.“*))
522
ACTA
in size of the coherently internal twinning, It
has
been
scattering
calculated
that
METALLURGICA,
k = 2n’
the
and
patterns
forbidden
contain
according
I=
Metallography
et a1.(14)
2n” + 1 Some of the
reflections
which
are
to this rule, e.g. the retlections
{021), (321) in Fig. 17(c). may be explained diffraction.
1963
orthorhombic
where n, n’ and n” are any integers. diffraction
11,
crystal due to the
superlattice of y’ proposed by Kurdjumov requires reflections hkl to be forbidden if h = 3n,
VOL.
However,
satisfactorily,
these reflections
as the result of double
of mixtures of /I; and y’
Two alloys with 13 per cent and 13.7 per cent Al were observed
to
Unfortunately,
these compositions
contain
mixtures
lower and upper concentration
of &’
and
y’.
were just at the
limits of the two phase
region, and it was difficult to find the areas showing both phases. In
both
alloys
distinguishable
the
and
two
may
martensites occur
plates or both may be contained both ,!ll’ and y’ are contained
are readily
either
as separate
within one plate.
If
in one plate the transi-
tion is abrupt (see Fig. 18). The faulting frequency y’ is clearly less than it is in PI’. indicates
in
The microstructure
that many plates have begun transforming
to ,Q1’and have then continued
to grow lengthwise
by
Although the type of forming y’, or vice versa. martensite may change during the lengthwise growth of a plate no further change is observed in the sidewise growth.
In a single two-phase
plate the rate of side-
wise growth is the same for either phase and the habit plane of the plate is only slightly deviated. Hardness measurements
of /II’ and y’
The metallographic
and crystallographic
tions were supplemented over the composition was realized
that
by hardness
range lo-13 the hardness
domain size depend sensitively
wt.% and
observa-
measurements Al.
Since it
the antiphase
on the quenching
rate,
all specimens were cut to the same size (2 x 2 x 0.2 cm3) and quenched at two different rates. The specimens of each set were treated
simultaneously.
The
results have been plotted in Fig. 19 and it is shown that hardness decreases with increasing quenching rate and with aluminum contents approaching the The composition dependence composition Cu,Al. agrees with that found by previous investigators.‘3p22) DISCUSSION
Martensite crystallography Experimental high-temperature
observations b.c.c.
have
shown
phase /? decomposes
that
the
during
FIG. 1’7. Quadrants of diffraction patterns of y’ martensite with zone axes parallel to the twinning plane. Note the streaking in all patterns normal to the twinning -plane (121). l e-main and superlattice reflections of twin I. C? o-main and sunerlattice reflections of twin II. n n 0 O-reflections due to double diffraction.
(4 The [234] zone. (b) The [274] zone.
Note the splitting of higher order reflections, i.e. 281, due to streaking inclined to the normal of the pattern resulting from second order twinning or faulting.
(0) The [012] zone. Showing forbidden reflections due to multiple diffraction.
SWANN
quenching
WARLIMONT:
AND
to form martensites
COPPER-ALUMINUM
which have basically
either f.c.c. (p’ and p,‘) or h.c.p. (y’) structures.
MARTENSITES
their superstructures.
The relevant
Fig. 20. Using the data shown in Table 1 the principal
an extent
strains of the Bain distortion
placed
almost
certain
diffraction
to hexagonal
maxima
positions.
are dis-
is expected content,(23)
to
decrease
independent
with
of composition.
that a constant
amount
increasing
of faults
vi = aJ2/g,
aluminum
was found
to be
From this it is inferred
of faulting is inherent in the
an undistorted pi’.
i.e. the faulting
deformation Also,
occurred
interface
intrinsic
dominate
necessary
the
between
rather
and f.c.c. structures. From Q and q2 it was calculated shear necessary
of
growth
faults
plane strain is
and y’ Al alloy.
The density
of stacking
faults
is then
given by
gd(111) ‘%h
pre-
=
~
b
= 0.355
that the faulting has to the face
In the case of the h.c.p. y’ invariant deformation is
provided by twinning. It is possible to calculate
g = 0.251.
of the b.c.C.
that the total slip
to attain an invariant
/3 and @’ or ,di and
than
and thus it is probable
production
after the lattice transformation
centered cubic structure. martensite the lattice
twinning
is the lattice-invar-
for
= 0.8910
where ab and af are the lattice parameters
18. Showing the junction between pl’ portions of a martensite plate in a Cu-13.0 wt.% Transmission electron micrograph.
iant
are
q2 = aJa, = 1.2600
FIG.
transformation,
in
and
the energy of stacking faults in #?’ and ,!lr’ the density
are illustrated
This has led
previous X-ray investigators to interpret the structure of p’ and pi’ as distorted h.c.p.(%‘), monoclinic or triclinic.(*) Although
of the transformation
crystallographic
The B’ and pi’ martensites were found to be faulted to such that
relationships
523
the extent of faulting
in /l’ and &’ martensites
or
using the theories
is the where4111,
spacing between slip planes and b is
experimentally.
The discrepancy
and
stacking
the Burgers vector of a partial dislocation (a,/6)[112]. This value is to be compared with sex = 0.44 found theoretical
arises from
fault
the assumption
between densities
observed probably
in the calculation
that
of Wechsler et a1.(24)or of Mackenzie and Bowles.(25) In the present case the former analysis was used. It was assumed that stacking faults extend completely across martensite plates and that the b.c.c. matrix and
faults extend completely across martensite plates. This assumption is not fully supported by the experimental observations since partial dislocations are observed within the martensite plates (see Fig. 4), and
the f.c.c. martensites are related through the Bain correspondence. In the case of & + pl’ this latter
thus more faults are required to achieve the necessary
assumption
was verified by the relationship
between
shear. In y’
martensite
the
lattice
invariant
shear
is
ACTA
524
METALLURGICA,
VOL.
11,
1963
260 240
22
I II
23 I
ALUMINUM
or.-%
24
I 12
25
I 26 13
wt.-% CONdENTRATlON
FIG. 19. Hardness of martensitic Cu-Al alloys after quenching at different rates. C--Quenched in water at 100°C. @-Quenched in brine at 25°C.
accomplished mainly by twinning on {201}, (121) of the orthorhombic lattice and with small amounts of twinning and slip on {202}, (122) and (OOl}, respectively. Assuming the twinning plane to be (121) or {201}, Mackenzie and Bowles(25) have calculated the ratio of the thickness of the two twin orientations in a y’ plate to be 3.5 : 1. The experimentally observed ratios are between 1.3 : 1 and 3.3 : 1. This discrepancy between the theoretical and the lower experimental values may again be explained by the fact that some of the twins do not extend completely across the martensite plates. The habit, planes of both /J’ and y’ have been determined experimentally by Greninger .(7) Mackenzie and Bowles(25) have calculated the habit plane of y’ and have obtained agreement with the experimental results. The habit plane of /II’, however, was not determined theoretically because of the uncertainty
h
.%Ob_ -
COlOlf COlOlb 4 I
I
I
IO
20
FIG. 21. Stereographic representation of the habit plane of p’ martensite showing the extent of the experimentally determined ( 1m )“1 and calculated (0) poles in relation to the @ lattice.
In the present study the WechslerLieberman-Read theory has been used to determine the habit plane of PI’ on the basis that the transformation is b.c.c. -+ f.c.c. Using the values given above for qI and qZ the direction cosines of the habit plane normal are found to be in its structure.
-0.1791 0.7252 0.6648 [ IB The position of this pole agrees well with Greninger’s results which are plotted in Fig. 21. The diffraction results show that only four variants of a possible twelve partial dislocations may create stacking faults during the /I1 to PI’ transformation. The preference for certain partial dislocations may also be deduced from consideration of the types of bonds changed during the motion of each of the twelve possible partial dislocations. For example, from Fig. 13 it may be seen that the partial dislocation &[lli] does not disturb nearest or next nearest neighbor relations, whereas the partials ,%[&.21], &2&l] do; see also Table 4. Since the interaction energies of next nearest neighbors in close packed structures are small,(26127)the motion of the parrtials &[321], &2&l] would create high energy antiphase boundaries and is therefore avoided. This restriction TABLE 4. Change in bonding per atom across the faulting plane (112) resulting from the three possible faulting shears in PI’ martensite Faulting shear f[ llT1
OlREGTlON
Neighbor relation
Al-Al
cu-cu
nearest
0
next nearest
0
~___
Cu-Al
0
0
0
0
OF SHEAR
FIG. 20. Crystallographic relationships of the &B distortion. Drawing is projected on to the (fOl)f or (001)b plane.
h[421]
nearest
+a
++
-4
$.J%l]
next nearest
-4
-4
+1
SWANN
WARLIMONT:
AND
COPPER-ALUMINUM
in the shearing process limits the number of possible habit plane variants
generated
during the pi to &
The ordering
of ,3
The superlattice
in &’ martensites is always formed
transformation from twenty four to eight. However, since the selected-area diffraction patterns in this
in the high temperature
investigation
appearance
were taken from martensite
plates with
transformation
a width of at least two microns, it is possible that the
lattice
remaining
boundaries
16 habit plane variants
are found
among
formation
tion of the high energy stacking faults.
temperature
plane variants (excluding
the number
is 24 since three partial
those
producing
extrinsic
were shown to be absent) operating close packed
planes
of habit
dislocations faults,
which
on four different
may each generate
two undis-
torted planes.
It was found 13.0-13.7
that the transition
occurs
from
/3r’ to y’
in the concentration range from It is interesting to consider this
per cent Al.
result in relation to the composition
dependence
stacking
copper-aluminum
alloys,
fault which
boundaries
of the
indiscriminately plates.
super-
across
The Cu,Al
the
super-
lattice is analogous to Fe,Si, and the low temperature of Fe,Al,
which
have
DO,
of a superlattice form
structures.
analogous
of Fe,Al
The
to the high
(B2 structure)
was not
observed. The grown-in
antiphase
show no tendency crystallographic
domain
boundaries
in /3i’
to be parallel with any specific and form a continuous plane,
network (above 11 .O per cent Al) with three-boundary junctions predominating. These observations are in agreement with calculations concerning antiphase
The relative stability of /?i and y’ martensite
extend
of martensite
driving force may be high enough- to allow the formamartensite
b.c.0. phase p prior to the This is evidenced by the
pi’.
of antiphase
form
In the disordered
to
which
the smaller plates. This might occur since the smaller plates form at lower temperatures where the chemical
525
MARTENSITES
energy
of
is shown
f.c.c.
in Fig.
1.
There
of the
is some
boundaries of the DO, structures by Marcinkowski Brown.(2g)
and
It has been shown that the ordering of ,!l occurs by the growth of isolated domains of #J into a disordered matrix.(30)
Alloys nearer the stoichiometric
tion
a larger
have
domain
ordering temperature is faster.
size, since
composi-
the
critical
is higher and, therefore, growth
The domain
size may also be increased
by
justification for considering the stacking fault energy of the f.c.c. structure to represent the difference in free
lowering
energy between f.c.c. and h.c.p. phases,(23s28) i.e. when
proceed further with less undercooling.
the stacking fault energy is zero, the f.c.c. and h.c.p. phases are in equilibrium. By extrapolation into the
num concentration
metastable
and are enclosed in a disordered or short range ordered
f.c.c.
region
the
stacking
fault
energy
the
domains
quenching
rate,
is reduced
of the superlattice
because
growth
may
If the alumi-
below
11.0 wt.%
the
are no longer in contact
(Fig. 1) is found to be zero at 13.5 -& 0.5 per cent Al where &’ and y’ do in fact coexist. The fact that ,&’ is
matrix.
ordered thus appears to have little effect on the free
(M,) is only a few degrees for the 11 per cent Al alloy,
energy of the f.c.c. structure.
and presumably the ordering reaction is interrupted
Since the transformation expect
the pi’-y’
composition 13.0-13.7 the
is diffusionless,
boundary
to occur
we would
at a unique
rather than over the composition
per cent Al.
stacking
fault
However,
energy
range
it may be seen that
is rather
insensitive
to
The difference between the ordering temper-
ature (To) and the martensite
the martensitic transformation. range
order
quenched
should
In an alloy containing
tant
this /3’ martensite
martensites periphery
are formed. of a growing
growth to be continued compositions is supported
whether
the
f.c.c.
or
h.c.p.
Thus the strain field at the martensite
plate may favor
by the alternate
structure
where it is chemically less stable. by the experimental observations
at
This that
,!?i’ and y’ may both be contained in one plate. At compositions outside the range 13.0-13.7 per cent Al the nonchemical factors such as strain energy are presumably too weak to cause the chemically stable martensite to form.
less
not
by
With a slight extrapo-
is above
in the
10 per cent aluminum the M, however,
contains
it is believed
regions
of short
This may be deduced by considering
which some of the larger martensite mid-region
slowly
less than 10.5 per cent Al.
To (see Fig. 1) and the super-
lattice is not detectable; order.
be present
alloys containing
temperature
determining
temperature
lation of the results in Fig. 7 it may be seen that long
composition in alloys containing more than ~6 per cent Al and thus nonchemical factors may be imporin
formation
that range
Fig. 8, in
plates contain
a
which appears lighter than the rest of the
plate. By examining the edges of the thin foils it was shown that these regions are thinner and must therefore dissolve faster during the electropolishing process. This difference in rate of electrochemical attack in the larger plates is associated with a varying degree of short-range order across them. The occurrence of such a variation may be explained by supposing that the
526
ACTA
METALLURGICA,
VOL.
11, 1963
long light bands are the first martensite plates to form
structurally
from disordered ,!I, i.e. at about 430°C.
hardness is predominantly
identical
martensite,
pi’, it is seen that
The lengthwise growth of these initial plates continues until collision
superlattice.
with
and stacking fault density do not change measurably
other
similar
plates;
however,
their
sidewise
growth occurs more slowly and at temperatures short range ordering smaller martensite temperatures mid-regions
is taking
plates which are nucleated at lower
should
therefore
The large martensite have
contrast
contain
thinner
strongly
This is shown to be the
plates in the 10 per cent Al
curved
boundaries.
which are constrained It is suggested
that
This is in
with higher Al-
to have planar interthe relaxation
of this
constraint in the 10 per cent Al martensite is connected with the short range diffusion
process which we have
shown to occur during the formation More specifically, development loops
of the curved boundary
behind
CS!.(~~))accounts
8(b)).
of vacancies
or ahead
Such a mechanism dislocation martensite
of these plates.
the shape change accompanying
by the condensation
(originally high
the
proposed
interface.
by Baker et
density
of elongated
In the 10 per cent Al alloy the smaller marten-
expected plates
have
straight
by the above are
diffusion
formed
interfaces reasoning,
at
is more difficult.
be inferred not
lower
which
is to be
since the smaller
temperatures
where
The results also show that
the sidewise growth of p’ plates can proceed
at rates
which are much lower than those measured for ferrous
contribute
composition
studies
concerned
martensites
effect of solid-solution of carbon,
microstructure
does
in hardness
with
to the variations plotted
in Fig.
19.
metric composition,
Cu,Al,
superlattice.(34) the variation
increase
in
is similar to the behavior of the Fe,Al
(DO,)
This behavior may be associated with of the antiphase
superlattice.
The
from the stoichio-
By comparison
domain
size of the
of Figs. 7 and 19 it is
seen that upon lowering the aluminum
concentration
from 25 at.% the hardness increases whereas the domain size decreases. A corresponding relationship between the resistance the “antiphase Flinn(35)
to dislocation
boundary
space,”
r=-
motion, t, was
7, and
given
by
(r
t where
(r is the energy
boundary
of the additional
antiphase
area created by the passage of dislocations.
There is a complication hardness
and
in this interpretation
domain
size are both
that the
increased
by
decreasing the quenching rate. an increase quenching
in antiphase
This may be related to boundary energy at lower
rates which overrides the weakening
due to an increase in the domain size. the short range order component at lower quenching
effect
An increase in
of the superlattice
rates may, also, contribute
to an
increase in strength. ACKNOWLEDGMENTS We
Thestrength of p’ and PI’ martensites Recent
range from 10-13 wt.% Al it will
hardness with increasing deviation
martensites.(32$33)
ferrous
since the microstructure
that the martensitic
is accommodated
loops found near the interface of the plates in the 10 per cent Al alloys (see Fig.
site plates
in the concentration
to form dislocation
of the advancing
for the
Furthermore,
found in flow stress measurements
to the plates of martensite
content faces.
not
and should not be in contact with the thin
regions of the larger plates. case (see Fig. 8(a)). alloy
where
place in the j3. The
due to the presence of the
with
the
have emphasized hardening
strength
either,
of
(1) the
due to the presence
or (2) the presence of a fine internal twin
structure which presents a strong barrier to dislocation motion. In the case of copper-aluminum martensites, solid solution hardening is unimportant since the hardness decreases with increasing
helpful
The correlation of the strength of the /?’ and /!li martensites with their micro- and crystal-structures depends on several factors. It may be supposed that some work hardening has occurred due to the passage of the partial dislocations during the transformation. However, by comparing the hardness of the disordered martensite, @‘, with that of the ordered but otherwise
suggestions,
performed
members
of this
and to K. D. Fike who kindly
the hardness measurements.
to thank S. Amelinckx,
Centre d’Etude
Also we wish de 1’Energie
Nucleaire, Mol-Donk, Belgium; and D. W. Pashley, Tube Investments Laboratory, Hinxton Hall, U.K. REFERENCES
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