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Phase transformations in metastable β′ CuZn alloys—II. Isothermal transformations
PH.4SE
TRANSFORMATIONS ISOTHERMAL I.
CORNELISt
IN METASTABLE p’ CuZn TRANSFORM.4TIONS* and
C.
11.
ALLOYS-II.
WAYMhNt
The present investigation was carried out with the primary purpose of establishing the nature of the isothermal @’ - c+ transformation in Cu-Zn alloys containing -40 wt.?; Zn. The morphology, habit substructure, and orientation relationship of the plate plane, shape deformation, crystal structure. and scanning electron shaped x1 transformation product hew been studied by optical, transmission microscopy. and by electron and S-ray diffraction. The inltlal 8’ - z, transformation ww. found to exhibit all of the characteristics generally ascribed Contrary to previous suggestions, diffusion. i.e. partitionmg of Cu to a martensitic transformation. and Zn, only influences the characteristics of the %,-plates after their formation in a di&sionless manner. The habit piane, orientation relationship, and shape strain associated with the /?’ + xx transformation are in ver>- good agreement with predictions of these quantities obtained by employing the martensite crystallography theory. The structure of the initially formed x,-plates can be described as a 3R stacking sequence involving close.packecl planes. It is emphasized that the 3R stacking is a result of the inhomogeneous shear of the phase transformation. The of the crystallography theory, and therefore is a consequence “overannealing” of the x,-plates during longer (isothermal) transformation times results in a disordered (f.c.c.) phase with non-crystallographic interfaces. TR_~SSFOR~I~TIO~S
DE PHASES DASS LES ALLIAGES C&n /?’ YETASTABLES-II. TRdSSFORSI_+.TIOSS ISOTHERXES Cette Etude a pour principal objet l’btablissement de la nature de la transformation isotherm0 8’ - )I, se produisant dans les slliages Cu-Zn contenant environ 40 96 en masse de Zn. La morphologic, le plan d’accolement, le changement de forme, la structure cristalline, la sous-structure et la relation d’orientation du produit de la transformation en forme de plaquettes z1 ont BtB Btudids par microscopic optique, par microscopic Bleotronique en transmission et 8. balayage, ainsi que par diffraction Blectronique et RX. Les auteurs trouvent que la transfo~ation initial9 fi’ -+ CL~p&sent0 tous Ies caract&es g&&alement ~ontrairement aux suggestions antbrieures, la diffusion. attribues b une transformation martensitique. c‘eat&-dire la s&pax&ion de Cu et Zn n’influe sur Ies caraot&istiques des plaquettes 01~qu’apr&s leur formation, qui s’effectue sans diffusion. Le plan d’accolement, la relation d’orientation et le changement de form0 associ6s B Is transformation J’ -+ x1 sont en bon accord avec les rCsultats p&us par la thPorie criatallographique de la martensite. La structure des plaquettes x1 form&es au debut peut 4tre d&rite par une st;quence d’empilement 3R comprenant des plans compacts. Les auteurs soulignent que l’empilement 3R r&&e du cisaillement hdtBrog&ne de la thborie cristallographique, et qu’il est done une con&quence de la transformation de phase. Cn recuit isotherme prolong6 deu plaquettes 11, produit une phase f.c.e. non ordorm& presentant des interfaces non cristallographiques. P~~SE~L~lW_~~DLUNGES
I?; JIETASTABILES ,!7’-CuZn-LEGIERUSGES.-IL: ISOTHERME UMWASDLUSGEX Die vorliegende Arbeit wurde mit dem Hauptziel durchgefiihrt, die Xatur der isothermen fi’ -+ I~. Umwandlung in Cu-Zn-Legierungen mit etwa 40 Gew.% Zn herauszuf?nden. Morphologic, Habitusebene, Gestalt~deru~, Kristaflstruktur, Substruktur und Orientierungsbe-zieh~g des plattenfijrmigen z~-Um~andlun~pr~~tes wurden mit iicht-, elektronenund ~ter-elekt~nenm~oskopischen Xethoden sowie durch Elektronenund Rantgenbeugungsexperimente untersucht. Die zunschst stettfindende ,3’ -f a,-Ummandlung besitzt alle Me&male einer marten&t&hen Umwandlung. Im Gegensatz zu friiheren Vorschl%gen bee&l&t die Diffusion, d.h. die Aufteilung van Cu und Zn, die Eigenschaften der a,.Platten erst nach ihrer diffusionsloaen Entstehung. Die mit der 8 * x,-Cmwandlung verbundene Hsbitusebene, Orietierungsbeziehung und Gestaltsdehnung sind in sehr guter t’bereinstimmung mit den anhand der Theorie der Martensitkristallographie vorhergesagten Werten. Die Struktur der am Beginn der TJmwtlndlung gebildeten %,-Flatten kann ah 3~.Stapelfolge dichresr gepaekter Ebenen beschrieben werden. Es nird darauf hingewiesen, da13 die 3R-Stapeifolge das Theorie und somit eine Folge der Ergebnis der inhomogenen Scherung nach der kristaliographischen Phasenummandlung ist. Das “UberanIassen“ der r,-Platten bei lilngeren (isothermen) Umwandlungszeiten fiihrt zu einer ungeordneten (f.c.c.) Phase mit nicht-kristallographischen Grenzfilchen. INTRODUCTION
products formed in a 41.3 wt. % Zn alloy: are platelike below 3BO”C, and that particles of irregular shape appeared in addition to the plates at slightly higher temperatures. At the upper temperature limit of his investigation, 470°C, the transformation product took the form of rods (or needles). This
In addition to the martensitic t~nsformation obtained by cooling CuZn alloys below X8(1-2) or by deformation below Xd,f3) the /l’ phase also transforms isothermally at higher temperatures.(A*j) Garwoocl(‘J found that the isothermal transformation * Received May 29, 1973; revised July 30, 1953. 7 Department of JLetallurgy and Mining Engineering and Materials Research Laboratorv, University of Illinois, Urbana-Champaign, Urbana, IllinoA 61801, U.S.A. ACTX
JIETALLTTRGICA,
VOL.
52, MARCH
197.4
change Flewitt
in morphology has been confirmed and Towner,@) who examined a number
brasses
within
also found 301
that
the 40544.1 by plotting
wt.%
Zn range.
the incubation
by of
They periods
ACT-1
30-2
>lETdLLCRGIC.1,
for visible transformation on a time/temperature diagram, the plates and rods gave rise to two separate C curves. The T-T-T diagram for a 40.5 wt.% Zn alloy is sholvn in Fig. 1. The lower C-curve, n-hich corresponds to the plate-like product,, shows a sharp cutoff at an upper temperature limit, B,, above which only rods form. Just below B,, a mixed plate-rod product is formed (region M). The incubation time for the plate formation is increased and the percentage of rods decreases, as the temperature is decreased below B,. The transition temperature, B,, varies with composition in approximately the same manner as the M, temperature which is -6OO’C lower,(s) although Repas and Hehemannt6) found that this parallelism does not hold for the higher Cu alloys. The B, values from both references(5+6) are given in Fig. 2. The plates which form below B, frequently appear as V-shaped pairs separated by an obtuse angle. This is to be contrasted with the subzero martensite plates Jvhich form as acute-angled V-pairs. For a given composition, the plates become finer with decreasing transformation temperature; the density of the plates becomes higher with increasing transformation time, leading ultimately to a closed network arrangement. The plate-shaped transformation product has been referred to as bainite+8) because of certain similarities with bainite in ferrous alloys. However, because of the existing disagreement concerning definition, nucleation, growth kinetics, crystal structure, etc. for bainite in steels, the terminology x,-plates has beejb chosen in the present work for the plate-shaped isothermal transformation product. The rod-shaped procluct will be referred to as a-rods. A characteristic feature of the x,-plates is the presence of internal markings which have been
0.
IO
;SE
I03
I04
TIME,
SECONDS
IO3
ii16
FIG. 1. T-T-T diagrem for the isothermal transformation products in a Cu-40.5 wt.% Zn according to Flenitt and Tow-ner.cs’ R. P and ~!f indicate the regions where rod-shaped, plate-shaped end mixed transformation products are formed.
VOL. 22, 1951
zoo+ L-_-
’
35
FIG.
2. Upper formation
\
I
_
\
I
~_5-~~_
ATOMIC
40
?Z?CShT
ZINC
temperature limit (B,) for r,-plate as a function of composition.i5~B~
observed by electron microscopy of thin foils(s) as well as replicas.(g) These striations are reported(*) to be similar to the stacking faults observed in martensite plates. As with the martensite forming from b’ CuZn, S-ray diffraction studies(4.10*11) have led to several proposed crystal structures for the a,-plates. The more recent electron diffraction studiea(**is) indicate a 3R stacking sequence of close-packed planes for the The tendency for the faults initially formed plates. to “anneal” out with longer transformation times(*) was found to be accompanied by a transition to an f.c.c. stacking sequence (i.e. 1R structure). Single surfaces analysis(4*13) indicates that the habit plane of the a,-plates is close to (2, 11, 12)b.c.c., which corresponds to the habit plane reported for martensite plates in CuZn alloys.(14~15) The surface relief observed on a pre-polished surface after the formation of the al-plates may also be similar to that observed with martensite. Interference microthat an invariant-plane strain graphs(7*16) indicate type of relief exists for the a,-plates. However, the small size of the initially formed plates (4.2 p thick) has precluded the observation of scratch displacements by conventional optical microscopy. The present investigation was undertaken in order to more clearly define the nature of the a,-plates, and, insofar as possible, establish the mechanism Separate sections of the ,!?I+ a1 transformation. of the paper are devoted to morphology-, habit plane, surface relief, electron metallography, crystal structure, orientation relationship, ancl application of the martensite crystallography theory. EXPERIMENTAL
PROCEDURE
Twelve CuZn alloys, ranging from 38.1 to 44.0 wt. % Zn, were prepared. The quenched metastable p’specimens were isothermally transformed in 50 : 50
.~.diuru nitrate,/potassium nitrate salt baths in the ~i.w--&SP)‘C: temperature range, and in a molten had bath for the higher temperat~e range. The specimens used for surface relief studies were electropolkhed and scratched by a very light abrasion with 0.05 ,rt.Al,O, particles prior to the isothermal transformation. These specimens were then isothermally transformed, at the appropriate temperature, by holding them with the non-polished surface on top of a salt bath for time intervals which resulted in to rather low density of plates, so that the surface relief of individual plates could be studied rrithout the interference of neighboring plates. Since the smalt size of the x,-plates prevented scratch displacement observations by opt,icel microof the pre-polished scopic means, carbon replication and transformed surfaces was performed. Several replication techniqueso’) were used. First, the txo stage method using a cellulose sheet was tried. Several variants of carbon evaporation and shadowing (with platinum or chromium) of the negative cellulose replica \vere employed, but the desired contrast efkcts were not obtained. Therefore the single stage for direct) method which consists of evaporating carbon (with or nithout preshado~ving~ immediately on the surface x-as employed. The best resolution n-as finally obtained by using only carbon evaporation at an angle of 1%30”, allowing only thin layers of carbon to be formed on the surface. The carbon Brn was then divided in small squares 1-2 x 1 mm), xvhile still on the surface of the specimen, and afterwards removed by immersing the specimen in a 2 y0 solution of nitric acid in ethyl alcohol. These replicas as Tvell as the thin foils, prepared as described in part I, lvere inveskigated with a Hitachi HZ;-11B or HU-19 electron microscope. Finally, a special preparation technique was used on the specimens intended for two surface analysis. In this analysis, a very sharp edge is required betn-een the two surfaces in order to follow the traces of the small x,-plates in both surfaces; moreover the surface from which the back reffection Laue S-ray is taken cannot be deformed. Therefore, a cylindrical specimen (4G in. diameter and ~0.3 in. thick) was electropolished, etched and the required grain was sectioned with a slow speed rotary saxv. The edge was sharpened up by carefully trail polishing the cut surface with A&O, (0.03 ~1. This left the pre-electropolished surface undeformed and suitable for S-ray diffraction. A Hilger microfocus S-ray unit xvith copper target \vas used at 40 1;V and 350 mA, to determine the 6’ grain orientations. The desired grain was positioned
in the S-ray
beam
RESULTS
Jiror&203y
by using ASD
an z--y positioner.41s~
DISCUSSIOS
of the ianthermal ty~~~,~~~~~tat~~~~ produ&
Jletallographic study of specimens containing mere than 40 wt. 7.~;Zn, showed general agreement with the T-T-T curves and consequently the 23, temperatures determined by Flewitt and Towner.(3j In the lower zinc region a noticeable deviation from their estrapolated straight line towards lower temperatures was found; the deviation is not as large as claimed by Repas and Hellemann(6) as shown in between the 3, temFig. 2. A strict comparison peratures of different investigators is difbcult since many factors may influence the nature of the transespecially at 10~ zinc conformation products, Some of the important variables are; centrations. specimen size, solution treatment temperature and time, quenching rate (and consequently the amount of massive ~1 formed), grain size, and the method Severtheless, used to transform the specimens. it may be concluded from the present and previous morphology studier; that the concentrationdependence of .B, is roughly parallel with X$ for the higher zinc region, and that B, levels off towards lox-er temperatures for the low zinc region (i.e. < 40 wt. % Zn). In this respect it should be noted that the al-plates were never observed to form at temperatures above the reported ordering temperature of the p-phase (in the a - 6 region), -n-hich is .-4WC. An impedimel~t to many experimental observations is the small size of the x,-plates (lo-15 $6 Iong and 0.2-l p thick depending on the composition and transformation temperature). Several attempts were made to obtain larger plates. One method which was partially successful involved a step heat treatment, It was found that the %,-plates are larger Then isothermal transformation occurs close to B,. However a mixed product of al-plates and r-rods resulted at these temperatures and the initial substructure within the plates disappeared very quickly, as -+vas verified by transmission electron microscopy. This indicates that these products are not representative of initially formed plates, which vere desired for this work. It was observed, however, that near B,, plates formed before rods. To obtain somewhat larger plates without the presence of rods, a step treatment was performed which consisted of a very short heat treatment just below 3, (e.g. 10 see at 4OO’C for the iO’% Zn alloy) followed by a longer isothermal transformation at a lower temperature (e.g. W-300%). This was finally followed by quenching to room temperature. Such a procedure
allo-~ed the nucleation of fen-er plated in the hiph temperature region, folloned by their growth in the lower temperature region without “overannealing” and nithout the formation of small rods between the plates which would obscure the surface relief due to isolated plates in the $-matris. Another attempt to produce larger plates through the use of a ternary alloying element was made. An aIloF was prepared containing 40 at. % Zn, 18 at.. % Cu and 2 at. % dg. A slight increase in the size of the al-pfates in this alloy was found, and is probably due to the higher B, temperature (close to 510°C). This increase in the upper temperature limit for the plate formation is likely to be due to the increase in ordering temperature. However, the addition of a third element makes the interpretation of some results more diflicult and estrapolation to the binary case is not always without danger. The step treat,ment method for obtaining larger rx,-plates was therefore preferred to alloying, and was successfully used in the ctl habit plane and surface relief analysis.
To date? only a single surface technique(i9) has been used to infer the habit plane of the a1-plates.(4*13) With the combination of the previously mentionecl step heat treatment (to increase the plate size) and a special specimen preparation technique, it was possible to perform the more reliable two-surface analysis. The habit plane traces in the two surfaces were measured on a. series of matched-up photographs taken from both surfaces. Figure 3 show a composite (dark field illumination) from both surfaces of a grain in a 10.5 wt. ‘A Zn alloy, after transformation for
10 xc at -Et!‘C and 30 5ec at 330°C. Direct measurements were also made on the enlarged image projected on the ground plas* screen of the metallograph. The direction cosines found for x,-plates in two 40.5 wt. T{ Zn specimens and three 11 v-t. y.6 Zn specimens are plotted in a common unit triangle. Fig. 4. The (2, 11: l?), pole and the theoretical habit plane from caleulationsr using the phenomenological crystallographic theory of martensitic transformations (discussed in Part I). are also shown. The observed habit plane poles vary in a region around the theoretical and fP> i 1: 12) habit planes and are close to formerly reported results for both ‘A~-and martensite plates. The two-surface analysis also made it possible to determine the relative location of the habit plane variants in a unit triangle n-hich correspond to the two plates of an obtuse angled V-shaped plate pair (e.g. plate pair x-x’ in Fig. 5). It was found that these variants are situated near the same (llcI), pole and on the same ride of the great circle connecting the tao surrounding {ill): poles. These relative locations are in agreement with a suggestion by Flewittts”) based on geometrical observations. Surfuce re2ief due to a,-plates Ati invariant plane strain type of surface relief for the x,-plates was expected, on the basis of earlier work.“.ie) The small plate size preTented, however, the use of the scratch displacement technique as determined by optical microscopy. Moreorer, the small increase in plate size obtained by the step heat treatment or the addition of a ternary element was not sufficient to use the scratch displacement t,echnique and optical microscopy. Thus, a scan&g
Fta. 4. Espwimentaliy observed habit pisne plates ahown i:l K common unit sternographic
fur zl. trianqie.
electron microscope study was attempted. Figure 5 show a back scattering image of z,-plates formed in a 40.5 w-t. “/:, Zn alloy at MtX. As can be seen, both plates in an obtuse angled V-shaped pair (e.g. I and z’) exhibit a surface tilt in the same sense. The tilt, associated with plates y and y’ is again in the same sense for both plates but opposite to the tilt associated xith the .r--1’ pair. The detection of scratch displacements by scanning electron microscop.failed due to the rather low contrast obtained from the small surface relief and also due to the preferential formation of the r*,-plates at the scratches, as is illustrated in Fig. 6.
Fx. 5. Scanning eketrot microgmph (beck scetterinpj shcwing surface relief produced by q-plates form& in a Cu-4K.i wxO, Zn atlq- at 400’C after 15 Sec. -i
Fro. 6. Scanning illustrating the
A
clear
associated
electron micrograph (back ecattering) preferenti81 nw&aiion of r,-plates at scratches.
indication with
the
of an nomination
invariant of the
plane z,-plates
strain xas
using the interference microscope on pre-polished and transformed 2 “/6 -J-g ternary alloy specimens. This alloy made the obserrarions of the fringe displacements easier due to the slightly Iurger plate size obtained. Figures i and S show the fringe disp~ace~lents and associated micrographs of the same region in the ternary alloy transformed at 430°C for 10 and 30 set respectivel-. Only a plateshaped transformation product is seen in Fig. r and the invariant plane strain type of relief is clcarlv indicated by the fringe displacement mode. For longer transformation periods (Fig. S) smalt rodi; from in-between the initial x,-plates which for longer times become thicker and less straight. The clear fringe displacements observed after 10 set, become levs evident due to an or-oral1 rumpling of the surface produced b:- the man-c_ small rods and also because of the non-crystallographic thick. ening of the plates themselves? the latter caused by a long range diffusion process G&r the initial plate formation.czl) The negative influence of the rods could be aroided by the two step heat treatment, which was used to increase the plate size in the binary CuZn alloys. Xeasurements on the latter were required for the calculations of the shear angles, which were used in conjunction with other crystallographic data found for these alloys. The measurements of the fringe displacements resulted in shear angles ranging from IOR- rakes up to 11.7.~‘. obtained
FTF:. 7. Fringe dkplttcement~ and corresponding surface relief micrographs of q-plates formed in B 2t0 _1g ternary allo_v transformed at 43O’C for 10 sec.
Although rather
the fringe displacements
could he measured
accurately,
detailed
the small plate measurements of the plate
the
obtained
=I>
only.
Carbon
shear
replication
isothermally
electron
a!so performed. bp the q-plates, ohserred be Seen.
Depending
an
scratched
specimens,
microscopic
accuraq-
followed
inrestigations
of and b,v xvas
The displacement of the scratches obserred using this technique, is
in Fig. 9. for
have
of pre-polished,
transformed
EransmisGon
illustrated
angles
size pferented width: so that
Displacements
martenvite
plates
similar (see
Part
to those I)
can
FIG. 8. Fringe tlisplrtcements and corresponding surface relief micrographs of the same region as in Fig. 7 after 10 set more of isothermal transformation at 43O’C.
direction 13”
of
(=O._“‘)
slightly
larger
orientation
of plates
and the
scratches,
than
displacements
measured. that
This obsert-ed
up
to
surface tilt is for martensite
plates, which x-as -11.3” for the 39 xt. o/0Zn allo-s and corresponds rert_ well with the predicted ralue from the martensite theories as will be discwed later. The surface tilt obserred for the q-plates is highly unlikely to result from a transformation process which is in some degree diffusion controlkd.1T2J Trcm~m i.s.sionelectron ,micr-o.scopy Qml detraction The
microprobe
inclicated on the
the were
that
microanal-sis
the x,-plates
a change in composition;
formed
of
thin
foik,‘.a)
initially
without
a partitioning
of Cu and Zn
However, this mixed structure could not be confirmed after careful tilting of the specimenzr for moat Therefore, the striatiow x,-plates investigated. inside the q-plates are beliered to be disordered stacking faults, and not coherent. boundaries between 1R and 312 lamellae. Moreover, the 1R intensity maxima observed in newly formed plates may indicate a slight orerannealing of the x,-plates, since the partitio~~i~~~ of Cu and Zn atoms occurs yuickly after their initial formation.(21~ Twin related intensity ma_xima, as obaerx-ed in the martensite plates (see Part I)! were never found in the case of the x,-plates.
FIG. 9. Typical example of the scratch clisplacements CMWX~b_ the fnfmation of x,-plates in a Cu-40 wt.So Zn aflcty {carbon replica). hetn-een the pIates and the surrounding a’-matris occurs only offer the plates are formed in a diffusionless manner. The change in morphology of the x,-plates with increasing isothermal transformation time is illustrated in Figs. lO(a-e) for a -kO.5x%. % Zn alloy transformed at 3.5O’C for 301 90 and 210 r;ec rt>sIxctiveIy. The initially thin plates (after 30 set) ~Jecam! gradud~y thicker with increasing transformation times; the interface also becomes more irre@ar. This “OTeratltlealiflg” of the q-plates is accompanied by a gradual, altl~ougl~ not u[~iform, decrease in the density of striations in the q-plates. The electron diffraction patterns obtained from the ne%Yly formed q-plates generally indicated a _A typical selected area diffraction 322 structure. pattern obtained from such plates is giren in Fig. 11. Besides the 38 intensity maxima a continuous streaking in the c*-direction (i.e. direction perpendicular to the close-packed planes) is observed. This streaking cannot be due to an ordered fault and therefore must be caused by distribution, randomly distributed faults in the close-packed arrangement. The diffraction patterns obtained from slightly orerannealed plates contain f.c.c. intensity maxima in addition to the 3R spots, as illustrated in Fig. 12. maxims became stronger, These f.c.e. intensity and ~o~espondingly the 3R spots became ireaker, vith increasing transformation periods. On a few occasions, f.c.c. intensity maxima were present in diffraction patterns obtained from newly formed plates. This may indicate that a “mixed structure” of IR and 3E lamellae was present in these plates.
FIG. 10 (P.-C). Transmission electron microgrnphs illus. trating the chttnge in morpholoc of the z,-plates in a Cu-40.5 wt.“, Za alloy- with increwina isotherm&l trensformation time (36. 90 and 210 i& St 330% respectid>-).
12 FIG. 10(c).
Lrcttice orientrrtion relatiotulbip
Selected
diffraction of tlcljacent q-plates was used tf.) determine the lattice orientation relationship AS in the case of martensite and correspondence. (Part I). only t!le x,-plates oriented such that the ?;tacking plane was parallel to the beam direction orientations coulcl were considered. since other easily leacl to a fake interpretation. The $‘-matrix adjacent to the piatw lvith this required orientation showed a zone axis orientation close to zL ,‘lll; direction. The orientation relationship, or 100 rchgions
of
area
the
electron
iJ’-matrix
and
Fra. from
12. Typical electron diffraction pattern obtained a nlightly ~‘overarmraled” x,.plete. The indesing ia given in Fig. 13.
measured from a series of diffraction patterns with zone axis orientation. can be summarized a (llli,b.,.,, as: (OOO),, (= 3tacking
plane) 5.5’ from (lOl),,.,,,,
(ii4),, // (,r,.;S’) with (Oll)h,,y ,,.., (ilz),, 5.53(E0.5”) from (iiO),_,~.,T.. The beam direction is assumed to be cxactl~~ parallel to both zone axes! i.e. B.D. 11[lli)],E jj [iii],,,.,,. In the cases n-here f.c.c. intenait? masima are present, the orientation relationship lJetW?en the f.c.c. and b.c.c. lattices is similar to that found for martensite (see Part I), i.e. (ill),,,.,,
(=(009),,)
53
from (IOl),,,.,.
(iii),,,.,, 5.5'(&O.Y) from (O1l),,,J,C, (~00~~.
I/ (ilo),.,.,..
These relationships. shown that the lattice orientation p” and rl is identical to that martensite.
FIG. 11. Typical electron diffraction pattern obtained from a freshly formelI x,-plute with stacking plane parallel to the beam direction. The indexing of this typical of the 3R structure. is ~liffractinn pattern, schemstically showu in Fig. 13.
in Fig. 13, indicate relationship between found between p’ and
A major problem is whether or not long range &Fusion is invoked in the formation of the x,-plates. The results of the present work shoxr that q-plate formation is essentially a shear-like prows, ancl that long range difYusion mainly influences the plate morpholog)and crystal structure after the initial transformation. The shear-like nature of the transformation is suggested from a number
CORSELIS
ASD
ISOTHERMAL
i\-_iY3I_iS:
TRASSFOR~IATIOSS
0
hkf
INDICES
OF
f c c WITH
0
hkf
INDICES
OF
3R
.
iii
INDICES
OF
WITH
bee
ZONE ZONE
WITH
ZONE
AXIS AXIS AXIS
IS
JLETASTABLE
3’
CuZn
309
[Oil] [IlO] [Tll] (112)
iII(oll) -2 ,I -E
zoo “\
Jo__-------
(iTor
-
-c
I
’
\ \ iii
-?_
____------Q2
\
\
I/ ,
I’ /’
/
/
/
/
/-
/-
(101)
____-------
w
4.
_iII
0%
[1111
[0011 -
iTOT FIG. 13. Schematic
representation
of the lattice orientation relationship and 3R structures.
of experimental observations of the present work; (1) The displacements of scratches and interference fringes clearly indicate invariant plane strain surface relief. (2) The habit plane is close to (2, 11, 12), which is ident,ical to that reported for the martensite plates in CuZn alloys. (3) The structure of the al-plates (3R stacking sequence) can be regarded as an f.c.c. structure (disregarding small deviations) containing regularly distributed stacking shifts. Therefore, the only basic difference comparing a1 and martensite is the different mode of inhomogeneous shear; i.e. slip and twinning were observed for the aI- and martensite plates respectively. (4) An identical orientation relationship was found between the /Y-matrix and the al- and martensite plates. (5) The preferential formation of al-plates at scratches or other inhomogeneities on a pre-polished surface, indicates that their nucleation may be facilitated by stresses. The nucleation of plates at, the edges of existing plates also indicates that the a,-plates are more easily formed in a stress An autocatalytic reaction may therefore field. explain the rapid increase in plate density once the first few plates are formed. These observations are in agreement with those of Weisner and Hornbogen(?3’ which showed that the incubation periods for the al-plate formation were reduced by applying esternal stresses. This is similar to the effect of stresses on martensite formation; at X, no external stress is required to start the transformation, while above NS (and below NJ an external stress can initiate the transformation.
between the @‘-matrix
and the f.c.c.
(6) The microprobe microanalysis cf the al-plates described in detail elsewhere,(21J indicated that these formed without a change in composition; a partitioning of Cu and Zn between the /Y-matrix and the plates occurs, but starts immediately after the initial plate formation. It may be concluded from these observations that the ,6’ -+ a1 transformation resembles a martensite transformation process. All characteristics generally used in the description of a martensite transformation appear to apply; (a) an invariant-plane strain relief effect at a free surface; (b) an irrational habit plane and orientation relationship; (c) a lattice correspondence which implies the absence of long range diffusion; and (d) the systematic presence of internal inhomogeneities such as twins or stacking faults. Crystallography
of the 8’ + a1 transformation
In view of the crystallographic features of the ,6’ + a1 transformation just described, calculations were carried out using the martensite crystallography theory to compare predictions with experimentally obtained results. Even though the z1 plates exhibit the 3R structure, the transformation is basically (neglecting order) a b.c.c. to f.c.c. transformation, the stacking modulation being effected by the inhomogeneous shear of the crystallography theory. Referring to the earlier discussion on martensite (part I) the same argument applies concerning the choice of the inhomogeneous shear for the for a given (111) shear B’ - a1 transformation: (stacking) plane only one of the three (112) directions in the shear plane is possible because of neighbor violation restrictions imposed by ordering. Thus
310
ACTA
JlETALLI7RGIC.1,
the specific case was chosen involving the system (111) [ii2ji,c.c., or equivalently the system (101) when referred to the parent phase for [w,.,.,. computational convenience. Thus the theoretical habit plane, orientation relationship, and shape deformation for the 8’ + a1 transformation are identical to those predicted for the martensite case. The only difference is that the inhomogeneity for the x1 case is slip, lvhereas the martensite is internally twinned (on the same plane). The experimental habit planes and orientation relationships are identical considering both the x,-phase and martensite. The theoretical predictions are in good a,oreement with the observed quantities, as shown in Part I for the martensitic transformation. The large surface tilt (~13”) found for the a,-plates is also in good agreement with t.he theoretically predicted value, and indeed the surface relief itself is the most convincing proof that the B’ -+ aI transformation is martensitic in nature. A reproducible invariant plane strain is quite unlikely to result from a diffusion controlled process. Phenomenologically, the 3R structure can be regarded as an f.c.c. structure (disregarding small deviations) containing regularly spaced stacking shifts (faults). It is interesting that the magnitude of ‘-slip” to generate the 3R structure is given by a displacement of a/G[iiZ] on every third (111) plane (other variants would apply to other variants of (ill}), and for the appropriate f.c.c. lattice parameter is a shear of 13.26”. This is in remarkable agreement with the theoretical magnitude of the inhomogeneous shear (see Fig. 9, Part I) m2 = 0.234 (considering the b.c.c.-f.c.c. transformation) which corresponds to an angle of 13.15”. In other words, a certain magnitude of inhomogeneous shear is required to produce an undistorted habit plane, and it is just this amount which also effects the f.c.c. -+ 3R change in stacking sequence. In addition to the regularly distributed stacking disordered stacking faults shifts, some “extra” were observed. On average, the frequency of such faults w-as one in every twelve planes, a relatively minor component because of their low density. It is probable that the extra faults are formed by accommodation, or else are required in addition to the stacking shifts to produce a macroscopically undistorted habit plane. The internal striations in the al-plates and the continuous streaks in the corresponding diffraction patterns are considered to result from the disordered faults. However, the possibility has been raised that the fringe patterns
VOL.
27,
1974
within the plates correspond to rerv tie lamellae of f.c.c. material ls-ithin an otherwise 3R structure.“Q Weak f.c.c. intensity maxima were occasionally observed in the newly formed plates, but ma)- hare been due to a slight ‘-overannealing” of the x,-plates. A detailed study of the estinction contour configuration, as proposed by Delaey,“j) to determine the origin of the striations uniquely is rather difficult since the width of the initially formed x,-plates is typically only ~0.2 ,u. Stability of the x,-plates The irreversible nature of the x,-plates has been explained as due to diffusion involved in the plate formation process (e.g. a segregation of zinc to the “ordered stacking faults” which pins them in a It is considered here that a stable position@)). diffusion process during a1 formation is not a necessity to render the plates non-reversible. The plates can form by a (diffusionless) marteusitic reaction, and the “overannealing” which starts immediately after their formation may be responsible for their stability. Moreover, the partitioning of Cu and Zn between the forming plates and the matrix may even be responsible for their small size, i.e. the martensitic reaction is halted due to what is in effect an immediate tempering reaction. Xote that al-plates are ordered in that they form martensitically from a CsCl parent.? The “overannealing” also destroys the order, and hence glissile nature of the interface in a crystallographic sense. The “bulging” +pe of interface advancement during overannealing is analogous to the movement of a boundary during recrystallization, where the atom transfer is not coordinated. It is to be noted that the 3R structure possessed by the freshly formed x,-plates is considered here to have resulted from the transformation per se. That is, the inhomogeneous shear of the crystallography theory is responsible for the stacking shift on each third plane. In addition to the 3R structure, other long period stacking sequences have been observed in martensite in other materials. It may be in these cases as well that t,he long period structures exist only because the transformation has occurred. That is, such structures are not intrinsically stable in the sense that they do not form upon solidification. Finally, the present results indicate that an 7 The q-plates were never observed to form above the reported ordering temperature of the p-phase. It has also been verified by electron diffraction in the present nork that the untransformed parent phase remains ordered even after extensive isothermal transformation periods.
CORSELIS
&SD %-_~~>~.\IAS:
ISOTHERXAL
TRSSYFORXATIOSS
of giren composition can transform martensitically in two different temperature regimes. At the 8’ -+ u1 transformation higher temperatures, exhibits “C”’ curve nucleation kinetics; the product phase can be regarded as an internally slipped martensite. The athermal martensite which forms at. much lower temperatures is basicaliy an internaily twinned f.c.c. phase. These different modes of inhomogeneities may be temperature dependent, i.e. twinning at lower temperatures and slip at higher temperatures. It remains to be determined, howerer, why there are two temperake regimes inrolting basically the same tra~formation. Finally, sinee the 8’ + CQ transformation does not, involve diffusion, at least in the early crystallographic stages, it is suggested here that the terminology, “bainitic,” as applied to this transformation, not be used. allo-j-
ACKNOWLEDGEMENTS
This work, based on the Ph.D. thesis of I. Cornelis, was supported by the US. Atomic Energy Commission through the Materials Research Laboratory at. the University of Illinois (Contract AT-11-l-1195). REFERENCES
1. A. L. TITCHESER and 31. B. BEVER. Trans. AINE 200, 303 (1954). 4. H. POPS and T. B. 3l.~ssr~s~1, Trans. AIM.?? 230, 1662 (196-1).
IS
XETdSTSBLE
3’
CuZn
311
3. T. B. Jlass~~zrrr and C. S. BJLRRETT,Trans. _-lIME 209,453
(1955).
4. R. D. GSWOOD, J. Inst. -Ilet& 22, 6-i (19%). 5. P. E. J. FLEW-ITTand J. Y. TOWSEB, J. Inst. Xetat.s
95, 273 (1967). 6. P. E. REPAS and R. F. HEEEXUS, Technical Report So. 6 to the Office of Naval Research, June (1965). 7. R. D. G.~RWOOD,Iron Steel Inst. Spec. Rep. 93, 90 (1965). S. E. HORX~OGEX and H. W_UU,IXOIOST, dcta i%fet. 15, 943 (1967). 9. P. E. J. FLE~ITT and J. JI. TOWSER, lcta Xet. 14, 1013 (1966). 10. G. Bassr and B. STR~X, 2. .MetaZ&. 47, 16 (1936). 11. E. HORX~OGEX, 2. .~fe~al~~.50, 70 (19Zi9). 12. K. T~KEz~w.~ and S. SATO, Sippon Kimoh GaM. 35, 469 (1971). 13. G. R. SRI[SI~_~SATand JI. T. HEPWORTH, Icta Net. 19, 1141 (1971). 1-I. R. D. GUWOOD and D. HAL, The mechanism of phase tra~formations in metals, Inst. XetaZ8, Zlonogr. No. IS, 219 (1935). 1.5. R. D. GMWOOD and D. HL-LL, Rcta Jfet. 6, 98 (1959). 16 H. >I. CLMX and C. JI. W~TX~S, Phase transformations, Am. Sot. Xet. 59, (1968). 17. D. E. BRADLEY,J. a&. P&s. 27, 1395 (1936). IS. L. C. ;\ZICEEL~and C. 11. ~V~YXAS, Rec. Sci. In&r. 33, 552 (1962). 19. J. BOWLES, Trans. .4IME 191, 14 (1951). 20. P. E. J. FLEWITT, Scripta Xet, 2, 161 (196s). 21. f. CORSELIS and c. l\iL.WAY&&X, Scriptn JIet. 7, 579 (1973) 22. J. W. &MSTISX:, Decomposition of dudenite by IXffusio?tal Processes, edited by Z_uxuy and &ROSSOS, D. 371 (19631. 23. c. L. I?. W&XEB and E. HOBNBOGES,Scripta Xet. 3, 513 (1969). 2-1. L. DEUEY, private communication (19i3). 2% L. DEL.IEY, Phys. Status Sol& 29, 697 (196s).
TRANSFORMATIONS ISOTHERMAL I.
CORNELISt
IN METASTABLE p’ CuZn TRANSFORM.4TIONS* and
C.
11.
ALLOYS-II.
WAYMhNt
The present investigation was carried out with the primary purpose of establishing the nature of the isothermal @’ - c+ transformation in Cu-Zn alloys containing -40 wt.?; Zn. The morphology, habit substructure, and orientation relationship of the plate plane, shape deformation, crystal structure. and scanning electron shaped x1 transformation product hew been studied by optical, transmission microscopy. and by electron and S-ray diffraction. The inltlal 8’ - z, transformation ww. found to exhibit all of the characteristics generally ascribed Contrary to previous suggestions, diffusion. i.e. partitionmg of Cu to a martensitic transformation. and Zn, only influences the characteristics of the %,-plates after their formation in a di&sionless manner. The habit piane, orientation relationship, and shape strain associated with the /?’ + xx transformation are in ver>- good agreement with predictions of these quantities obtained by employing the martensite crystallography theory. The structure of the initially formed x,-plates can be described as a 3R stacking sequence involving close.packecl planes. It is emphasized that the 3R stacking is a result of the inhomogeneous shear of the phase transformation. The of the crystallography theory, and therefore is a consequence “overannealing” of the x,-plates during longer (isothermal) transformation times results in a disordered (f.c.c.) phase with non-crystallographic interfaces. TR_~SSFOR~I~TIO~S
DE PHASES DASS LES ALLIAGES C&n /?’ YETASTABLES-II. TRdSSFORSI_+.TIOSS ISOTHERXES Cette Etude a pour principal objet l’btablissement de la nature de la transformation isotherm0 8’ - )I, se produisant dans les slliages Cu-Zn contenant environ 40 96 en masse de Zn. La morphologic, le plan d’accolement, le changement de forme, la structure cristalline, la sous-structure et la relation d’orientation du produit de la transformation en forme de plaquettes z1 ont BtB Btudids par microscopic optique, par microscopic Bleotronique en transmission et 8. balayage, ainsi que par diffraction Blectronique et RX. Les auteurs trouvent que la transfo~ation initial9 fi’ -+ CL~p&sent0 tous Ies caract&es g&&alement ~ontrairement aux suggestions antbrieures, la diffusion. attribues b une transformation martensitique. c‘eat&-dire la s&pax&ion de Cu et Zn n’influe sur Ies caraot&istiques des plaquettes 01~qu’apr&s leur formation, qui s’effectue sans diffusion. Le plan d’accolement, la relation d’orientation et le changement de form0 associ6s B Is transformation J’ -+ x1 sont en bon accord avec les rCsultats p&us par la thPorie criatallographique de la martensite. La structure des plaquettes x1 form&es au debut peut 4tre d&rite par une st;quence d’empilement 3R comprenant des plans compacts. Les auteurs soulignent que l’empilement 3R r&&e du cisaillement hdtBrog&ne de la thborie cristallographique, et qu’il est done une con&quence de la transformation de phase. Cn recuit isotherme prolong6 deu plaquettes 11, produit une phase f.c.e. non ordorm& presentant des interfaces non cristallographiques. P~~SE~L~lW_~~DLUNGES
I?; JIETASTABILES ,!7’-CuZn-LEGIERUSGES.-IL: ISOTHERME UMWASDLUSGEX Die vorliegende Arbeit wurde mit dem Hauptziel durchgefiihrt, die Xatur der isothermen fi’ -+ I~. Umwandlung in Cu-Zn-Legierungen mit etwa 40 Gew.% Zn herauszuf?nden. Morphologic, Habitusebene, Gestalt~deru~, Kristaflstruktur, Substruktur und Orientierungsbe-zieh~g des plattenfijrmigen z~-Um~andlun~pr~~tes wurden mit iicht-, elektronenund ~ter-elekt~nenm~oskopischen Xethoden sowie durch Elektronenund Rantgenbeugungsexperimente untersucht. Die zunschst stettfindende ,3’ -f a,-Ummandlung besitzt alle Me&male einer marten&t&hen Umwandlung. Im Gegensatz zu friiheren Vorschl%gen bee&l&t die Diffusion, d.h. die Aufteilung van Cu und Zn, die Eigenschaften der a,.Platten erst nach ihrer diffusionsloaen Entstehung. Die mit der 8 * x,-Cmwandlung verbundene Hsbitusebene, Orietierungsbeziehung und Gestaltsdehnung sind in sehr guter t’bereinstimmung mit den anhand der Theorie der Martensitkristallographie vorhergesagten Werten. Die Struktur der am Beginn der TJmwtlndlung gebildeten %,-Flatten kann ah 3~.Stapelfolge dichresr gepaekter Ebenen beschrieben werden. Es nird darauf hingewiesen, da13 die 3R-Stapeifolge das Theorie und somit eine Folge der Ergebnis der inhomogenen Scherung nach der kristaliographischen Phasenummandlung ist. Das “UberanIassen“ der r,-Platten bei lilngeren (isothermen) Umwandlungszeiten fiihrt zu einer ungeordneten (f.c.c.) Phase mit nicht-kristallographischen Grenzfilchen. INTRODUCTION
products formed in a 41.3 wt. % Zn alloy: are platelike below 3BO”C, and that particles of irregular shape appeared in addition to the plates at slightly higher temperatures. At the upper temperature limit of his investigation, 470°C, the transformation product took the form of rods (or needles). This
In addition to the martensitic t~nsformation obtained by cooling CuZn alloys below X8(1-2) or by deformation below Xd,f3) the /l’ phase also transforms isothermally at higher temperatures.(A*j) Garwoocl(‘J found that the isothermal transformation * Received May 29, 1973; revised July 30, 1953. 7 Department of JLetallurgy and Mining Engineering and Materials Research Laboratorv, University of Illinois, Urbana-Champaign, Urbana, IllinoA 61801, U.S.A. ACTX
JIETALLTTRGICA,
VOL.
52, MARCH
197.4
change Flewitt
in morphology has been confirmed and Towner,@) who examined a number
brasses
within
also found 301
that
the 40544.1 by plotting
wt.%
Zn range.
the incubation
by of
They periods
ACT-1
30-2
>lETdLLCRGIC.1,
for visible transformation on a time/temperature diagram, the plates and rods gave rise to two separate C curves. The T-T-T diagram for a 40.5 wt.% Zn alloy is sholvn in Fig. 1. The lower C-curve, n-hich corresponds to the plate-like product,, shows a sharp cutoff at an upper temperature limit, B,, above which only rods form. Just below B,, a mixed plate-rod product is formed (region M). The incubation time for the plate formation is increased and the percentage of rods decreases, as the temperature is decreased below B,. The transition temperature, B,, varies with composition in approximately the same manner as the M, temperature which is -6OO’C lower,(s) although Repas and Hehemannt6) found that this parallelism does not hold for the higher Cu alloys. The B, values from both references(5+6) are given in Fig. 2. The plates which form below B, frequently appear as V-shaped pairs separated by an obtuse angle. This is to be contrasted with the subzero martensite plates Jvhich form as acute-angled V-pairs. For a given composition, the plates become finer with decreasing transformation temperature; the density of the plates becomes higher with increasing transformation time, leading ultimately to a closed network arrangement. The plate-shaped transformation product has been referred to as bainite+8) because of certain similarities with bainite in ferrous alloys. However, because of the existing disagreement concerning definition, nucleation, growth kinetics, crystal structure, etc. for bainite in steels, the terminology x,-plates has beejb chosen in the present work for the plate-shaped isothermal transformation product. The rod-shaped procluct will be referred to as a-rods. A characteristic feature of the x,-plates is the presence of internal markings which have been
0.
IO
;SE
I03
I04
TIME,
SECONDS
IO3
ii16
FIG. 1. T-T-T diagrem for the isothermal transformation products in a Cu-40.5 wt.% Zn according to Flenitt and Tow-ner.cs’ R. P and ~!f indicate the regions where rod-shaped, plate-shaped end mixed transformation products are formed.
VOL. 22, 1951
zoo+ L-_-
’
35
FIG.
2. Upper formation
\
I
_
\
I
~_5-~~_
ATOMIC
40
?Z?CShT
ZINC
temperature limit (B,) for r,-plate as a function of composition.i5~B~
observed by electron microscopy of thin foils(s) as well as replicas.(g) These striations are reported(*) to be similar to the stacking faults observed in martensite plates. As with the martensite forming from b’ CuZn, S-ray diffraction studies(4.10*11) have led to several proposed crystal structures for the a,-plates. The more recent electron diffraction studiea(**is) indicate a 3R stacking sequence of close-packed planes for the The tendency for the faults initially formed plates. to “anneal” out with longer transformation times(*) was found to be accompanied by a transition to an f.c.c. stacking sequence (i.e. 1R structure). Single surfaces analysis(4*13) indicates that the habit plane of the a,-plates is close to (2, 11, 12)b.c.c., which corresponds to the habit plane reported for martensite plates in CuZn alloys.(14~15) The surface relief observed on a pre-polished surface after the formation of the al-plates may also be similar to that observed with martensite. Interference microthat an invariant-plane strain graphs(7*16) indicate type of relief exists for the a,-plates. However, the small size of the initially formed plates (4.2 p thick) has precluded the observation of scratch displacements by conventional optical microscopy. The present investigation was undertaken in order to more clearly define the nature of the a,-plates, and, insofar as possible, establish the mechanism Separate sections of the ,!?I+ a1 transformation. of the paper are devoted to morphology-, habit plane, surface relief, electron metallography, crystal structure, orientation relationship, ancl application of the martensite crystallography theory. EXPERIMENTAL
PROCEDURE
Twelve CuZn alloys, ranging from 38.1 to 44.0 wt. % Zn, were prepared. The quenched metastable p’specimens were isothermally transformed in 50 : 50
.~.diuru nitrate,/potassium nitrate salt baths in the ~i.w--&SP)‘C: temperature range, and in a molten had bath for the higher temperat~e range. The specimens used for surface relief studies were electropolkhed and scratched by a very light abrasion with 0.05 ,rt.Al,O, particles prior to the isothermal transformation. These specimens were then isothermally transformed, at the appropriate temperature, by holding them with the non-polished surface on top of a salt bath for time intervals which resulted in to rather low density of plates, so that the surface relief of individual plates could be studied rrithout the interference of neighboring plates. Since the smalt size of the x,-plates prevented scratch displacement observations by opt,icel microof the pre-polished scopic means, carbon replication and transformed surfaces was performed. Several replication techniqueso’) were used. First, the txo stage method using a cellulose sheet was tried. Several variants of carbon evaporation and shadowing (with platinum or chromium) of the negative cellulose replica \vere employed, but the desired contrast efkcts were not obtained. Therefore the single stage for direct) method which consists of evaporating carbon (with or nithout preshado~ving~ immediately on the surface x-as employed. The best resolution n-as finally obtained by using only carbon evaporation at an angle of 1%30”, allowing only thin layers of carbon to be formed on the surface. The carbon Brn was then divided in small squares 1-2 x 1 mm), xvhile still on the surface of the specimen, and afterwards removed by immersing the specimen in a 2 y0 solution of nitric acid in ethyl alcohol. These replicas as Tvell as the thin foils, prepared as described in part I, lvere inveskigated with a Hitachi HZ;-11B or HU-19 electron microscope. Finally, a special preparation technique was used on the specimens intended for two surface analysis. In this analysis, a very sharp edge is required betn-een the two surfaces in order to follow the traces of the small x,-plates in both surfaces; moreover the surface from which the back reffection Laue S-ray is taken cannot be deformed. Therefore, a cylindrical specimen (4G in. diameter and ~0.3 in. thick) was electropolished, etched and the required grain was sectioned with a slow speed rotary saxv. The edge was sharpened up by carefully trail polishing the cut surface with A&O, (0.03 ~1. This left the pre-electropolished surface undeformed and suitable for S-ray diffraction. A Hilger microfocus S-ray unit xvith copper target \vas used at 40 1;V and 350 mA, to determine the 6’ grain orientations. The desired grain was positioned
in the S-ray
beam
RESULTS
Jiror&203y
by using ASD
an z--y positioner.41s~
DISCUSSIOS
of the ianthermal ty~~~,~~~~~tat~~~~ produ&
Jletallographic study of specimens containing mere than 40 wt. 7.~;Zn, showed general agreement with the T-T-T curves and consequently the 23, temperatures determined by Flewitt and Towner.(3j In the lower zinc region a noticeable deviation from their estrapolated straight line towards lower temperatures was found; the deviation is not as large as claimed by Repas and Hellemann(6) as shown in between the 3, temFig. 2. A strict comparison peratures of different investigators is difbcult since many factors may influence the nature of the transespecially at 10~ zinc conformation products, Some of the important variables are; centrations. specimen size, solution treatment temperature and time, quenching rate (and consequently the amount of massive ~1 formed), grain size, and the method Severtheless, used to transform the specimens. it may be concluded from the present and previous morphology studier; that the concentrationdependence of .B, is roughly parallel with X$ for the higher zinc region, and that B, levels off towards lox-er temperatures for the low zinc region (i.e. < 40 wt. % Zn). In this respect it should be noted that the al-plates were never observed to form at temperatures above the reported ordering temperature of the p-phase (in the a - 6 region), -n-hich is .-4WC. An impedimel~t to many experimental observations is the small size of the x,-plates (lo-15 $6 Iong and 0.2-l p thick depending on the composition and transformation temperature). Several attempts were made to obtain larger plates. One method which was partially successful involved a step heat treatment, It was found that the %,-plates are larger Then isothermal transformation occurs close to B,. However a mixed product of al-plates and r-rods resulted at these temperatures and the initial substructure within the plates disappeared very quickly, as -+vas verified by transmission electron microscopy. This indicates that these products are not representative of initially formed plates, which vere desired for this work. It was observed, however, that near B,, plates formed before rods. To obtain somewhat larger plates without the presence of rods, a step treatment was performed which consisted of a very short heat treatment just below 3, (e.g. 10 see at 4OO’C for the iO’% Zn alloy) followed by a longer isothermal transformation at a lower temperature (e.g. W-300%). This was finally followed by quenching to room temperature. Such a procedure
allo-~ed the nucleation of fen-er plated in the hiph temperature region, folloned by their growth in the lower temperature region without “overannealing” and nithout the formation of small rods between the plates which would obscure the surface relief due to isolated plates in the $-matris. Another attempt to produce larger plates through the use of a ternary alloying element was made. An aIloF was prepared containing 40 at. % Zn, 18 at.. % Cu and 2 at. % dg. A slight increase in the size of the al-pfates in this alloy was found, and is probably due to the higher B, temperature (close to 510°C). This increase in the upper temperature limit for the plate formation is likely to be due to the increase in ordering temperature. However, the addition of a third element makes the interpretation of some results more diflicult and estrapolation to the binary case is not always without danger. The step treat,ment method for obtaining larger rx,-plates was therefore preferred to alloying, and was successfully used in the ctl habit plane and surface relief analysis.
To date? only a single surface technique(i9) has been used to infer the habit plane of the a1-plates.(4*13) With the combination of the previously mentionecl step heat treatment (to increase the plate size) and a special specimen preparation technique, it was possible to perform the more reliable two-surface analysis. The habit plane traces in the two surfaces were measured on a. series of matched-up photographs taken from both surfaces. Figure 3 show a composite (dark field illumination) from both surfaces of a grain in a 10.5 wt. ‘A Zn alloy, after transformation for
10 xc at -Et!‘C and 30 5ec at 330°C. Direct measurements were also made on the enlarged image projected on the ground plas* screen of the metallograph. The direction cosines found for x,-plates in two 40.5 wt. T{ Zn specimens and three 11 v-t. y.6 Zn specimens are plotted in a common unit triangle. Fig. 4. The (2, 11: l?), pole and the theoretical habit plane from caleulationsr using the phenomenological crystallographic theory of martensitic transformations (discussed in Part I). are also shown. The observed habit plane poles vary in a region around the theoretical and fP> i 1: 12) habit planes and are close to formerly reported results for both ‘A~-and martensite plates. The two-surface analysis also made it possible to determine the relative location of the habit plane variants in a unit triangle n-hich correspond to the two plates of an obtuse angled V-shaped plate pair (e.g. plate pair x-x’ in Fig. 5). It was found that these variants are situated near the same (llcI), pole and on the same ride of the great circle connecting the tao surrounding {ill): poles. These relative locations are in agreement with a suggestion by Flewittts”) based on geometrical observations. Surfuce re2ief due to a,-plates Ati invariant plane strain type of surface relief for the x,-plates was expected, on the basis of earlier work.“.ie) The small plate size preTented, however, the use of the scratch displacement technique as determined by optical microscopy. Moreorer, the small increase in plate size obtained by the step heat treatment or the addition of a ternary element was not sufficient to use the scratch displacement t,echnique and optical microscopy. Thus, a scan&g
Fta. 4. Espwimentaliy observed habit pisne plates ahown i:l K common unit sternographic
fur zl. trianqie.
electron microscope study was attempted. Figure 5 show a back scattering image of z,-plates formed in a 40.5 w-t. “/:, Zn alloy at MtX. As can be seen, both plates in an obtuse angled V-shaped pair (e.g. I and z’) exhibit a surface tilt in the same sense. The tilt, associated with plates y and y’ is again in the same sense for both plates but opposite to the tilt associated xith the .r--1’ pair. The detection of scratch displacements by scanning electron microscop.failed due to the rather low contrast obtained from the small surface relief and also due to the preferential formation of the r*,-plates at the scratches, as is illustrated in Fig. 6.
Fx. 5. Scanning eketrot microgmph (beck scetterinpj shcwing surface relief produced by q-plates form& in a Cu-4K.i wxO, Zn atlq- at 400’C after 15 Sec. -i
Fro. 6. Scanning illustrating the
A
clear
associated
electron micrograph (back ecattering) preferenti81 nw&aiion of r,-plates at scratches.
indication with
the
of an nomination
invariant of the
plane z,-plates
strain xas
using the interference microscope on pre-polished and transformed 2 “/6 -J-g ternary alloy specimens. This alloy made the obserrarions of the fringe displacements easier due to the slightly Iurger plate size obtained. Figures i and S show the fringe disp~ace~lents and associated micrographs of the same region in the ternary alloy transformed at 430°C for 10 and 30 set respectivel-. Only a plateshaped transformation product is seen in Fig. r and the invariant plane strain type of relief is clcarlv indicated by the fringe displacement mode. For longer transformation periods (Fig. S) smalt rodi; from in-between the initial x,-plates which for longer times become thicker and less straight. The clear fringe displacements observed after 10 set, become levs evident due to an or-oral1 rumpling of the surface produced b:- the man-c_ small rods and also because of the non-crystallographic thick. ening of the plates themselves? the latter caused by a long range diffusion process G&r the initial plate formation.czl) The negative influence of the rods could be aroided by the two step heat treatment, which was used to increase the plate size in the binary CuZn alloys. Xeasurements on the latter were required for the calculations of the shear angles, which were used in conjunction with other crystallographic data found for these alloys. The measurements of the fringe displacements resulted in shear angles ranging from IOR- rakes up to 11.7.~‘. obtained
FTF:. 7. Fringe dkplttcement~ and corresponding surface relief micrographs of q-plates formed in B 2t0 _1g ternary allo_v transformed at 43O’C for 10 sec.
Although rather
the fringe displacements
could he measured
accurately,
detailed
the small plate measurements of the plate
the
obtained
=I>
only.
Carbon
shear
replication
isothermally
electron
a!so performed. bp the q-plates, ohserred be Seen.
Depending
an
scratched
specimens,
microscopic
accuraq-
followed
inrestigations
of and b,v xvas
The displacement of the scratches obserred using this technique, is
in Fig. 9. for
have
of pre-polished,
transformed
EransmisGon
illustrated
angles
size pferented width: so that
Displacements
martenvite
plates
similar (see
Part
to those I)
can
FIG. 8. Fringe tlisplrtcements and corresponding surface relief micrographs of the same region as in Fig. 7 after 10 set more of isothermal transformation at 43O’C.
direction 13”
of
(=O._“‘)
slightly
larger
orientation
of plates
and the
scratches,
than
displacements
measured. that
This obsert-ed
up
to
surface tilt is for martensite
plates, which x-as -11.3” for the 39 xt. o/0Zn allo-s and corresponds rert_ well with the predicted ralue from the martensite theories as will be discwed later. The surface tilt obserred for the q-plates is highly unlikely to result from a transformation process which is in some degree diffusion controlkd.1T2J Trcm~m i.s.sionelectron ,micr-o.scopy Qml detraction The
microprobe
inclicated on the
the were
that
microanal-sis
the x,-plates
a change in composition;
formed
of
thin
foik,‘.a)
initially
without
a partitioning
of Cu and Zn
However, this mixed structure could not be confirmed after careful tilting of the specimenzr for moat Therefore, the striatiow x,-plates investigated. inside the q-plates are beliered to be disordered stacking faults, and not coherent. boundaries between 1R and 312 lamellae. Moreover, the 1R intensity maxima observed in newly formed plates may indicate a slight orerannealing of the x,-plates, since the partitio~~i~~~ of Cu and Zn atoms occurs yuickly after their initial formation.(21~ Twin related intensity ma_xima, as obaerx-ed in the martensite plates (see Part I)! were never found in the case of the x,-plates.
FIG. 9. Typical example of the scratch clisplacements CMWX~b_ the fnfmation of x,-plates in a Cu-40 wt.So Zn aflcty {carbon replica). hetn-een the pIates and the surrounding a’-matris occurs only offer the plates are formed in a diffusionless manner. The change in morphology of the x,-plates with increasing isothermal transformation time is illustrated in Figs. lO(a-e) for a -kO.5x%. % Zn alloy transformed at 3.5O’C for 301 90 and 210 r;ec rt>sIxctiveIy. The initially thin plates (after 30 set) ~Jecam! gradud~y thicker with increasing transformation times; the interface also becomes more irre@ar. This “OTeratltlealiflg” of the q-plates is accompanied by a gradual, altl~ougl~ not u[~iform, decrease in the density of striations in the q-plates. The electron diffraction patterns obtained from the ne%Yly formed q-plates generally indicated a _A typical selected area diffraction 322 structure. pattern obtained from such plates is giren in Fig. 11. Besides the 38 intensity maxima a continuous streaking in the c*-direction (i.e. direction perpendicular to the close-packed planes) is observed. This streaking cannot be due to an ordered fault and therefore must be caused by distribution, randomly distributed faults in the close-packed arrangement. The diffraction patterns obtained from slightly orerannealed plates contain f.c.c. intensity maxima in addition to the 3R spots, as illustrated in Fig. 12. maxims became stronger, These f.c.e. intensity and ~o~espondingly the 3R spots became ireaker, vith increasing transformation periods. On a few occasions, f.c.c. intensity maxima were present in diffraction patterns obtained from newly formed plates. This may indicate that a “mixed structure” of IR and 3E lamellae was present in these plates.
FIG. 10 (P.-C). Transmission electron microgrnphs illus. trating the chttnge in morpholoc of the z,-plates in a Cu-40.5 wt.“, Za alloy- with increwina isotherm&l trensformation time (36. 90 and 210 i& St 330% respectid>-).
12 FIG. 10(c).
Lrcttice orientrrtion relatiotulbip
Selected
diffraction of tlcljacent q-plates was used tf.) determine the lattice orientation relationship AS in the case of martensite and correspondence. (Part I). only t!le x,-plates oriented such that the ?;tacking plane was parallel to the beam direction orientations coulcl were considered. since other easily leacl to a fake interpretation. The $‘-matrix adjacent to the piatw lvith this required orientation showed a zone axis orientation close to zL ,‘lll; direction. The orientation relationship, or 100 rchgions
of
area
the
electron
iJ’-matrix
and
Fra. from
12. Typical electron diffraction pattern obtained a nlightly ~‘overarmraled” x,.plete. The indesing ia given in Fig. 13.
measured from a series of diffraction patterns with zone axis orientation. can be summarized a (llli,b.,.,, as: (OOO),, (= 3tacking
plane) 5.5’ from (lOl),,.,,,,
(ii4),, // (,r,.;S’) with (Oll)h,,y ,,.., (ilz),, 5.53(E0.5”) from (iiO),_,~.,T.. The beam direction is assumed to be cxactl~~ parallel to both zone axes! i.e. B.D. 11[lli)],E jj [iii],,,.,,. In the cases n-here f.c.c. intenait? masima are present, the orientation relationship lJetW?en the f.c.c. and b.c.c. lattices is similar to that found for martensite (see Part I), i.e. (ill),,,.,,
(=(009),,)
53
from (IOl),,,.,.
(iii),,,.,, 5.5'(&O.Y) from (O1l),,,J,C, (~00~~.
I/ (ilo),.,.,..
These relationships. shown that the lattice orientation p” and rl is identical to that martensite.
FIG. 11. Typical electron diffraction pattern obtained from a freshly formelI x,-plute with stacking plane parallel to the beam direction. The indexing of this typical of the 3R structure. is ~liffractinn pattern, schemstically showu in Fig. 13.
in Fig. 13, indicate relationship between found between p’ and
A major problem is whether or not long range &Fusion is invoked in the formation of the x,-plates. The results of the present work shoxr that q-plate formation is essentially a shear-like prows, ancl that long range difYusion mainly influences the plate morpholog)and crystal structure after the initial transformation. The shear-like nature of the transformation is suggested from a number
CORSELIS
ASD
ISOTHERMAL
i\-_iY3I_iS:
TRASSFOR~IATIOSS
0
hkf
INDICES
OF
f c c WITH
0
hkf
INDICES
OF
3R
.
iii
INDICES
OF
WITH
bee
ZONE ZONE
WITH
ZONE
AXIS AXIS AXIS
IS
JLETASTABLE
3’
CuZn
309
[Oil] [IlO] [Tll] (112)
iII(oll) -2 ,I -E
zoo “\
Jo__-------
(iTor
-
-c
I
’
\ \ iii
-?_
____------Q2
\
\
I/ ,
I’ /’
/
/
/
/
/-
/-
(101)
____-------
w
4.
_iII
0%
[1111
[0011 -
iTOT FIG. 13. Schematic
representation
of the lattice orientation relationship and 3R structures.
of experimental observations of the present work; (1) The displacements of scratches and interference fringes clearly indicate invariant plane strain surface relief. (2) The habit plane is close to (2, 11, 12), which is ident,ical to that reported for the martensite plates in CuZn alloys. (3) The structure of the al-plates (3R stacking sequence) can be regarded as an f.c.c. structure (disregarding small deviations) containing regularly distributed stacking shifts. Therefore, the only basic difference comparing a1 and martensite is the different mode of inhomogeneous shear; i.e. slip and twinning were observed for the aI- and martensite plates respectively. (4) An identical orientation relationship was found between the /Y-matrix and the al- and martensite plates. (5) The preferential formation of al-plates at scratches or other inhomogeneities on a pre-polished surface, indicates that their nucleation may be facilitated by stresses. The nucleation of plates at, the edges of existing plates also indicates that the a,-plates are more easily formed in a stress An autocatalytic reaction may therefore field. explain the rapid increase in plate density once the first few plates are formed. These observations are in agreement with those of Weisner and Hornbogen(?3’ which showed that the incubation periods for the al-plate formation were reduced by applying esternal stresses. This is similar to the effect of stresses on martensite formation; at X, no external stress is required to start the transformation, while above NS (and below NJ an external stress can initiate the transformation.
between the @‘-matrix
and the f.c.c.
(6) The microprobe microanalysis cf the al-plates described in detail elsewhere,(21J indicated that these formed without a change in composition; a partitioning of Cu and Zn between the /Y-matrix and the plates occurs, but starts immediately after the initial plate formation. It may be concluded from these observations that the ,6’ -+ a1 transformation resembles a martensite transformation process. All characteristics generally used in the description of a martensite transformation appear to apply; (a) an invariant-plane strain relief effect at a free surface; (b) an irrational habit plane and orientation relationship; (c) a lattice correspondence which implies the absence of long range diffusion; and (d) the systematic presence of internal inhomogeneities such as twins or stacking faults. Crystallography
of the 8’ + a1 transformation
In view of the crystallographic features of the ,6’ + a1 transformation just described, calculations were carried out using the martensite crystallography theory to compare predictions with experimentally obtained results. Even though the z1 plates exhibit the 3R structure, the transformation is basically (neglecting order) a b.c.c. to f.c.c. transformation, the stacking modulation being effected by the inhomogeneous shear of the crystallography theory. Referring to the earlier discussion on martensite (part I) the same argument applies concerning the choice of the inhomogeneous shear for the for a given (111) shear B’ - a1 transformation: (stacking) plane only one of the three (112) directions in the shear plane is possible because of neighbor violation restrictions imposed by ordering. Thus
310
ACTA
JlETALLI7RGIC.1,
the specific case was chosen involving the system (111) [ii2ji,c.c., or equivalently the system (101) when referred to the parent phase for [w,.,.,. computational convenience. Thus the theoretical habit plane, orientation relationship, and shape deformation for the 8’ + a1 transformation are identical to those predicted for the martensite case. The only difference is that the inhomogeneity for the x1 case is slip, lvhereas the martensite is internally twinned (on the same plane). The experimental habit planes and orientation relationships are identical considering both the x,-phase and martensite. The theoretical predictions are in good a,oreement with the observed quantities, as shown in Part I for the martensitic transformation. The large surface tilt (~13”) found for the a,-plates is also in good agreement with t.he theoretically predicted value, and indeed the surface relief itself is the most convincing proof that the B’ -+ aI transformation is martensitic in nature. A reproducible invariant plane strain is quite unlikely to result from a diffusion controlled process. Phenomenologically, the 3R structure can be regarded as an f.c.c. structure (disregarding small deviations) containing regularly spaced stacking shifts (faults). It is interesting that the magnitude of ‘-slip” to generate the 3R structure is given by a displacement of a/G[iiZ] on every third (111) plane (other variants would apply to other variants of (ill}), and for the appropriate f.c.c. lattice parameter is a shear of 13.26”. This is in remarkable agreement with the theoretical magnitude of the inhomogeneous shear (see Fig. 9, Part I) m2 = 0.234 (considering the b.c.c.-f.c.c. transformation) which corresponds to an angle of 13.15”. In other words, a certain magnitude of inhomogeneous shear is required to produce an undistorted habit plane, and it is just this amount which also effects the f.c.c. -+ 3R change in stacking sequence. In addition to the regularly distributed stacking disordered stacking faults shifts, some “extra” were observed. On average, the frequency of such faults w-as one in every twelve planes, a relatively minor component because of their low density. It is probable that the extra faults are formed by accommodation, or else are required in addition to the stacking shifts to produce a macroscopically undistorted habit plane. The internal striations in the al-plates and the continuous streaks in the corresponding diffraction patterns are considered to result from the disordered faults. However, the possibility has been raised that the fringe patterns
VOL.
27,
1974
within the plates correspond to rerv tie lamellae of f.c.c. material ls-ithin an otherwise 3R structure.“Q Weak f.c.c. intensity maxima were occasionally observed in the newly formed plates, but ma)- hare been due to a slight ‘-overannealing” of the x,-plates. A detailed study of the estinction contour configuration, as proposed by Delaey,“j) to determine the origin of the striations uniquely is rather difficult since the width of the initially formed x,-plates is typically only ~0.2 ,u. Stability of the x,-plates The irreversible nature of the x,-plates has been explained as due to diffusion involved in the plate formation process (e.g. a segregation of zinc to the “ordered stacking faults” which pins them in a It is considered here that a stable position@)). diffusion process during a1 formation is not a necessity to render the plates non-reversible. The plates can form by a (diffusionless) marteusitic reaction, and the “overannealing” which starts immediately after their formation may be responsible for their stability. Moreover, the partitioning of Cu and Zn between the forming plates and the matrix may even be responsible for their small size, i.e. the martensitic reaction is halted due to what is in effect an immediate tempering reaction. Xote that al-plates are ordered in that they form martensitically from a CsCl parent.? The “overannealing” also destroys the order, and hence glissile nature of the interface in a crystallographic sense. The “bulging” +pe of interface advancement during overannealing is analogous to the movement of a boundary during recrystallization, where the atom transfer is not coordinated. It is to be noted that the 3R structure possessed by the freshly formed x,-plates is considered here to have resulted from the transformation per se. That is, the inhomogeneous shear of the crystallography theory is responsible for the stacking shift on each third plane. In addition to the 3R structure, other long period stacking sequences have been observed in martensite in other materials. It may be in these cases as well that t,he long period structures exist only because the transformation has occurred. That is, such structures are not intrinsically stable in the sense that they do not form upon solidification. Finally, the present results indicate that an 7 The q-plates were never observed to form above the reported ordering temperature of the p-phase. It has also been verified by electron diffraction in the present nork that the untransformed parent phase remains ordered even after extensive isothermal transformation periods.
CORSELIS
&SD %-_~~>~.\IAS:
ISOTHERXAL
TRSSYFORXATIOSS
of giren composition can transform martensitically in two different temperature regimes. At the 8’ -+ u1 transformation higher temperatures, exhibits “C”’ curve nucleation kinetics; the product phase can be regarded as an internally slipped martensite. The athermal martensite which forms at. much lower temperatures is basicaliy an internaily twinned f.c.c. phase. These different modes of inhomogeneities may be temperature dependent, i.e. twinning at lower temperatures and slip at higher temperatures. It remains to be determined, howerer, why there are two temperake regimes inrolting basically the same tra~formation. Finally, sinee the 8’ + CQ transformation does not, involve diffusion, at least in the early crystallographic stages, it is suggested here that the terminology, “bainitic,” as applied to this transformation, not be used. allo-j-
ACKNOWLEDGEMENTS
This work, based on the Ph.D. thesis of I. Cornelis, was supported by the US. Atomic Energy Commission through the Materials Research Laboratory at. the University of Illinois (Contract AT-11-l-1195). REFERENCES
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IS
XETdSTSBLE
3’
CuZn
311
3. T. B. Jlass~~zrrr and C. S. BJLRRETT,Trans. _-lIME 209,453
(1955).
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