- Email: [email protected]
Modulated structures in AuNi alloys
MODULATED
J.
E.
STRUCTURES
WOODILLA
JR.t$
IN
and
B.
L.
Au-Ni
ALLOYS*
AVERBACEEt
A modulated structure has been shown to develop on {IOO} planes in certain gold-nickel alloys on aging at temperatures in the vicinity of lOO-200°C. The critical composition is at 45 at.% Ni and the maximum temperature at which these structures ocours is 220°C. The boundary of the modulated structure region is nearly symmetric, with extreme compositions at approximately 20 and 60 at. O/eNi. This structure has been observed by means of electron diffraction and is characterized by the formation of satellite reflections on (100) axes around each fundamental peak. The modulated structures exhibited a composition amplitude of approxixnately 5 at. “/band characteristic wavelengths which varied from 6.5 A at 54 at-“/b Ni to 13 A at 22 at.% Ni. An activation energy of 13,500 eel. per mol. was calculated for the rate limiting step in the low temperature region, corresponding to the energy of migration for vacancies in this composition range. The satellite reflections disappeared on aging just above the critical temperature; on reaging at a lower temperature, the satellites did not reappear. This is taken to indicate that the presence of a large concentration of quenched-in excess vacancies is required for the development of the modulated structure. The formation of the modulated structure may be associated with the postulated spinodal decomposition in this system, although the theories of spinodal deeompositioi~ do not yet adequately account for the role of vacancies.
STRUCTURES
MODULEES
DANS
LES
ALLIAGES
Au-h?
Une structure modulee se developpe sur les plans {IOO] de eertains alliages or-nickel soumis a un vieillissement dans un domaine de temperatures voisin de lOO-200°C. La composition critique se situe a 4.5 at.“/b de Ni et la temperature maximum d~apparition de ces structures est 220°C. La timite de la region a structure mod&&e est presque symetrique, avec des compositions extremes 8.environ 20 et 60 at. y0 de Ni. On a observe cette structure par diffraction Blectronique; elle est oara&risee par la formation de reflexions auxiliaires sur les axes (100) autour de chaque pit fondamental. Les structures modulees montrent une amplitude de composition d’environ 5 at. o/’et des longueurs d’onde caracteristiquas variant de 6,5 A pour 54 at.% de Ni a 13 A pour 22 at.% de Ni. On calcule pour lo stade limitant la vitesse une energie d’activation de 13 500 Cal. par mole, correspondant 8, l’energie de migration des laeunes dans ee domaine de compositions. Les &flexions auxili&es disparais~nt quand on fait un vieillissement juste au-dassus de la temp6rature critique; en faisant un nouveau vieillissement a temp&atura inferieure on no les fait pas tiapparaitre. Ceci indique que la croissance de la structure modul&e requiart la presence d’une grande concentration de lacunes trempees en exces. On neut associer la formation de la structure modulee b la decomposition spinodale supposee de oe systeme,- bien que les theories de la decomposition spinodale ne rend&t cependant pas oor%tament compte du role des lacunes.
MODULIERTE
STRUKTUREN
IN
Au-Ni-LEGIERUNGEN
Eine modulierte Struktur zeigt sich auf {lOOf-Ebenen in einigen Gold-Nickel-Legierungen nach Anlassen bei lOO-200%. Die kritischa Zusammensetzung ist bei 45 At.-% Ni. Die Hijchsttemperatur fiir das Entstehen dioser Strukturen betragt 22O’C. Die Grenze der modulierten Struktur ist nahezu symmetrisch bei extromen Zusammensetzungen von 20 und 60 At.-% Ni. Diese Struktur wurde mittels Elektronenbeugung beobachtet. Sie ist charakterisiert durch die Bildung von Sattelitreflaxen auf (lOO)-Achsen bei jedem Fundamentalmaximum. Die modulierten Strukturen zeigten eine Zusammensetz~gsamplitude von etwa 5 At.-% und charakteristische WelIenlangen, die von 6,5 A bei 64 At.-% Ni bis 13 A bei 22 At.-% Ni variieren. Eine Aktivierungsenergie van 13 500 cal/MoI wurde fur den geschwindigkeitsbestimmenden Sohritt im Tieftemperaturbersich berechnet; das entspricht der Wanderungsenergie von Leerstellen in diesem Zusammensetzungsbereich. Die Sattolitenreflexe verschwanden beim Altern gerade oberhalb der kritischen Temperatur. Beim weiteren Anlassen wieder bei tieferen Temperaturen traten die Satteliten nioht wieder auf. Zur Bildung der modulierten Strukturen scheint also eine hohe Konzentration iiberschtisiger Leerxtellen erforderlich zu sein. Diese Bildung mag mit der vorgeschlagenen spinodalen ~ntm~schung in diesem System zusamme~~ngen, obwohl die Theorien der spinodalen Entmischung die Leerstellen nicht in angemessener Weise beriicksichtigen.
* Received May 6, 1967. t Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, 2 Now at: The Torrington Co., Torrington, Connecticut. ACTA
METALLURGI~A,
VOL.
16, FEBRUARY
1968
255
Cambridge, Massachusetts.
ACTA
‘756
METALLURGICA,
1. INTRODUCTION
The gold-nickel temperatures metric
miscibility
temperatures
gap
above
this
chemical
lower
and
temperatures.
spinodal
process
is
alloys
originating Several
associated
with
the
miscibility
t’o
of the free energy
composition,
a2G/aX2,
is
zero.
with
Cahn@)
concluded
the
It
aaG/aX2 < 0, and postulated
in a state of metsstable
The segregation tinue,
limited
equilibrium cibility
gap
defined
excursions.
of the latter
decomposition
spinodal
wit,h experimental
t,he spinodal gold-nickel effects
alloys,
which
was
scattering
the
to determine
200°C
within
the
two-phase
nucleated
heterogeneously
modulation reflections
fundamental predicted
of
of temperature
an asymmetry by the
satellite
In this
investigation,
and More
temperature experimentally of satellite
fluctuations
of the
remain coherent
tension of
the resultant
effects,
such
with the matrix. elastic
it was predicted
a coherent
spinodal
Taking
energy and surface that
would
aging.
about
all of the
the
model, than
origin.
In
intensity
with
the
t,he high
is low
angle
a modulated
reflections
satellite of
the boundary
determined
be
satellite intensities
depressed
well below (approximately 2000°C in the gold-nickel system) the chemical spinodal defined by Borelius.
structures
determined
on electron diffraction wavelength
structure
gold-nickel
was
alloys on low
The region of the phase diagram
in which the modulated
recently, Cahn(3s4) has considered that the spinodal should be defined only for cases where the composition into account
intense
size
factor,
should lead to the
in the satellit’e
Hargreavcs
more
in atomic
including
shown to develop in certain
bhe
form,
reflections,
the
alloys,
satellite.
the
Furthermore
angle
and
in scattering
of satellite
to show any
at grain boundaries.
difference
composition
addition,
in a pearlitic
is both a large difference
an appreciable
of
top
parameter
formation
with the position
region.
was lamellar,
and
lattice
In the case of gold-nickel
a periodic
the role of
the
where there
and formulated
in the case of a simultaneous
the
and
rate of precipitation
below
factor.
dif-
expanded
of the fundamental
alloys.
decomposition
miscibilit)y gap and nearly independent precipitate
of the
rates
in Au-Pt
but he was unable
approximately
spinodal.
boundary
equilibrium
needed
around
and electron
Hargreaves@)
for the amplitude
fluct,uations
nucle-
to the
structure
reflections
of Daniel and Lipson”)
the expressions
the solid
leading
modulated
in X-ray
experiments.(7-s)
the treatment
certain fluctua-
throughout
of satellite peak,
reflections
the
The maximum
fraction
that
in
of this type may be observed
formation
of both
could be associated
of the spinodal.
usual
the
each fundamental
modulation
precipitation
attempted in the
the
from
(100)
equilibrium.
structures
and satellite
to find some correlat’ion
Underwood(2)
mis-
Modulated
to be less probable
within
to relate
the
with finite
homogeneous
composition
attempted
until
incoherent,
contrast’.
Thus,
Borelius
would thus con-
fluct,uations
t#ppe appears
t,han the infinitesimal
appeared
In
require
fluc-
be stable.
processes,
by the
were reached.
produce
composition should
of each component
processes
composition
to
that alloys
only by diffusion phases
nucleation ation
in such
on
is a minimum
to develop,
sinusoidally
the
respect
composition
everywhere
formation
occurring
a
fluctuations
of these
would continue of
within with
was also assumed
wavelengths
which occur
with the peak
solutions
energy
com-
spinodal
Ni and 812°C.
be unstable
elastic
directions.
preferred
solid
would
to the
critical
coherent
at 71 at.%
composition
since
solution,
relative
in the
of the
that
spinodal
Borelius pointed out that within the chemical spinodal, tuations
spinodal
a shift
spinodal
infinitesimal
tions,
a non-uniform
Ni and O”C, in comparison
coherent
(100)
of the elastic
caused
coherent
and temperature
of the chemical
by
spinodal is defined as the locus of
composition
of the
t)o 40 at.%
planes,
The variation
were used.
spinodal
position
to
the
1968
with
chemical
whet,her
influenced
points where the second derivative respect
At
to decompose
imperfections.
process
The chemical
with
depression
have been made to determine
decomposition
gap.
at
precipitation
boundaries
attemptso2)
at elevated
field with an asym-
gap are observed
by a discontinuous grain
constants
3OO”C, approximately,
within the miscibilit,y at
composition
solid solubility
and a two-phase
16,
by a single
system is characterized
phase field with complete
VOL.
about
the
the fundamental
photographs.
reflections
peaks
From the position
in reciprocal
composition
as a function
developed has been
by observing the formation
space,
modulation
of composition,
provided information
and
the was the
on the ampli-
tude of the composition modulation. Measurements of the rate of development of the satellite reflections as a function of temperature, showed that vacancy
A more recent attempt to locate the boundary of the coherent spinodal was made by Golding and Moss.(s) This calculation was similar to the one
migration is the rat,e-controlling step for the development of the modulated structure in the low tem-
performed
perature
elastic
by Cahn, but experimental
constants
and the entropy
values
of the
as a function
of
region.
We have thus located the boundary of a region within
tT’C~ODILL.4,
JR.
.%li~ AVER138CH:
MODt-LATED
STR.lTCTITRES
IS
Au-Ni
25:
40 x binocular microscope, with a light source arranged so that a narrow beam was reflected through the electrolyte onto the bottom surface of the disise. A flash of light observed within the periphery of the disc indicated a perforation at the base of the cavity and the polishing was immediately terminated. A small hole about 25 microns in dia. was praduced, surrounded by thin area suitable for efectron transmission. 2. EXPERIMENTAL PROCEDURE Electron diffraction patterns were obtained using JEM-6AS and JEM-7 electron microscopes operating A series of gold-nickel alloys was cast contaiuing t>he following atomic percentages of nickel : 8.57, at 100 kV; the greater portion of data was obtained 22.24, 34.99, 46.08, 53.93, 64.73, 69.85 and 89.85. with the JEM-7 instrument. The liquid nitrogen cold finger electively eli~~inated specimen contaminaThe alloys were prepared fram 99.995 wt.% gold tion, and it was possible to obtain well oriented sponge and 99.9 wt.% electrolytic nickel. The alloys were induction melted in a re~~ystallized alumina dj~ra~tio~ patterns with the -&20” tilt, 36QO” crucible under a helium atmosphere and the melt was rotation stage. The majority of thin foilswere oriented bottom-poured int:o a copper mold, producing a chill with an [OOl] axis pa,rallel to the electron beam; the remainder were [01 I ] orientations. Electron cast rod 2 in. dia. and 4 in. long. Afi&eran homodi f&_&ion patt,erns were taken from aa ama limited genizing anneal for one ~;eek at 900°C within evacuated Vycor capsules, discs approximately 0.1 in. by a 70 micron intermediate aperture near the edge of thick were cut from the rods and rolled into strips the perforation. A Joyce-Loebl Mark III-C microdensitometer was 0.005 in. thick, with several intermediate anneals used to record the position and the intensity of the required during the processing. A large reduction on the final pass resulted in reery~alIjzation with a satellite and the fundament‘al reflections. The first satellites observable on the plates were very weak oube texture on subsequent solution anuealing. Disos 4 in. dia. were punched from the cold rolled and diffuse, and were recorded only as an asymmetry strip, sealed in evacuated Vycor capsules and solution to the fundamental reflection. Satellites were resolved with the microdensitometer when they reached an annealed at 900°C for two hours in a vertical tube intensity approximately 0.1 that of the satellite Rapid quenches, estimated to be in the furnace. in&n&y co~esponding to the most developed moduvicinity of 10,OOO°C per sec. i were achieved by ra.mming the holder of t.he Vycor capsule through the aluminum lated structure. In most instances, a low density optical wedge was used to increase the resolving foil cover at the botbom encl of the furnace capability of the instrument with respect to the first into a pan of cold mater. The Vycor capsule broke immediately upon impact with the bottom of t,he satellite reflectictns, and consequently the fundamental peaks were cut off at8a low intensity. Each diffraction pan. Aging treat#ments were initia,ted imnlediately after the solution anneal at temperatures between pat;tern was traced from the high angle side of (%O) 100°C and 350°C. A silicone oil bath was used below through (000) to the high angle side of (ZOO). The 250°C and a molten salt, bath at higher temperatures. separation of the low angle satellites from their In eaoh case the discs were supported in small wire fundamental reflections was used to calculate the baskets in direct contact wit,h the bath. characteristic wavelength of the modulation, using Thin foils for electron transmission were prepared by the Daniel a,nd Lipson expression in the form : a jet machining t,echnique. A 30’34 nitric acid stream (1) from a 1 mm orifice was used to el~~trol~tieally etch a depression near the center of one side of the disc; where /z is the modulation wavelength corresponding SO-SO% of the thickness of the disc was removed in to the peak of the satellite, d, is the spacing between this manner. The remaining material was electro(ZOO) planes in the alloy and the quantity, f = &, polished away. The electrolyte used in alloys with is the ratio of the separation of the satellite from the fundamental peak, p, to the separation between 54 at.% Ni and less consisted of 35 gm chromium (~00) reflections along the cube axes in reciprocal t&oxide, 210 ml glacial acetic acid and 10 ml distilled water; an electrolyte with 53 ml perohlorio acid and space. In practice, the separation of the (200)-(200) 177 ml glacial acetic acid was used in the high nickel fundamental peaks and the separation of the correalloys. The elec~,ro~olishin~ was observed through a sponding low angle sat*ellites were measured along
the miscibility gap wherein a coherent modulated structure forms. This structure is met&stable and has no evident relationship to the equilibrium precipitate which forms at higher temperatures. Kowever, the formation of the modulated structures is apparently entirely dependent on the presence of excess vacancies, and the thermodynamic basis for the formation of these coherent structures is thus not yet clear.
ACTA
258
a line through
the origin
in order
METALLURGICA,
to improve
the
VOL.
16,
of the alloy, and the streaks became longer and more intense.
accuracy. Electron diffraction
Fortunately,
these streaks did not cause any
difficulty in observations
RESULTS
3.
1968
patterns of solution treated and
rapidly
quenched
specimens
exhibited
amounts
of low angle diffuse scattering.
varying
This diffuse
the (100) directions. diffraction
However,
pattern
the fundamental
of the diffuse intensity along
intensity was greatest in the 35 and 46 at. o/0 Ni alloys,
densitometer
on the traces was thus subject
alloys.
This
low
angle
observed in the 9,65, interesting
diffuse
intensity
was not
70 or 90 at.O/, Ni alloys, and it is
to note
that
modulated
structures.
attributed
to the partial
these
alloys
do not form
The low angle scattering decomposition
solution as it passed through the modulated
structure
region during the quench, and it was concluded the solid solution above the miscibility In addition, served
however,
narrower
to lie in (110)
pattern.
directions
was
of the solid that
gap is random.
streaks
were ob-
on the diffraction
These streaks were more prominent
low angle side of each fundamental
on the
reflection,
where
traces.
The location
reason the fundamental The principal
centered on that portion of the phase diagram between Upon
the diffuse intensity
present
became
more
intense
originated
The formation tropolishing
of gold films was observed
when clear electrolytes
films were observed of the electrolyte surface despite with
an
solution. the matrix
to a gold surface layer
in the electropolishing
operation. during elec-
were used.
Gold
on all alloy thin films, regardless used, and these remained
attempts
ammonium
to remove them by etching persulphate-sodium
The disregistry increased
on the cyanide
between the gold layer and
with increasing
nickel
content
low temperature
and
reciprocal
space.
intensity,
satellite reflections
proceeded.
Within
aging,
in the quenched extended
alloy
farther
into
this broad band of diffuse developed
These satellite reflections
as the aging were found on
the (100) axes on both sides of each fundamental
spot
in the reciprocal lattice, regardless of the order of the reflection.
pure gold and was attributed
plates.
22 and 54 at. “/o Ni.
the origin and displayed
which
of the peak center
to error, and for this
area of interest in this investigation
they were found to emanate from a diffuse spot just as
to
peak separation was measured
directly on the diffraction
next to the fundamental.
This spot was indexed
an asymmetry
peak near the base on the micro-
with lesser amounts observed
in the 22 and 54 at. %Ni
this gold spot on the
did introduce
The satellites were also observed an asymmetry
around
in intensity,
being much more intense on the low angle side of each fundamental.
The
high
angle
only after very long aging times. intensity distribution
in alloys with different
Satellites were never observed
with compositions A diffraction
developed
No change in the
between the high and low angle
satellites was observed position.
satellites
outside pattern
com-
when alloys
of this range were aged. representing
the
situation
when the low angle satellites were first resolved by the microdensitometer
is shown in Fig. 1, and Fig. 2 shows
.
FIG. 1. (100) electron diffraction pattern of a 35 at. “/ Ni alloy aged at 240 min at 150°C.
FIG. 2. (100) electron diffraction pattern of a 35 at.% alloy aged 48 hr at 150°C.
Ni
WOODILLA,
MODCLATED
*JR. AND AVERBACH:
From
STRUCTURES
IN
259
Au-Ni
these data, an activation
energy for the rate
limiting
step
of the development
culated
to be 13,500 cal. per mol. for both
This is in close agreement Sivertsen and Wertol) energy alloy.
was calalloys.
with the determination
of
of 15,000 cal. per mol. for the
of migration
of vacancies
The transformation
quantitatively
process
in a 30 at.%
Ni
kinetics were not followed
in the 22 and 54 at. y0 Ni alloys except
to note that aging times in excess of 10,000 min were required
to develop
the modulated
produce observable
structure
At temperatures
above the C-curve
low angle satellites never developed resolved
by
diminished
the
observable
a diffraction
in intensity
structure
microdensitometer
resolve
and
However,
full to
diffraction
tracing,
was
of the thin foil in the
M&orientation
satellites
the
The ability
microdensitometer
to the orientation
microscope. others.
peaks.
A
diffraction
3, showing
either on the electron
or on the
sensitive
extent.
of the latter
in Fig.
of the fundamental a satellite,
pattern
some
tracing
is presented
intensity
to its maximum
enhanced the intensity of
diminished
the
intensity
of
by using foils which were as close
as possible to either (001) or (011) and which possessed satellites of equal intensity along the (100) directions on opposite obtain
sides of the origin,
reproducible
development
it was possible
measurements
of the
on the microdensitometer
observation
of t.he first resolvable
determined
as
a
function
to
satellite
traces.
The
satellibe was thus
of
aging
time
and
temperature. The time to produce
the first resolvable
confirmed
The
as the temperature at which
on the diffraction
to be
satellites increased,
they were no longer
pattern after aging times
10,000 min was used to define the boundary
of the modulated
pattern from a specimen aged to develop
the modulated pattern
exceeding
in Fig. 4, the
sufficiently
microdensitometer.
and the temperature FIG. 3. Microdensitometer tracing of electron diffraction pattern presented in Fig. 2, including full intensity of fundamental peaks.
and to
satellites.
structure.
This temperature
was
by aging at 150°C for 48 hr to develop
strong satellite reflections,
similar to Figs. 2 and 3,
and then reverting t,his structure by aging for one or two days at or above the temperature the first technique.
determined
by
If a specimen first aged at 15O’C
for 48 hr was then aged at a higher temperature
still
within the boundary
the
of the modulated
structure,
satellite reflect,ions were found to correspond same weak and barely observable characteristic
of direct
aging
Aging above the boundary produced
no satellites,
intensity
similar
to,
satellite reflections
a#t that
temperature.
of the modulated
only a broad but
more
to the
structure
band of diffuse
intense
than,
that
observed in the solution treated and rapidly quenched specimens.
This diffuse intensity gradually diminished
as the tempera’ture observable
was increased,
but- it was still
at 350°C where the grain boundary
tate had begun to develop.
Evidently,
still present in the solid solution,
precipi-
clustering was
but there was no
low angle
satellite tracing as a function
of temperature is shown in Fig. 4 for the 35 and 46 at. o/0 Ni alloys. The dashed curve represents the approximate
time for both alloys
when the low angle satellite first became on the electron diffraction optical
and
densitometer capability next.
mechanical
resolvable
patterns.
Use of the same
parameters
on the micro-
reproduced
the
of the instrument
maximum
resolving
from one plate to the
The linear portion of the C-curve suggests that
the rate of development in this temperature
of the modulated
range is governed
structure
by an equation
of the form : l/t(d)
=
A exp
(--Q/RF)
(2)
FIG. 4. Time to resolve the first low angle satellites as a function ot agmg temperature.
ACTA
2RO
evidence
of a satellite
intensity
to
modulation The
peak in this band
indicate
that
a periodic
phase
diagram
modulated
of the
structure
and nickel-rich
to the incoherent
1965
of diffuse
y/oNi
comparison,
and
the
in which
the
observed
compositions
precipitates
miscibility
The critical composition
Golding
16,
composit.ion
was experimentally
shown in Fig. 5. The equilibrium
at 45 at.
VOL.
existed.
region
gold-rich
METALLURGICA,
is
of the
corresponding
gap are also represented.
and temperature
220 & 5”C,
coherent
are placed
respectively.
spinodal
For
calculated
by
and Moss is also plobted in this figure, with
a critical composition
and temperature
L-
of 40 at#.gb Ni
0
~ I
2
and 0°C.
XNI
The wavelength calculated
of the composition
was
lattice and these are shown in Fig. 6.
These values were obtained on the first resolvable sitometer
modulation
FIG.
from calculations
based
peak traced by the microden-
as well as values obtained
after long aging
No growth of the wavelength,
a change in satellite position, given composition,
as detected
by
was ever observed for a
either as a function of temperature
It should structures
be mentioned
that transmission
were examined
foil.
micro-
during the course of this
investigation.
Striations
were observed
in thin foils oriented with their (loo),
of composition
modulation.
beam.
as in the
indicated
that
aged these
specimens
specimens.
Further
were Moire
images
as
analysis produced
by the gold surface layers and the matrix of the thin
on alloy
observed.
These
wavelength
of the composition
are considerably
longer
modulation patterns.
these
quite
Moire
orientation
patterns
were
to the
micrographs
modulated
4.1 Forrnution
of satellite
Decomposition characteristic modulated
observed
in the
were not
at all
structures
in the alloy.
process
rejlectiom
during the quench
feature
of alloy
structures
diffuse intensity has
The X-ray
at
been
appears to be a
systems
low
which
along \
, .
o
appeared
b
observed
in the
aluminum-zinc
: -60
complete
\
in
‘i7 \ \
I
\
0
\
i, .I
2
b 3
4
.5
quenched
of
8d
single crystal,
but
visible satellite formation.
aging was used in our study in order to
the decomposition
process.
The formation
6
7
the
composition
modulation
on
of a periodicity (100)
planes.
Aging of the 35 and 46 at.% Ni alloys at 150°C for 48 hr developed the satellites to the corresponding
\
P
LI!--
modulations
a wavelength
as direct evidence of t’he development
3\
:
-180
composition
with
of the satellite reflections along (100) axes was accepted
/
-120
directions
in a slowly
Isothermal
,d \
study showed that satellites,
to periodic
along these axes, without
coherent splnodol (Goldng and Moss) A
investigation
system by Moss and Averbach.(13)
rapid quenching produced only diffuse intensity streaks
llmlt of modulated structure observot~on modulated structure not observed
0’
(100)
form
temperatures.
The with the decomposition
associated
transmission
corresponding modulated structure
the
4. DISCUSSION
of the gold-nickel
of
to
microscope.
system,(12) and in an X-ray transmission
reglo”
the
determined
sensitive
that the striations
electron
were
than
Furthermore,
of the foil in the electron
transmission related
ranging from
composition,
from the electron diffraction
These striations, giving the appearance of a modulated were visible in the quenched
with wavelengths
depending
parallel to (100) directions
(110) and (211) axes parallel to the electron structure,
Moire! patterns
25 to 90 8,
It was concluded
or for different times at a given temperature.
well
Wavelength
G.
from the position of the satellite reflections
in the reciprocal
times.
.-___j 678910
345
.8
9
IO
XN,
FIG. 5. Region of phase diagram with modulat,ed structure and calculated coherent gpinodal.
maximum intensity values and these were unchanged with further aging. It was observed that the diffuse intensity
between
the
reflect(ions was reduced
fundamental
and
as the satellites
satellite
developed,
WOODILLA,
JR.
and it was concluded the
wavelength
that
were
converted
wavelength
to
the
all of the wavelengths
rather
composition
position
remained tures
and
differed
composition
only
with
modulation
modulated
dependence
infinity
(Fig.
was un-
in Fig.
5.
with the predicted the
wavelength
spinodal.
of the modulation
the temperature
6).
of the
with the temperature,(4’
requires
at the coherent
our measurements
alloy
shown
is not in agreement
calculation
(Fig.
wavelength
region
of the wavelength
the
approach
space
was raised to the limit of
structure
This behavior
composition
for a given
to the values
considered periods peaks,
indicating
However,
wavelength
predicted
intensities
in
for the
of the wavelength
The
apparent
of the modulated
respect to aging temperature
stability
structure
was typical
with
of all of the
alloys in this system.
Hargreaves scattering reflections
in the
modulation parameter
Satellites,
of the fundamental
case
of a sinusoidal
with a simultaneous
:
h *
The
ratios
which
for the
for the
and
b is the
these
modulation composition
tion,
to
fundamental
(000)
and
aged
to a far greater
factor,
the
and gold-
than the modulation
were
values of the composition
with
the
calculated
using
75 at.%
Compositions Ni
modulation
are
local
intensities
modulatlion predicted
phases.
to
average
the satellite
The relative
reflections
5 and
influences
associated
in composition.
of the
corresponding
between
extent
parameter
spinodal
(200)
48 hr at
of the alloy and the nickel-rich
satellite
and
with the experimental
in composition
lattice
coherent
the
from Fig. 3. The amplitude
rich regions in the modulation,
the
specimens
Ni alloy
of the scattering
difference
which
This assumption
for the
were compared
obtained
of the
for the
of approxi-
postulated,
i.e.,
of approximately
with 61 percent (200)
composition
intense than the observed
variation
of lattice
factor :
modulation ratios.
(3)
modulation from
of the scattering to
the
amplitude
factor modulation
composition of the
(4)
modulation
lattice
parameter
with the composition
h is the order of the reflection,
modula-
Q is the wave-
the
in Table
This
a
35 at. 7;
that
intensity
a sinusoidal
composition
amplitude
of 15 at. oh Ni
alloy
resulted
in
ratios which are in good agreement values.
more
value, and the composition
with a maximum
These
results
satellite with the
are summarized
1.
The apparent
value
is felt
correct,ions Inelastic
of the funda-
is considerably
was varied to fit the measured
homogeneous
amplitude
to the electron
experimental the
of the intensity
reflection.
It was found
intensity
associated
transformation
quenched
35 at.74
mental
J&h&b/a) It Jl(hQW)+ i J&h&b/a)
modulation
values of the satellite
satellite
sateWe
1 /Q :
corresponds
of the
reflect,ions
of the
the
affect our conclusions.
were calculated
the
than
and the calculations
intensities
intensities
of diffuse
fundamental
other
to complete
and satellite
JoV4?VQ
A is the amplitude
wavelengths
structure,
does not greatly
experimental
t
and
from the average, and this should result in a low angle
and scattering
Fundamental
that
was
aged for long
absence
satellite
valid for severely
composition
derived the following expressions
amplitudes
a near
the
corresponded
is probably
mately
4.2 Satellite intensities
to our data
follow are based on this assumption.
variation
were unchanged.
equations
value for a given alloy were not present.
to the modulated
even though
of the most characteristic
261
Au-Ni
the specimens
revealed
between
intensity
satellites
since
of time
initial stages of the decomposition.(i4) The intensities of the satellites changed with temperature, however, the positions
of these
valid
intensity
150°C;
to
range below the nose of the C-curve
4) correspond
The application
IN
It was assumed that maximum
in reciprocal
characteristic
STRUCTTTRES
characteristic
aging times.
satellites
changed as the temperature
since
short
characteristic
t’he same for all aging times and tempera-
This well-established
the
process
well-defined
modulation
of the
in
as the
of the decomposition
of the alloy from the earliest The
MODULATED
were disappearing
Eventually,
present at the beginning
AVERBACH:
the longer components
spectrum
aging proceeded.
AND
major
to
of the measured be too
large
experimental scattering
satellite correction
data
probably
peak
heights
arises
from
composition
because
several
are required. enhances somewhat, the
the but
non-linear
length of the modulation expressed in unit cells, and a is the lattice parameter of the alloy. J,, J,, and J, are Bessel functions of orders 0, 1 and 2. The asym-
relationship between the photographic density and the exposure for the very intense fundamental peaks.
metry in the satellite intensities
low because
and high angle equations.
satellites
These equations
of complete transformation
may
between the low angle be noted
from these
were derived for the case to the modulated structure.
The recorded
fundamental
peak
of the photographic
heights
are all tco
saturation
at high
intensities and the apparent satellite-to-fundamental intensity ratio is thus greater than the true ratio. An approximate
saturation
correction
was made, and
ACTA
262 Table 1. Satellite
intensity
ratios for a 35 at.% 150°C
Calculated 35 at.%, modulation
Satellite ratio
METALLURGICA, Ni alloy at
VOL. same
16,
for the simultaneous
and the corresponding amplitude,
Calculated 15 at.% modulation
1968
However,
I(OO0)
0.12*
0.30
the amplitude
factor modulation, term
amplitude,
is accompanied
with a small composition f(200)
0.12
0.11
enough
diffracted
distinct
satellite
observed 0.07
0.36
I(200)
fraction
* Approximate.
that the composition
less than 5 at. %. modulated
structure develops
composition
modulation
Limitations tension
We thus conclude
imposed
to accommodate
in these alloys, but the
amplitude is less than 5 at. %. probably
value of the composition ampitudes
is
that a periodic
by the coherency
requirements
position
amplitude
account
modulation.
approaching
the
prevalent
short
A more complete
wavelengths
amplitude,
wavelength
distribution
would only
t’ion were much greater.
com-
the regions
of
Although
a nucleation
barrier for the production
This is similar to the observa-
of the initial composition
fluctuations
tion by Hargreaves@)
of a limiting value of the misfit
not exist, the development
of the modulated
matrix in the copper-nickel-iron
structure with the
system.
In the latter
requires atomic migrations available
diffusion
case the term, &b/a, attained nearly the same limiting
The experimental
value during aging at different temperatures.
the act(ivation
case of the gold-nickel
a decrease in the composition length aging
of
the
at
modulated
higher
wavelengths
require
wavelengths
requires
as the wave-
increases
Thus, the amplitudes
less than the amplitudes The
in the
by
structure
composition
wavelengths.
observed
amplitude
temperatures.
are proportionally short
In the
system, this condition
the
presence
of
copper-nickel-iron
increase
in the
with longer which of the
the
longer
alloys
diffuse
were
intensity
modula-
4.3 Influence of vacancies
different composition.
for the coherence of the modulated
most
as the aging proceeds.
for the low stress
and we
of the
be observed in these alloys if the composition
Larger
of the
in the wavelength
population
wavelength
the
satellite
to a smaller
shorter
still have the greatest composition the relative
as a in
the boundary
may be attributed
Nevertheless, only
observed decrease
and surface
require a larger coherency the misfit between
The
of the short wavelength
observe
in
do not produce
be
of the short wavelength
structure
spectrum. it was concluded
to
reflection.
intensity
modulated
in this
sharp decrease
amplitude
intensity
with temperatures
0.06
and a decrease
by a very
of
For this reason, longer wavelengths
satellite intensity. 0.61
in other
of the scattering
A, is a very sensitive function
the composition 1.64
in the composition
in analogy with the observations
systems.
Experimental
growth of the wavelength
decrease
energy
in the solid solution.
of 13,500 cal. per mol. for
associated
ment of the modulated
does
structure
which are limited by the
mechanisms value
probably
with the develop-
structure
below the nose of
the C-curve in Fig. 4 agrees quite well with the value of 15,000 cal. per mol.
for the energy
migration
gold-nickel
composition
observed
in
In several of our experiments just
above
in
this
range.‘ll) the modulated
ture was formed at a low temperature, appearance
of vacancy
alloys
the
critical
temperature
of the modulated
structure
struc-
and then aged the
dis-
(Fig. 5).
for
The
between the fundamental and satellite reflection as aging proceeded. ‘r5) In the gold-nickel system,
treatment,
although diffuse streaks corresponding
to a
however,
clustering
of nickel-rich
in a
the satellites
fundamental peratures, satellites
reflection but
the
did not move upon
diffuse
aging intensity
and the fundamentals
closer to the
at higher between
increased
temthe
on aging.
However, the composition modulation in the goldnickel alloys is so low that a lengthening of the wavelength would wipe out the modulation. The gold-platinum systemda) appears to behave similarly to the copper-nickel-iron
system.
non-periodic lower
(h&b/a), remains the
disappeared
fashion
temperature,
with this reversion
and gold-rich
remained. however,
regions
On reaging the satellites
at the did not
reappear, even with aging times 1000 times longer than the time required to develop satellites when aging specimens
directly
after the quench.
The low angle
diffuse intensity observed for these reaged specimens was similar to that observed for specimens which had been
We may thus assume that for a given reflection the argument of the Bessel function,
sat’ellite reflections
quenched
temperature
and aged directly
without
the modulated
at the reversion
any intermediate
structure.
The absence
formation
of
of satellite
WOODILLA,
reflections
on reaging
JR.
AND
AVERBACH:
at the low temperature
reversion indicates that the modulated not form
again.
Thus,
the large
quenched-in
vacancies
temperature
is apparently
at which the
from
vacancies
concentration
the solution
structure
for the rate
develops.
a substantial
number
allows
this structure
to develop,
structure
would
of
annealing
of
modulated
after
structure does
responsible
the modulated
presence
MODULATED
Only
of
excess and
a
not have been observed
without them. 5. SUMMARY
formation
in gold-nickel
as follows.
structure
alloys may be summarized
Periodic modulated
a metastable
miscibility
structures form within
gap with a peak at about
45 at. ‘$4 Ni and 220°C ; the modulations observed for alloys with compositions 60 at.%
Ni.
composition
The
predominant
modulation
varied between 6.5 and 13 A,
perature or the time of aging. composition
of 5 at.%
of the
for each alloy.
The did not seem to vary with either the tem-
maximum
above
The amplitude
modulation
of the
was of the order
and below the average composition
of the alloy and did not change during aging. The modulated severely was
structures formed on aging only in
quenched
apparently
alloys and the rate of formation controlled
with an activation below
limited
vacancy
by
annihilated
by
of about
temperatures temperature
vacancy
migration,
13,500 cal. per mol.
At
150°C the rate of formation migration,
and
by the rate at which by annealing;
thus observed
These
entire process
of modulated
thus critically
dependent
was
above
this
vacancies
were
a C-curve
for the kinetics
behavior
of the process. structure
formation
on the presence
was The is
of excess
vacancies,
It appears that the parent solid solution
is probably
random
at the solution temperature
that clustering in quenched
modulated
connection
alloys is observed
and
only in
263
Au-Ni
through
the region
structure formation. structures
have
with the equilibrium
no
apparent
precipitates
which
form on aging at higher temperatures.
They are, in a
sense,
a rather
coherent
superstructures
established
periodicity
composition
modulation
lattice.
and
with a
well
well-defined
extending through the parent and
non-reversible,
dependent
The resultant properties
phenomenon
on
superimposed
may greatly affect the mechanical magnetic
fairly
It appears that these structures are metastable vacancy
periodicity
properties
and the
in certain alloy systems and the
deserves
cussion of properties
are only
between 20 and
wavelength
and appeared to be characteristic wavelength
of modulated
migration.
features of the modulated
IN
those alloys which are quenched
and
The principal
STRUCTURES
careful
consideration
in dis-
in systems of this hype.
ACKNOWLEDGMENTS
The authors are very grateful to the Atomic Energy Commission
for the support
also like to acknowledge cussions with Professors
of this work.
the many
We would
interesting
dis-
J. W. Cahn, S. C. Moss and
J. F. Breedis. REFERENCES 1. G. BORELIUS, J. Metals 3, 477 (1951). 2. E. E. UNDERWOOD, Precipitation in gold-nickel alloys. SC. D. Thesis, Mass. Institute of Technology (1954). 3. J. W. CAHN, Acta Met. 10, 907 (1962). Acta Met. 9, 795 (1961). 4. J. W. CAHN, B. GOLDINQ and S. C. Moss, Acta Met., 15, 1239 (1967). :: J. W. CAHN, Acta Met. 10, 179 (1962). 7. V. DANIEL and H. LIPSON, Proc. phys. Sot. A181, 368 (1943). M. E. HABOREAVES, Acta Cry8taZZogr.4, 301 (1951). :: Y. FUKANO, J. phys. Sot. Japan 16,1195 (1961). G. F. DAY, J. Inst. Metals 89, 296 (1961). ::: J. SIVERTSEN and C. WERT, Acta Met. 7, 275 (1959). 12. K. B. RUNDMAN and J. E. HILLIARD, Dept. of Materials Science, Northwestern University, Evanston, Illinois (1966). 13. S. C. Moss and B. L. AVERBACH, Local atomic configurations in a gold-nickel alloy. Proc. Conf. Small Angle X-Ray Scattering, Syracuse, N.Y. (1965), edited by H. BRUMBERQER. Gordon & Breach. 14. J. W. CAHN, private communication. 15. M. HILLERT, M. COHEN and B. L. AVERBACH, Acta Met. 9, 536 (1961). 16. T. J. TIEDEMA, J. BOUMAN and W. G. BURGERS, Acta Met. 5, 310 (1957).
J.
E.
STRUCTURES
WOODILLA
JR.t$
IN
and
B.
L.
Au-Ni
ALLOYS*
AVERBACEEt
A modulated structure has been shown to develop on {IOO} planes in certain gold-nickel alloys on aging at temperatures in the vicinity of lOO-200°C. The critical composition is at 45 at.% Ni and the maximum temperature at which these structures ocours is 220°C. The boundary of the modulated structure region is nearly symmetric, with extreme compositions at approximately 20 and 60 at. O/eNi. This structure has been observed by means of electron diffraction and is characterized by the formation of satellite reflections on (100) axes around each fundamental peak. The modulated structures exhibited a composition amplitude of approxixnately 5 at. “/band characteristic wavelengths which varied from 6.5 A at 54 at-“/b Ni to 13 A at 22 at.% Ni. An activation energy of 13,500 eel. per mol. was calculated for the rate limiting step in the low temperature region, corresponding to the energy of migration for vacancies in this composition range. The satellite reflections disappeared on aging just above the critical temperature; on reaging at a lower temperature, the satellites did not reappear. This is taken to indicate that the presence of a large concentration of quenched-in excess vacancies is required for the development of the modulated structure. The formation of the modulated structure may be associated with the postulated spinodal decomposition in this system, although the theories of spinodal deeompositioi~ do not yet adequately account for the role of vacancies.
STRUCTURES
MODULEES
DANS
LES
ALLIAGES
Au-h?
Une structure modulee se developpe sur les plans {IOO] de eertains alliages or-nickel soumis a un vieillissement dans un domaine de temperatures voisin de lOO-200°C. La composition critique se situe a 4.5 at.“/b de Ni et la temperature maximum d~apparition de ces structures est 220°C. La timite de la region a structure mod&&e est presque symetrique, avec des compositions extremes 8.environ 20 et 60 at. y0 de Ni. On a observe cette structure par diffraction Blectronique; elle est oara&risee par la formation de reflexions auxiliaires sur les axes (100) autour de chaque pit fondamental. Les structures modulees montrent une amplitude de composition d’environ 5 at. o/’et des longueurs d’onde caracteristiquas variant de 6,5 A pour 54 at.% de Ni a 13 A pour 22 at.% de Ni. On calcule pour lo stade limitant la vitesse une energie d’activation de 13 500 Cal. par mole, correspondant 8, l’energie de migration des laeunes dans ee domaine de compositions. Les &flexions auxili&es disparais~nt quand on fait un vieillissement juste au-dassus de la temp6rature critique; en faisant un nouveau vieillissement a temp&atura inferieure on no les fait pas tiapparaitre. Ceci indique que la croissance de la structure modul&e requiart la presence d’une grande concentration de lacunes trempees en exces. On neut associer la formation de la structure modulee b la decomposition spinodale supposee de oe systeme,- bien que les theories de la decomposition spinodale ne rend&t cependant pas oor%tament compte du role des lacunes.
MODULIERTE
STRUKTUREN
IN
Au-Ni-LEGIERUNGEN
Eine modulierte Struktur zeigt sich auf {lOOf-Ebenen in einigen Gold-Nickel-Legierungen nach Anlassen bei lOO-200%. Die kritischa Zusammensetzung ist bei 45 At.-% Ni. Die Hijchsttemperatur fiir das Entstehen dioser Strukturen betragt 22O’C. Die Grenze der modulierten Struktur ist nahezu symmetrisch bei extromen Zusammensetzungen von 20 und 60 At.-% Ni. Diese Struktur wurde mittels Elektronenbeugung beobachtet. Sie ist charakterisiert durch die Bildung von Sattelitreflaxen auf (lOO)-Achsen bei jedem Fundamentalmaximum. Die modulierten Strukturen zeigten eine Zusammensetz~gsamplitude von etwa 5 At.-% und charakteristische WelIenlangen, die von 6,5 A bei 64 At.-% Ni bis 13 A bei 22 At.-% Ni variieren. Eine Aktivierungsenergie van 13 500 cal/MoI wurde fur den geschwindigkeitsbestimmenden Sohritt im Tieftemperaturbersich berechnet; das entspricht der Wanderungsenergie von Leerstellen in diesem Zusammensetzungsbereich. Die Sattolitenreflexe verschwanden beim Altern gerade oberhalb der kritischen Temperatur. Beim weiteren Anlassen wieder bei tieferen Temperaturen traten die Satteliten nioht wieder auf. Zur Bildung der modulierten Strukturen scheint also eine hohe Konzentration iiberschtisiger Leerxtellen erforderlich zu sein. Diese Bildung mag mit der vorgeschlagenen spinodalen ~ntm~schung in diesem System zusamme~~ngen, obwohl die Theorien der spinodalen Entmischung die Leerstellen nicht in angemessener Weise beriicksichtigen.
* Received May 6, 1967. t Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, 2 Now at: The Torrington Co., Torrington, Connecticut. ACTA
METALLURGI~A,
VOL.
16, FEBRUARY
1968
255
Cambridge, Massachusetts.
ACTA
‘756
METALLURGICA,
1. INTRODUCTION
The gold-nickel temperatures metric
miscibility
temperatures
gap
above
this
chemical
lower
and
temperatures.
spinodal
process
is
alloys
originating Several
associated
with
the
miscibility
t’o
of the free energy
composition,
a2G/aX2,
is
zero.
with
Cahn@)
concluded
the
It
aaG/aX2 < 0, and postulated
in a state of metsstable
The segregation tinue,
limited
equilibrium cibility
gap
defined
excursions.
of the latter
decomposition
spinodal
wit,h experimental
t,he spinodal gold-nickel effects
alloys,
which
was
scattering
the
to determine
200°C
within
the
two-phase
nucleated
heterogeneously
modulation reflections
fundamental predicted
of
of temperature
an asymmetry by the
satellite
In this
investigation,
and More
temperature experimentally of satellite
fluctuations
of the
remain coherent
tension of
the resultant
effects,
such
with the matrix. elastic
it was predicted
a coherent
spinodal
Taking
energy and surface that
would
aging.
about
all of the
the
model, than
origin.
In
intensity
with
the
t,he high
is low
angle
a modulated
reflections
satellite of
the boundary
determined
be
satellite intensities
depressed
well below (approximately 2000°C in the gold-nickel system) the chemical spinodal defined by Borelius.
structures
determined
on electron diffraction wavelength
structure
gold-nickel
was
alloys on low
The region of the phase diagram
in which the modulated
recently, Cahn(3s4) has considered that the spinodal should be defined only for cases where the composition into account
intense
size
factor,
should lead to the
in the satellit’e
Hargreavcs
more
in atomic
including
shown to develop in certain
bhe
form,
reflections,
the
alloys,
satellite.
the
Furthermore
angle
and
in scattering
of satellite
to show any
at grain boundaries.
difference
composition
addition,
in a pearlitic
is both a large difference
an appreciable
of
top
parameter
formation
with the position
region.
was lamellar,
and
lattice
In the case of gold-nickel
a periodic
the role of
the
where there
and formulated
in the case of a simultaneous
the
and
rate of precipitation
below
factor.
dif-
expanded
of the fundamental
alloys.
decomposition
miscibilit)y gap and nearly independent precipitate
of the
rates
in Au-Pt
but he was unable
approximately
spinodal.
boundary
equilibrium
needed
around
and electron
Hargreaves@)
for the amplitude
fluct,uations
nucle-
to the
structure
reflections
of Daniel and Lipson”)
the expressions
the solid
leading
modulated
in X-ray
experiments.(7-s)
the treatment
certain fluctua-
throughout
of satellite peak,
reflections
the
The maximum
fraction
that
in
of this type may be observed
formation
of both
could be associated
of the spinodal.
usual
the
each fundamental
modulation
precipitation
attempted in the
the
from
(100)
equilibrium.
structures
and satellite
to find some correlat’ion
Underwood(2)
mis-
Modulated
to be less probable
within
to relate
the
with finite
homogeneous
composition
attempted
until
incoherent,
contrast’.
Thus,
Borelius
would thus con-
fluct,uations
t#ppe appears
t,han the infinitesimal
appeared
In
require
fluc-
be stable.
processes,
by the
were reached.
produce
composition should
of each component
processes
composition
to
that alloys
only by diffusion phases
nucleation ation
in such
on
is a minimum
to develop,
sinusoidally
the
respect
composition
everywhere
formation
occurring
a
fluctuations
of these
would continue of
within with
was also assumed
wavelengths
which occur
with the peak
solutions
energy
com-
spinodal
Ni and 812°C.
be unstable
elastic
directions.
preferred
solid
would
to the
critical
coherent
at 71 at.%
composition
since
solution,
relative
in the
of the
that
spinodal
Borelius pointed out that within the chemical spinodal, tuations
spinodal
a shift
spinodal
infinitesimal
tions,
a non-uniform
Ni and O”C, in comparison
coherent
(100)
of the elastic
caused
coherent
and temperature
of the chemical
by
spinodal is defined as the locus of
composition
of the
t)o 40 at.%
planes,
The variation
were used.
spinodal
position
to
the
1968
with
chemical
whet,her
influenced
points where the second derivative respect
At
to decompose
imperfections.
process
The chemical
with
depression
have been made to determine
decomposition
gap.
at
precipitation
boundaries
attemptso2)
at elevated
field with an asym-
gap are observed
by a discontinuous grain
constants
3OO”C, approximately,
within the miscibilit,y at
composition
solid solubility
and a two-phase
16,
by a single
system is characterized
phase field with complete
VOL.
about
the
the fundamental
photographs.
reflections
peaks
From the position
in reciprocal
composition
as a function
developed has been
by observing the formation
space,
modulation
of composition,
provided information
and
the was the
on the ampli-
tude of the composition modulation. Measurements of the rate of development of the satellite reflections as a function of temperature, showed that vacancy
A more recent attempt to locate the boundary of the coherent spinodal was made by Golding and Moss.(s) This calculation was similar to the one
migration is the rat,e-controlling step for the development of the modulated structure in the low tem-
performed
perature
elastic
by Cahn, but experimental
constants
and the entropy
values
of the
as a function
of
region.
We have thus located the boundary of a region within
tT’C~ODILL.4,
JR.
.%li~ AVER138CH:
MODt-LATED
STR.lTCTITRES
IS
Au-Ni
25:
40 x binocular microscope, with a light source arranged so that a narrow beam was reflected through the electrolyte onto the bottom surface of the disise. A flash of light observed within the periphery of the disc indicated a perforation at the base of the cavity and the polishing was immediately terminated. A small hole about 25 microns in dia. was praduced, surrounded by thin area suitable for efectron transmission. 2. EXPERIMENTAL PROCEDURE Electron diffraction patterns were obtained using JEM-6AS and JEM-7 electron microscopes operating A series of gold-nickel alloys was cast contaiuing t>he following atomic percentages of nickel : 8.57, at 100 kV; the greater portion of data was obtained 22.24, 34.99, 46.08, 53.93, 64.73, 69.85 and 89.85. with the JEM-7 instrument. The liquid nitrogen cold finger electively eli~~inated specimen contaminaThe alloys were prepared fram 99.995 wt.% gold tion, and it was possible to obtain well oriented sponge and 99.9 wt.% electrolytic nickel. The alloys were induction melted in a re~~ystallized alumina dj~ra~tio~ patterns with the -&20” tilt, 36QO” crucible under a helium atmosphere and the melt was rotation stage. The majority of thin foilswere oriented bottom-poured int:o a copper mold, producing a chill with an [OOl] axis pa,rallel to the electron beam; the remainder were [01 I ] orientations. Electron cast rod 2 in. dia. and 4 in. long. Afi&eran homodi f&_&ion patt,erns were taken from aa ama limited genizing anneal for one ~;eek at 900°C within evacuated Vycor capsules, discs approximately 0.1 in. by a 70 micron intermediate aperture near the edge of thick were cut from the rods and rolled into strips the perforation. A Joyce-Loebl Mark III-C microdensitometer was 0.005 in. thick, with several intermediate anneals used to record the position and the intensity of the required during the processing. A large reduction on the final pass resulted in reery~alIjzation with a satellite and the fundament‘al reflections. The first satellites observable on the plates were very weak oube texture on subsequent solution anuealing. Disos 4 in. dia. were punched from the cold rolled and diffuse, and were recorded only as an asymmetry strip, sealed in evacuated Vycor capsules and solution to the fundamental reflection. Satellites were resolved with the microdensitometer when they reached an annealed at 900°C for two hours in a vertical tube intensity approximately 0.1 that of the satellite Rapid quenches, estimated to be in the furnace. in&n&y co~esponding to the most developed moduvicinity of 10,OOO°C per sec. i were achieved by ra.mming the holder of t.he Vycor capsule through the aluminum lated structure. In most instances, a low density optical wedge was used to increase the resolving foil cover at the botbom encl of the furnace capability of the instrument with respect to the first into a pan of cold mater. The Vycor capsule broke immediately upon impact with the bottom of t,he satellite reflectictns, and consequently the fundamental peaks were cut off at8a low intensity. Each diffraction pan. Aging treat#ments were initia,ted imnlediately after the solution anneal at temperatures between pat;tern was traced from the high angle side of (%O) 100°C and 350°C. A silicone oil bath was used below through (000) to the high angle side of (ZOO). The 250°C and a molten salt, bath at higher temperatures. separation of the low angle satellites from their In eaoh case the discs were supported in small wire fundamental reflections was used to calculate the baskets in direct contact wit,h the bath. characteristic wavelength of the modulation, using Thin foils for electron transmission were prepared by the Daniel a,nd Lipson expression in the form : a jet machining t,echnique. A 30’34 nitric acid stream (1) from a 1 mm orifice was used to el~~trol~tieally etch a depression near the center of one side of the disc; where /z is the modulation wavelength corresponding SO-SO% of the thickness of the disc was removed in to the peak of the satellite, d, is the spacing between this manner. The remaining material was electro(ZOO) planes in the alloy and the quantity, f = &, polished away. The electrolyte used in alloys with is the ratio of the separation of the satellite from the fundamental peak, p, to the separation between 54 at.% Ni and less consisted of 35 gm chromium (~00) reflections along the cube axes in reciprocal t&oxide, 210 ml glacial acetic acid and 10 ml distilled water; an electrolyte with 53 ml perohlorio acid and space. In practice, the separation of the (200)-(200) 177 ml glacial acetic acid was used in the high nickel fundamental peaks and the separation of the correalloys. The elec~,ro~olishin~ was observed through a sponding low angle sat*ellites were measured along
the miscibility gap wherein a coherent modulated structure forms. This structure is met&stable and has no evident relationship to the equilibrium precipitate which forms at higher temperatures. Kowever, the formation of the modulated structures is apparently entirely dependent on the presence of excess vacancies, and the thermodynamic basis for the formation of these coherent structures is thus not yet clear.
ACTA
258
a line through
the origin
in order
METALLURGICA,
to improve
the
VOL.
16,
of the alloy, and the streaks became longer and more intense.
accuracy. Electron diffraction
Fortunately,
these streaks did not cause any
difficulty in observations
RESULTS
3.
1968
patterns of solution treated and
rapidly
quenched
specimens
exhibited
amounts
of low angle diffuse scattering.
varying
This diffuse
the (100) directions. diffraction
However,
pattern
the fundamental
of the diffuse intensity along
intensity was greatest in the 35 and 46 at. o/0 Ni alloys,
densitometer
on the traces was thus subject
alloys.
This
low
angle
observed in the 9,65, interesting
diffuse
intensity
was not
70 or 90 at.O/, Ni alloys, and it is
to note
that
modulated
structures.
attributed
to the partial
these
alloys
do not form
The low angle scattering decomposition
solution as it passed through the modulated
structure
region during the quench, and it was concluded the solid solution above the miscibility In addition, served
however,
narrower
to lie in (110)
pattern.
directions
was
of the solid that
gap is random.
streaks
were ob-
on the diffraction
These streaks were more prominent
low angle side of each fundamental
on the
reflection,
where
traces.
The location
reason the fundamental The principal
centered on that portion of the phase diagram between Upon
the diffuse intensity
present
became
more
intense
originated
The formation tropolishing
of gold films was observed
when clear electrolytes
films were observed of the electrolyte surface despite with
an
solution. the matrix
to a gold surface layer
in the electropolishing
operation. during elec-
were used.
Gold
on all alloy thin films, regardless used, and these remained
attempts
ammonium
to remove them by etching persulphate-sodium
The disregistry increased
on the cyanide
between the gold layer and
with increasing
nickel
content
low temperature
and
reciprocal
space.
intensity,
satellite reflections
proceeded.
Within
aging,
in the quenched extended
alloy
farther
into
this broad band of diffuse developed
These satellite reflections
as the aging were found on
the (100) axes on both sides of each fundamental
spot
in the reciprocal lattice, regardless of the order of the reflection.
pure gold and was attributed
plates.
22 and 54 at. “/o Ni.
the origin and displayed
which
of the peak center
to error, and for this
area of interest in this investigation
they were found to emanate from a diffuse spot just as
to
peak separation was measured
directly on the diffraction
next to the fundamental.
This spot was indexed
an asymmetry
peak near the base on the micro-
with lesser amounts observed
in the 22 and 54 at. %Ni
this gold spot on the
did introduce
The satellites were also observed an asymmetry
around
in intensity,
being much more intense on the low angle side of each fundamental.
The
high
angle
only after very long aging times. intensity distribution
in alloys with different
Satellites were never observed
with compositions A diffraction
developed
No change in the
between the high and low angle
satellites was observed position.
satellites
outside pattern
com-
when alloys
of this range were aged. representing
the
situation
when the low angle satellites were first resolved by the microdensitometer
is shown in Fig. 1, and Fig. 2 shows
.
FIG. 1. (100) electron diffraction pattern of a 35 at. “/ Ni alloy aged at 240 min at 150°C.
FIG. 2. (100) electron diffraction pattern of a 35 at.% alloy aged 48 hr at 150°C.
Ni
WOODILLA,
MODCLATED
*JR. AND AVERBACH:
From
STRUCTURES
IN
259
Au-Ni
these data, an activation
energy for the rate
limiting
step
of the development
culated
to be 13,500 cal. per mol. for both
This is in close agreement Sivertsen and Wertol) energy alloy.
was calalloys.
with the determination
of
of 15,000 cal. per mol. for the
of migration
of vacancies
The transformation
quantitatively
process
in a 30 at.%
Ni
kinetics were not followed
in the 22 and 54 at. y0 Ni alloys except
to note that aging times in excess of 10,000 min were required
to develop
the modulated
produce observable
structure
At temperatures
above the C-curve
low angle satellites never developed resolved
by
diminished
the
observable
a diffraction
in intensity
structure
microdensitometer
resolve
and
However,
full to
diffraction
tracing,
was
of the thin foil in the
M&orientation
satellites
the
The ability
microdensitometer
to the orientation
microscope. others.
peaks.
A
diffraction
3, showing
either on the electron
or on the
sensitive
extent.
of the latter
in Fig.
of the fundamental a satellite,
pattern
some
tracing
is presented
intensity
to its maximum
enhanced the intensity of
diminished
the
intensity
of
by using foils which were as close
as possible to either (001) or (011) and which possessed satellites of equal intensity along the (100) directions on opposite obtain
sides of the origin,
reproducible
development
it was possible
measurements
of the
on the microdensitometer
observation
of t.he first resolvable
determined
as
a
function
to
satellite
traces.
The
satellibe was thus
of
aging
time
and
temperature. The time to produce
the first resolvable
confirmed
The
as the temperature at which
on the diffraction
to be
satellites increased,
they were no longer
pattern after aging times
10,000 min was used to define the boundary
of the modulated
pattern from a specimen aged to develop
the modulated pattern
exceeding
in Fig. 4, the
sufficiently
microdensitometer.
and the temperature FIG. 3. Microdensitometer tracing of electron diffraction pattern presented in Fig. 2, including full intensity of fundamental peaks.
and to
satellites.
structure.
This temperature
was
by aging at 150°C for 48 hr to develop
strong satellite reflections,
similar to Figs. 2 and 3,
and then reverting t,his structure by aging for one or two days at or above the temperature the first technique.
determined
by
If a specimen first aged at 15O’C
for 48 hr was then aged at a higher temperature
still
within the boundary
the
of the modulated
structure,
satellite reflect,ions were found to correspond same weak and barely observable characteristic
of direct
aging
Aging above the boundary produced
no satellites,
intensity
similar
to,
satellite reflections
a#t that
temperature.
of the modulated
only a broad but
more
to the
structure
band of diffuse
intense
than,
that
observed in the solution treated and rapidly quenched specimens.
This diffuse intensity gradually diminished
as the tempera’ture observable
was increased,
but- it was still
at 350°C where the grain boundary
tate had begun to develop.
Evidently,
still present in the solid solution,
precipi-
clustering was
but there was no
low angle
satellite tracing as a function
of temperature is shown in Fig. 4 for the 35 and 46 at. o/0 Ni alloys. The dashed curve represents the approximate
time for both alloys
when the low angle satellite first became on the electron diffraction optical
and
densitometer capability next.
mechanical
resolvable
patterns.
Use of the same
parameters
on the micro-
reproduced
the
of the instrument
maximum
resolving
from one plate to the
The linear portion of the C-curve suggests that
the rate of development in this temperature
of the modulated
range is governed
structure
by an equation
of the form : l/t(d)
=
A exp
(--Q/RF)
(2)
FIG. 4. Time to resolve the first low angle satellites as a function ot agmg temperature.
ACTA
2RO
evidence
of a satellite
intensity
to
modulation The
peak in this band
indicate
that
a periodic
phase
diagram
modulated
of the
structure
and nickel-rich
to the incoherent
1965
of diffuse
y/oNi
comparison,
and
the
in which
the
observed
compositions
precipitates
miscibility
The critical composition
Golding
16,
composit.ion
was experimentally
shown in Fig. 5. The equilibrium
at 45 at.
VOL.
existed.
region
gold-rich
METALLURGICA,
is
of the
corresponding
gap are also represented.
and temperature
220 & 5”C,
coherent
are placed
respectively.
spinodal
For
calculated
by
and Moss is also plobted in this figure, with
a critical composition
and temperature
L-
of 40 at#.gb Ni
0
~ I
2
and 0°C.
XNI
The wavelength calculated
of the composition
was
lattice and these are shown in Fig. 6.
These values were obtained on the first resolvable sitometer
modulation
FIG.
from calculations
based
peak traced by the microden-
as well as values obtained
after long aging
No growth of the wavelength,
a change in satellite position, given composition,
as detected
by
was ever observed for a
either as a function of temperature
It should structures
be mentioned
that transmission
were examined
foil.
micro-
during the course of this
investigation.
Striations
were observed
in thin foils oriented with their (loo),
of composition
modulation.
beam.
as in the
indicated
that
aged these
specimens
specimens.
Further
were Moire
images
as
analysis produced
by the gold surface layers and the matrix of the thin
on alloy
observed.
These
wavelength
of the composition
are considerably
longer
modulation patterns.
these
quite
Moire
orientation
patterns
were
to the
micrographs
modulated
4.1 Forrnution
of satellite
Decomposition characteristic modulated
observed
in the
were not
at all
structures
in the alloy.
process
rejlectiom
during the quench
feature
of alloy
structures
diffuse intensity has
The X-ray
at
been
appears to be a
systems
low
which
along \
, .
o
appeared
b
observed
in the
aluminum-zinc
: -60
complete
\
in
‘i7 \ \
I
\
0
\
i, .I
2
b 3
4
.5
quenched
of
8d
single crystal,
but
visible satellite formation.
aging was used in our study in order to
the decomposition
process.
The formation
6
7
the
composition
modulation
on
of a periodicity (100)
planes.
Aging of the 35 and 46 at.% Ni alloys at 150°C for 48 hr developed the satellites to the corresponding
\
P
LI!--
modulations
a wavelength
as direct evidence of t’he development
3\
:
-180
composition
with
of the satellite reflections along (100) axes was accepted
/
-120
directions
in a slowly
Isothermal
,d \
study showed that satellites,
to periodic
along these axes, without
coherent splnodol (Goldng and Moss) A
investigation
system by Moss and Averbach.(13)
rapid quenching produced only diffuse intensity streaks
llmlt of modulated structure observot~on modulated structure not observed
0’
(100)
form
temperatures.
The with the decomposition
associated
transmission
corresponding modulated structure
the
4. DISCUSSION
of the gold-nickel
of
to
microscope.
system,(12) and in an X-ray transmission
reglo”
the
determined
sensitive
that the striations
electron
were
than
Furthermore,
of the foil in the electron
transmission related
ranging from
composition,
from the electron diffraction
These striations, giving the appearance of a modulated were visible in the quenched
with wavelengths
depending
parallel to (100) directions
(110) and (211) axes parallel to the electron structure,
Moire! patterns
25 to 90 8,
It was concluded
or for different times at a given temperature.
well
Wavelength
G.
from the position of the satellite reflections
in the reciprocal
times.
.-___j 678910
345
.8
9
IO
XN,
FIG. 5. Region of phase diagram with modulat,ed structure and calculated coherent gpinodal.
maximum intensity values and these were unchanged with further aging. It was observed that the diffuse intensity
between
the
reflect(ions was reduced
fundamental
and
as the satellites
satellite
developed,
WOODILLA,
JR.
and it was concluded the
wavelength
that
were
converted
wavelength
to
the
all of the wavelengths
rather
composition
position
remained tures
and
differed
composition
only
with
modulation
modulated
dependence
infinity
(Fig.
was un-
in Fig.
5.
with the predicted the
wavelength
spinodal.
of the modulation
the temperature
6).
of the
with the temperature,(4’
requires
at the coherent
our measurements
alloy
shown
is not in agreement
calculation
(Fig.
wavelength
region
of the wavelength
the
approach
space
was raised to the limit of
structure
This behavior
composition
for a given
to the values
considered periods peaks,
indicating
However,
wavelength
predicted
intensities
in
for the
of the wavelength
The
apparent
of the modulated
respect to aging temperature
stability
structure
was typical
with
of all of the
alloys in this system.
Hargreaves scattering reflections
in the
modulation parameter
Satellites,
of the fundamental
case
of a sinusoidal
with a simultaneous
:
h *
The
ratios
which
for the
for the
and
b is the
these
modulation composition
tion,
to
fundamental
(000)
and
aged
to a far greater
factor,
the
and gold-
than the modulation
were
values of the composition
with
the
calculated
using
75 at.%
Compositions Ni
modulation
are
local
intensities
modulatlion predicted
phases.
to
average
the satellite
The relative
reflections
5 and
influences
associated
in composition.
of the
corresponding
between
extent
parameter
spinodal
(200)
48 hr at
of the alloy and the nickel-rich
satellite
and
with the experimental
in composition
lattice
coherent
the
from Fig. 3. The amplitude
rich regions in the modulation,
the
specimens
Ni alloy
of the scattering
difference
which
This assumption
for the
were compared
obtained
of the
for the
of approxi-
postulated,
i.e.,
of approximately
with 61 percent (200)
composition
intense than the observed
variation
of lattice
factor :
modulation ratios.
(3)
modulation from
of the scattering to
the
amplitude
factor modulation
composition of the
(4)
modulation
lattice
parameter
with the composition
h is the order of the reflection,
modula-
Q is the wave-
the
in Table
This
a
35 at. 7;
that
intensity
a sinusoidal
composition
amplitude
of 15 at. oh Ni
alloy
resulted
in
ratios which are in good agreement values.
more
value, and the composition
with a maximum
These
results
satellite with the
are summarized
1.
The apparent
value
is felt
correct,ions Inelastic
of the funda-
is considerably
was varied to fit the measured
homogeneous
amplitude
to the electron
experimental the
of the intensity
reflection.
It was found
intensity
associated
transformation
quenched
35 at.74
mental
J&h&b/a) It Jl(hQW)+ i J&h&b/a)
modulation
values of the satellite
satellite
sateWe
1 /Q :
corresponds
of the
reflect,ions
of the
the
affect our conclusions.
were calculated
the
than
and the calculations
intensities
intensities
of diffuse
fundamental
other
to complete
and satellite
JoV4?VQ
A is the amplitude
wavelengths
structure,
does not greatly
experimental
t
and
from the average, and this should result in a low angle
and scattering
Fundamental
that
was
aged for long
absence
satellite
valid for severely
composition
derived the following expressions
amplitudes
a near
the
corresponded
is probably
mately
4.2 Satellite intensities
to our data
follow are based on this assumption.
variation
were unchanged.
equations
value for a given alloy were not present.
to the modulated
even though
of the most characteristic
261
Au-Ni
the specimens
revealed
between
intensity
satellites
since
of time
initial stages of the decomposition.(i4) The intensities of the satellites changed with temperature, however, the positions
of these
valid
intensity
150°C;
to
range below the nose of the C-curve
4) correspond
The application
IN
It was assumed that maximum
in reciprocal
characteristic
STRUCTTTRES
characteristic
aging times.
satellites
changed as the temperature
since
short
characteristic
t’he same for all aging times and tempera-
This well-established
the
process
well-defined
modulation
of the
in
as the
of the decomposition
of the alloy from the earliest The
MODULATED
were disappearing
Eventually,
present at the beginning
AVERBACH:
the longer components
spectrum
aging proceeded.
AND
major
to
of the measured be too
large
experimental scattering
satellite correction
data
probably
peak
heights
arises
from
composition
because
several
are required. enhances somewhat, the
the but
non-linear
length of the modulation expressed in unit cells, and a is the lattice parameter of the alloy. J,, J,, and J, are Bessel functions of orders 0, 1 and 2. The asym-
relationship between the photographic density and the exposure for the very intense fundamental peaks.
metry in the satellite intensities
low because
and high angle equations.
satellites
These equations
of complete transformation
may
between the low angle be noted
from these
were derived for the case to the modulated structure.
The recorded
fundamental
peak
of the photographic
heights
are all tco
saturation
at high
intensities and the apparent satellite-to-fundamental intensity ratio is thus greater than the true ratio. An approximate
saturation
correction
was made, and
ACTA
262 Table 1. Satellite
intensity
ratios for a 35 at.% 150°C
Calculated 35 at.%, modulation
Satellite ratio
METALLURGICA, Ni alloy at
VOL. same
16,
for the simultaneous
and the corresponding amplitude,
Calculated 15 at.% modulation
1968
However,
I(OO0)
0.12*
0.30
the amplitude
factor modulation, term
amplitude,
is accompanied
with a small composition f(200)
0.12
0.11
enough
diffracted
distinct
satellite
observed 0.07
0.36
I(200)
fraction
* Approximate.
that the composition
less than 5 at. %. modulated
structure develops
composition
modulation
Limitations tension
We thus conclude
imposed
to accommodate
in these alloys, but the
amplitude is less than 5 at. %. probably
value of the composition ampitudes
is
that a periodic
by the coherency
requirements
position
amplitude
account
modulation.
approaching
the
prevalent
short
A more complete
wavelengths
amplitude,
wavelength
distribution
would only
t’ion were much greater.
com-
the regions
of
Although
a nucleation
barrier for the production
This is similar to the observa-
of the initial composition
fluctuations
tion by Hargreaves@)
of a limiting value of the misfit
not exist, the development
of the modulated
matrix in the copper-nickel-iron
structure with the
system.
In the latter
requires atomic migrations available
diffusion
case the term, &b/a, attained nearly the same limiting
The experimental
value during aging at different temperatures.
the act(ivation
case of the gold-nickel
a decrease in the composition length aging
of
the
at
modulated
higher
wavelengths
require
wavelengths
requires
as the wave-
increases
Thus, the amplitudes
less than the amplitudes The
in the
by
structure
composition
wavelengths.
observed
amplitude
temperatures.
are proportionally short
In the
system, this condition
the
presence
of
copper-nickel-iron
increase
in the
with longer which of the
the
longer
alloys
diffuse
were
intensity
modula-
4.3 Influence of vacancies
different composition.
for the coherence of the modulated
most
as the aging proceeds.
for the low stress
and we
of the
be observed in these alloys if the composition
Larger
of the
in the wavelength
population
wavelength
the
satellite
to a smaller
shorter
still have the greatest composition the relative
as a in
the boundary
may be attributed
Nevertheless, only
observed decrease
and surface
require a larger coherency the misfit between
The
of the short wavelength
observe
in
do not produce
be
of the short wavelength
structure
spectrum. it was concluded
to
reflection.
intensity
modulated
in this
sharp decrease
amplitude
intensity
with temperatures
0.06
and a decrease
by a very
of
For this reason, longer wavelengths
satellite intensity. 0.61
in other
of the scattering
A, is a very sensitive function
the composition 1.64
in the composition
in analogy with the observations
systems.
Experimental
growth of the wavelength
decrease
energy
in the solid solution.
of 13,500 cal. per mol. for
associated
ment of the modulated
does
structure
which are limited by the
mechanisms value
probably
with the develop-
structure
below the nose of
the C-curve in Fig. 4 agrees quite well with the value of 15,000 cal. per mol.
for the energy
migration
gold-nickel
composition
observed
in
In several of our experiments just
above
in
this
range.‘ll) the modulated
ture was formed at a low temperature, appearance
of vacancy
alloys
the
critical
temperature
of the modulated
structure
struc-
and then aged the
dis-
(Fig. 5).
for
The
between the fundamental and satellite reflection as aging proceeded. ‘r5) In the gold-nickel system,
treatment,
although diffuse streaks corresponding
to a
however,
clustering
of nickel-rich
in a
the satellites
fundamental peratures, satellites
reflection but
the
did not move upon
diffuse
aging intensity
and the fundamentals
closer to the
at higher between
increased
temthe
on aging.
However, the composition modulation in the goldnickel alloys is so low that a lengthening of the wavelength would wipe out the modulation. The gold-platinum systemda) appears to behave similarly to the copper-nickel-iron
system.
non-periodic lower
(h&b/a), remains the
disappeared
fashion
temperature,
with this reversion
and gold-rich
remained. however,
regions
On reaging the satellites
at the did not
reappear, even with aging times 1000 times longer than the time required to develop satellites when aging specimens
directly
after the quench.
The low angle
diffuse intensity observed for these reaged specimens was similar to that observed for specimens which had been
We may thus assume that for a given reflection the argument of the Bessel function,
sat’ellite reflections
quenched
temperature
and aged directly
without
the modulated
at the reversion
any intermediate
structure.
The absence
formation
of
of satellite
WOODILLA,
reflections
on reaging
JR.
AND
AVERBACH:
at the low temperature
reversion indicates that the modulated not form
again.
Thus,
the large
quenched-in
vacancies
temperature
is apparently
at which the
from
vacancies
concentration
the solution
structure
for the rate
develops.
a substantial
number
allows
this structure
to develop,
structure
would
of
annealing
of
modulated
after
structure does
responsible
the modulated
presence
MODULATED
Only
of
excess and
a
not have been observed
without them. 5. SUMMARY
formation
in gold-nickel
as follows.
structure
alloys may be summarized
Periodic modulated
a metastable
miscibility
structures form within
gap with a peak at about
45 at. ‘$4 Ni and 220°C ; the modulations observed for alloys with compositions 60 at.%
Ni.
composition
The
predominant
modulation
varied between 6.5 and 13 A,
perature or the time of aging. composition
of 5 at.%
of the
for each alloy.
The did not seem to vary with either the tem-
maximum
above
The amplitude
modulation
of the
was of the order
and below the average composition
of the alloy and did not change during aging. The modulated severely was
structures formed on aging only in
quenched
apparently
alloys and the rate of formation controlled
with an activation below
limited
vacancy
by
annihilated
by
of about
temperatures temperature
vacancy
migration,
13,500 cal. per mol.
At
150°C the rate of formation migration,
and
by the rate at which by annealing;
thus observed
These
entire process
of modulated
thus critically
dependent
was
above
this
vacancies
were
a C-curve
for the kinetics
behavior
of the process. structure
formation
on the presence
was The is
of excess
vacancies,
It appears that the parent solid solution
is probably
random
at the solution temperature
that clustering in quenched
modulated
connection
alloys is observed
and
only in
263
Au-Ni
through
the region
structure formation. structures
have
with the equilibrium
no
apparent
precipitates
which
form on aging at higher temperatures.
They are, in a
sense,
a rather
coherent
superstructures
established
periodicity
composition
modulation
lattice.
and
with a
well
well-defined
extending through the parent and
non-reversible,
dependent
The resultant properties
phenomenon
on
superimposed
may greatly affect the mechanical magnetic
fairly
It appears that these structures are metastable vacancy
periodicity
properties
and the
in certain alloy systems and the
deserves
cussion of properties
are only
between 20 and
wavelength
and appeared to be characteristic wavelength
of modulated
migration.
features of the modulated
IN
those alloys which are quenched
and
The principal
STRUCTURES
careful
consideration
in dis-
in systems of this hype.
ACKNOWLEDGMENTS
The authors are very grateful to the Atomic Energy Commission
for the support
also like to acknowledge cussions with Professors
of this work.
the many
We would
interesting
dis-
J. W. Cahn, S. C. Moss and
J. F. Breedis. REFERENCES 1. G. BORELIUS, J. Metals 3, 477 (1951). 2. E. E. UNDERWOOD, Precipitation in gold-nickel alloys. SC. D. Thesis, Mass. Institute of Technology (1954). 3. J. W. CAHN, Acta Met. 10, 907 (1962). Acta Met. 9, 795 (1961). 4. J. W. CAHN, B. GOLDINQ and S. C. Moss, Acta Met., 15, 1239 (1967). :: J. W. CAHN, Acta Met. 10, 179 (1962). 7. V. DANIEL and H. LIPSON, Proc. phys. Sot. A181, 368 (1943). M. E. HABOREAVES, Acta Cry8taZZogr.4, 301 (1951). :: Y. FUKANO, J. phys. Sot. Japan 16,1195 (1961). G. F. DAY, J. Inst. Metals 89, 296 (1961). ::: J. SIVERTSEN and C. WERT, Acta Met. 7, 275 (1959). 12. K. B. RUNDMAN and J. E. HILLIARD, Dept. of Materials Science, Northwestern University, Evanston, Illinois (1966). 13. S. C. Moss and B. L. AVERBACH, Local atomic configurations in a gold-nickel alloy. Proc. Conf. Small Angle X-Ray Scattering, Syracuse, N.Y. (1965), edited by H. BRUMBERQER. Gordon & Breach. 14. J. W. CAHN, private communication. 15. M. HILLERT, M. COHEN and B. L. AVERBACH, Acta Met. 9, 536 (1961). 16. T. J. TIEDEMA, J. BOUMAN and W. G. BURGERS, Acta Met. 5, 310 (1957).