Materials Science and Engineering A356 (2002) 32 /36
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Precipitation in a Cu Cr Zr alloy
/
/
I.S. Batra *, G.K. Dey, U.D. Kulkarni, S. Banerjee
Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
Received 28 February 2002
Abstract
Transmission electron microscopy (TEM) investigations of the early stages of precipitation of a b.c.c. phase in f.c.c. matrix have
been carried out in a dilute Cu /Cr /Zr alloy. The initial product of decomposition, characterized by a state of order, which mimics a
cube-on-cube orientation relationship (OR) with the matrix appears prior to ordered b.c.c. phase precipitates showing Nishiyama /
Wassermann (N /W) OR.
# 2002 Published by Elsevier Science B.V.
Keywords: Transmission electron microscopy; Copper alloys; Microstructure; Nucleation and growth phase transformations; f.c.c. to b.c.c.
transformations
1. Introduction
Owing to their excellent thermal conductivity,
strength and fatigue resistance, aged dilute Cu /Cr
alloys and their minor modifications are considered to
be candidate materials for plasma facing components,
divertor plates and other heat-transfer elements of
future torroids [1 /6]. Precipitation of chromium in
dilute binary Cu /Cr alloys [7 /14] has been studied
extensively in the last three decades. It was found that
copper lattice expands due to the chromium in the
solution [15]. Upon precipitation from a Cu /0.3wt.% Cr
alloy, the chromium particles were seen by Hall et al. [7]
to exhibit with the copper matrix a range of orientation
relationships (ORs) from Nishiyama /Wassermann (N /
W) to Kurdjumov /Sachs (K /S). Luo et al. [11],
however, found that for well-grown Cr particles in a
Cu /0.33wt.% Cr alloy, the OR was always close to K /
S. This observation was in conformity with the invariant
line model proposed earlier by Dahmen [10]. It is also in
agreement with the recent model of Kelly and Zhang
[13] based on edge-to-edge matching which predicts that,
for b values (/baf:c:c:
=ab:c:c:
; a0 being the lattice para0
0
meter) in the range 1.2 /1.28, the K /S OR, wherein the
* Corresponding author
E-mail address: isbatra@aspara.barc.ernet (I.S. Batra).
close-packed directions in the two phases, viz. a
Ž1 1 0f.c.c. direction and a Ž1 1 1b.c.c. direction, are
parallel, should be favoured over an N /W type of OR.
In the latter OR, a close-packed Ž1 1 0f.c.c. direction is
parallel to a Ž1 0 0b.c.c. direction along which the interatomic spacing is about 1.15 times larger than closepacked Ž1 1 1b.c.c.. However, the crystal structure as
well as the OR of nano-sized chromium particles, during
the very initial stages of precipitation, continued to be
an enigma for a long time until the related controversies
were finally resolved by Fuji et al. [14]. The crystal
structure of the chromium particles was established to
be b.c.c. even for particle sizes smaller than 10 nm. Also,
two distinct ORs between the chromium particles and
the copper matrix were found */one near K/S and the
other near N/W. After prolonged aging, only the K/S
particles remained in the copper matrix. The relative
stability of the two types of particles could also be
explained by geometrical considerations. In contrast to
these detailed studies on Cu /Cr alloys, similar studies
[16 /18] related to morphology, composition and crystallography of precipitates in Cu /Cr /Zr alloys have been
very few. A number of reports, however, exist on the
evaluation of these alloys for their end uses [1,2,19].
These pertain to various aspects of these alloys, such as
physical and mechanical properties, corrosion and
irradiation behaviour and brazing and joining. In
0921-5093/02/$ - see front matter # 2002 Published by Elsevier Science B.V.
PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 8 5 2 - 3
I.S. Batra et al. / Materials Science and Engineering A356 (2002) 32 /36
investigations [16] related to the substructure of these
alloys, precipitates with lobe /lobe contrast and fringes
parallel to the diffraction vector have been seen. Tang et
al. [17] have identified the precipitates in a slightly
modified Cu /Cr/Zr /Mg alloy to be Cu4Zr and
CrCu2(Zr,Mg). To further understand the early stages
of precipitation in these alloys, an alloy having a
nominal composition of Cu /1wt.% Cr /0.1wt.% Zr
was examined in the present work using conventional
transmission electron microscopy (CTEM) as well as
high-resolution transmission electron microscopy
(HRTEM). Addition, as small as 0.1 wt.%, of Zr was
seen to drastically alter the process of precipitation. The
precipitates in their early stages of formation were found
to possess an ordered f.c.c. structure with a cube-oncube OR with the copper matrix.
2. Experimental
To prepare Cu /Cr /Zr alloy of the desired composition, small pieces of oxygen-free high conductivity
copper, electrolytic chromium and iodide zirconium,
weighed in an appropriate ratio, were melted together in
an induction furnace in a yttria-lined graphite crucible.
High-purity argon was used as a protective atmosphere
in the induction unit. The ingot weighing approximately
500 g was cut and cold-rolled to a thickness of 0.4 mm.
For TEM studies, a small piece of 0.4 mm thick strip
was further rolled to 0.2 mm thickness and then
chemically thinned to 0.1 mm thickness in a solution
of 5% HF, 45% HNO3 and 50% distilled water. Small
pieces from this strip were encapsulated in quartz in
helium, solutionized at 1000 8C for 1 h and then water
quenched by breaking the capsule. These pieces were
again encapsulated in helium, aged at 480 8C for 5 h
and water quenched. Discs of 3 mm in diameter were
punched out of these aged pieces, mechanically thinned
to about 0.07 mm and then jet thinned at room
temperature in a solution having a ratio of 75 g chromic
oxide, 375 ml acetic acid and 20 ml distilled water.
Voltage, during electrolytic polishing, was maintained at
about 50 V. The thinning process was terminated with
the help of a photocell. Prior to loading in TEM, the jetthinned discs were ion-milled in a Gaton Duomill for
about 10 min to remove the surface oxide. These foils
were examined in a JEOL 2000 FX microscope operating at 200 kV. HRTEM examinations were carried out
at 300 kV in a JEOL 3010 microscope having a point-topoint resolution of 0.21 nm.
3. Results and discussion
Precipitation in alloys of similar composition is
known to have a bimodal distribution of precipitate
33
sizes [16]. The coarse precipitates are the ones that form
during solidification of the alloy and do not dissolve
during solutionizing treatment as the Cr content of the
alloy is in excess of the equilibrium solubility. The finer
precipitates are the ones that form due to decomposition
of the supersaturated solid solution during aging.
The onset of decomposition of the supersaturated
matrix is characterized by the appearance of a mottled
contrast as seen in Fig. 1(a). In Fig. 1(c) and (d),
[1 1 0]f.c.c. and [1 1 1]f.c.c. SADPs from this region are
shown with their keys in Fig. 1(e) and (f), respectively.
From these, it appears that the initial decomposition
product has an ordered f.c.c. structure with a cube-oncube OR with the matrix. Superlattice reflections arising
from ordering can be seen midway between the {2 2 0}
reflections of the product and the transmitted spot. This
would imply solute enrichment on alternate {2 2 0}f.c.c.
planes. It also has the effect of dilating d2 2 0 of the
precipitate vis-à-vis that of the matrix giving rise to a
tetragonal distortion of the f.c.c. matrix. The [3 1 0]f.c.c.
pattern shown in Fig. 1(b) also reinforces the view that
an apparent cube-on-cube OR exists between the matrix
and the initial decomposition product. An important
observation in SADPs in Fig. 1(c) and (d) is that the
reflections due to the decomposition product exhibit
distinct streaking in specific directions. Ordinarily, a
true cube-on-cube OR, between the parent and the
product, would not be expected to give rise to different
orientational variants. It, therefore, appears that the
streaking seen in the patterns in Fig. 1(c) and (d) is a
consequence of the strain arising in the matrix due to the
formation of the ordered product phase in a very fine
state of subdivision. Indication of such coherency strains
in the matrix is obvious in the form of a mottled contrast
in Fig. 1(a). Such strains can influence the geometry of
the decomposition product and account for the observed
streaking. A large number of fine clusters of the ordered
product in various orientations lead to the observed
SADPs with the overall cube-on-cube OR as well as the
streaking.
The [1 0 0]f.c.c. SADP shown in Fig. 2(a) appears to
correspond to a more advanced stage of decomposition
in that all the reflections expected in a cube-on-cube OR
are not seen. Streaks oriented along Ž1 1 0* directions
as well as condensation of streaks into reflections of a
second phase can also be seen. A faint superlattice
reflection (marked by an arrow in SADP) can also be
seen midway between one of these reflections and the
transmitted spot. This indicates that the product phase
here is also ordered. The only common feature between
Fig. 2(a) and SADP in Fig. 1(c) and (d) is the
arrangement of spots along Ž2 2 0* directions. Associated with {2 2 0}f.c.c. reflections are streaked spots of
the decomposition product and also a superlattice
reflection lying midway between the latter and the
transmitted spot. A similar arrangement of spots along
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I.S. Batra et al. / Materials Science and Engineering A356 (2002) 32 /36
Fig. 1. (a) BF micrograph showing mottled contrast in the initial stages of decomposition, (b) [3 1 0], (c) [1 1 0], (d) [1 1 1] SADPs showing extra
reflections from the ordered f.c.c. product phase having a cube-on-cube OR with the f.c.c. matrix, (e) key to (c) and (f) key to (d). Keys (e) and (f)
show positions of the fundamental (/) and superlattice (m) reflections from the product and f.c.c. matrix reflections ( ). All other reflections in (c)
and (d) arise due to double diffraction.
Ž2 2 0* direction would also be expected in the case of
an ordered b.c.c. (having B2-type structure) exhibiting
an N/W type of OR with the f.c.c. matrix. The
[1 1 2]f.c.c. SADP with faint b.c.c. spots forming an
entire reciprocal lattice section and exhibiting N /W
OR between the matrix and the ordered B2 (CsCl type)
precipitates from some regions of the foil is shown in
Fig. 2(b) with the relevant key in Fig. 2(c). A B2-type
I.S. Batra et al. / Materials Science and Engineering A356 (2002) 32 /36
35
Fig. 2. (a) [1 0 0] SADP from the solutionized plus aged alloy showing streaks along Ž1 1 0* around the f.c.c. reflections. These arise due to the
solute segregation in the initial stages of the decomposition process. The streaks can also be seen to have condensed into reflections of the second
phase. A superlattice reflection midway between such a spot and the transmitted spot has been marked by an arrow, (b) [1 1 2] SADP from the matrix
showing extra reflections from the ordered b.c.c. precipitates. The two phases appear to follow the N /W OR, and (c) key to (b) showing positions of
the fundamental (D) and superlattice (k) reflections from the b.c.c. precipitates and f.c.c. matrix reflections ( ). Distinct precipitates showing lines of
no contrast can be seen in the BF micrograph in (d).
phase can undergo further ordering to form a CuMnAl2-type ternary ordered structure. The existence of
such an ordered phase has been reported by Tang et al.
[17] in quaternary Cu /Cr /Zr/Mg. This, however,
could not be ascertained in the present case. Fine but
distinct precipitates of the ordered b.c.c. phase could be
seen in these regions (Fig. 2(d)). Some of these
precipitates can be seen to exhibit a line of no contrast
arising due to the elastic strain associated with them.
The fact that the lines of no contrast from these
precipitates show various orientations strengthens the
observation that these precipitates have already acquired the b.c.c. structure [14,20]. In contrast to this,
the initial decomposition product exhibits only a
mottled contrast (Fig. 1(a)). Subtle differences are
observed between the transformation sequence here
and that observed in Cu /2wt.% Be alloy [21] */another
system showing B2 precipitation in f.c.c. Cu */in that
pronounced streaking in Ž1 0 0* directions is observed
in Cu /Be in the initial stages of decomposition.
Fine precipitates of the ordered b.c.c. phase (shown in
Fig. 2(d)) with a lobe /lobe contrast can also be seen in
the HRTEM micrograph in Fig. 3 showing clear
Fig. 3. HRTEM micrograph depicting (2 0 0)Cu lattice fringes, particles with lobe /lobe contrast showing Moiré fringes parallel to the lines
of no contrast can be seen. The Moiré fringes as well as the lines of no
contrast appear to be present in two perpendicular orientations along
Ž1 1 0.
(2 0 0)f.c.c. lattice fringes. The parallel Moiré fringes,
around the line of no contrast, having a spacing of
about 0.8 mm arise from the small difference in dspacing of a {2 2 0}f.c.c. reflection and that of the
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I.S. Batra et al. / Materials Science and Engineering A356 (2002) 32 /36
associated fundamental spot of the ordered B2 phase.
The lines of no contrast as well as the Moiré fringes were
found to be parallel to Ž1 1 0f.c.c. directions and at an
angle of 458 to [1 0 0]f.c.c..
A sequence of evolutionary stages during the decomposition of the Cu /1wt.% Cr /0.1wt.% Zr alloy is
apparent from the results presented above. The initial
decomposition product appears to have an ordered f.c.c.
structure with unit cell size larger than that of the
matrix. It possesses a cube-on-cube OR with the matrix
and appears to be a vital step in the overall process of
decomposition. In this respect, the decomposition in this
alloy appears to be similar to that in Cu /Be /Co alloy
system [22], in both the cases dimensional changes
brought about by ordering facilitate the formation of
the b.c.c. phase. The enlargement of d2 2 0 of f.c.c. on
ordering to d2 0 0 of the b.c.c. precipitates appears to
promote the development of the N/W OR. However,
the atomistic processes involved in the transformation
could not be clearly established in this investigation.
4. Conclusions
An important effect of the addition of a small amount
of zirconium in dilute Cu /Cr alloys appears to be on the
precipitation sequence. In contrast to the precipitation
sequence in binary Cu /Cr alloys, addition of zirconium
was found to promote the formation of ordered f.c.c.
solute-rich clusters, which eventually transformed into
ordered b.c.c. precipitates.
Acknowledgements
The authors thank Mrs. P. Agashe for the photographic work and Mrs. A.B. Menon for typing the
manuscript.
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