Microstructural Features of a New PrecipitationStrengthened Cu-8Cr-4Nb Alloy
Joanna R. Groza
Mechanical, Aeronautical and Materials Engineering Department, University of California
at Davis, Davis, CA 95616-5274
Microstructural characteristics of a new precipitation-strengthened copper alloy with 8 at. %
Cr and 4 at. % Nb (Cu-8Cr-4Nb) have been investigated using transmission electron microscopy. A bimodal distribution of Cr2Nb precipitates was found in extruded and aged alloys.
Specific particle microstructures were identified to distinguish between primary and secondary precipitates that were directly related to liquid and solid precipitation processes.
In the study of the microstructural changes upon aging, no significant coarsening of the
CraNb precipitates was observed during aging at 773 and 973K up to 100 h.
INTRODUCTION
of 2:1 of the line compound (Fig. 1 from
ref. 3). The complete solubility of Cr2Nb in
liquid copper and complete insolubility in
solid copper makes possible the formation
of a stable and uniform array of fine and hard
Cr2Nb precipitates by using rapid solidification (RS) techniques (soluble in the liquid,
insoluble in the solid, or SLIS, concept [4]).
Rapid solidification increases the solubility
limits of Cr and Nb in pure copper to produce controlled precipitation upon further
aging heat treatment. Rapidly quenched materials also exhibit highly homogeneous and
refined structures with minimal segregation,
which is critical for the subsequent aging
process.
Cu-Cr-Nb alloys with the Cr:Nb ratio of
2:1 were developed by NASA as the next
generation of high-temperature, highconductivity alloys [5, 6]. Preliminary mechanical testing of such an atomized alloy
that contains 8 at.% Cr and 4 at.% Nb (Cu8Cr-4Nb) in extruded and aged condition
revealed promising room and elevated temperature properties, as well as good thermal stability [6]. For instance, the tensile
strength of Cu-8Cr-4Nb alloy is 426MPa at
Cu-Cr-Nb is a new precipitation-strengthened,
high-temperature, high-conductivity alloy.
The selection of the alloying elements was
based on their strengthening effect on copper at room and elevated temperature and
minimal effect on copper thermal conductivity. The strengthening is provided by the
precipitation of an intermetallic compound,
Cr2Nb, that is stable up to its congruent
melting point at 2006K and is not soluble
in the solid copper. The good thermal conductivity of copper is preserved because both
chromium and niobium have negligible solubilities in copper at room temperature and
up to 1100K [1, 2]. Furthermore, because the
Cu-Cr-Nb alloy composition is selected such
that all Cr and Nb combine to form the
Cr2Nb compound, then the high thermal
conductivity of the copper matrix is maintained because no alloying elements will remain in solid solution. According to the CrNb phase diagram, this Cr2Nb compound
is formed in a composition range from 33.3
to 37.5 at.% Nb, thus allowing for some compositional deviations from the Cr:Nb ratio
133
Published by Elsevier Science Publishing Co., Inc., 1993
655 Avenue of the Americas, New York, NY 10010
MATERIALS CHARACTERIZATION 31:133-141 (1993)
1044-5803/93]$6.00
J. R. Groza
134
room temperature and 100MPa at 975K, substantially higher than the same properties of
NARloy-Z, the alloy currently used in aerospace applications. While a precipitationhardening effect was observed in Cu-8Cr4Nb after short aging times, there was no
signihcant decrease of mechanical strength
after long-time exposure to high temperature. For example, the tensile strength after
100 h exposure at 973K decreased less than
9.5% as compared to that of the 10-h-aged
specimen at the same temperature. However, for this atomized alloy, no data on the
microstructure are available. Therefore, the
goal of the present investigation is to provide a microstructural characterization of the
Cu-8Cr-4Nb alloys obtained from gas atomized powders that will serve as a baseline
for further understanding of their structure/
property relationship.
EXPERIMENTAL PROCEDURE
Extruded bars from conventionally gas
atomized Cu-8Cr-4Nb powders (<106~m),
as well as raw powders, were supplied by
NASA Lewis Research Center. The chemical composition of the initial atomized powders supplied to NASA by Special Metals,
Inc., was 6.0 wt.% (7.36 at.%) Cr and 5.8 wt.%
(3.98 at.%) Nb. Special attention was directed
to the oxygen content, which was at a level
typical for the argon atomization process
(640ppm). The extrusion was carried out in
mild steel cans at 1133K with 16:1 reduction
in area [6]. The extruded specimens were
aged for 1, 5, 10, 50, and 100 h at 773 and
973K. For the purpose of comparison, one
specimen was aged 100 h at 1073K.
As-atomized powders, as-extruded, and
extruded and aged specimens were mounted
and prepared by standard metallographic
techniques for light microscopy examination.
A solution of hydrogen peroxide and amm o n i u m and sodium hydroxides at room
temperature was used for etching.
Specimens for transmission electron microscopy were cut perpendicular to the extrusion direction using a mechanical saw.
The mechanical thinning to about 100~m
was carried out using a Gatan specimen
holder. The hnal electrolytical thinning was
performed using a methanol bath containing 20% nitric acid at 14V and a temperature
below 220K. Some thin foils revealed that
numerous oxides formed during or immediately after electrolytic thinning. These foils
were further cleaned in a cold-stage Gatan
ion mill using 5keV argon ions (current
0.5mA).
For transmission electron microscopy investigation, a Philips 400 T microscope operated at 120kV was used. Precipitate identification followed the method developed by
Ellis and Michal [5]. Bright-held images were
complemented by selected area diffraction
(SAD) patterns and dark-field images taken
with various Cr2Nb precipitate reflections.
Energy dispersive spectra and scanning electron microscopy analysis were also used to
conhrm particle composition. For particle
studies, the specimen was tilted to keep a
low dislocation contrast and to reveal distinctive faulty structure of particles.
RESULTS
PRECIPITATE STRUCTURE
AND MORPHOLOGY
Typical light microstructures of as-atomized,
and extruded and aged Cu-8Cr-4Nb alloys
are shown in Fig. 2. In the light micrographs,
only particles around 1 , m in size are distinguishable. The particles are fairly uniformly
distributed with only occasional oversized
particles. At the light microscope level, there
is no distinct difference between the particle structure in the initial atomized powder
[Fig. 2(b)] and that in the extruded and aged
material [Fig. 2(a)].
The detailed structure of the matrix and
Cr2Nb precipitates may be followed in the
transmission electron micrographs. At lower
transmission electron microscopy magnifications, a bimodal particle size distribution
is clearly observed (Fig. 3). Both large particles (shown at A), usually around l~m,
and small particles in tens to hundreds of
nanometers (shown at B) can be seen. In cer-
Precipitation-Strengthened Cu-8Cr-4Nb Alloy
135
2600
2400i
:4?fc
~2200
2ooo
L
1863'c
1800
1500
~73Yc
/ 20
.
.
.
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.
.
.
.
.
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.
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.
.
40
.
.
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.
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a/oCr 60
.
.
.
.
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.
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.
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~2~
f.......80.._._...,L00
FIG. 1. The Cr-Nb phase diagram.
tain cases, areas with a larger concentration
of small particles were found, such as s h o w n
at B in Fig. 3.
Figure 4 shows the large Cr2Nb particles
at higher magnihcation. The typical SAD
pattern of Cr2Nb particles is s h o w n in Fig.
4(c) and indicates a hexagonal C14 structure.
While some large particles are equiaxially
s h a p e d [Fig. 4(a)], most of t h e m exhibit
a very complex, irregular, dendritic-type
shape [Fig. 4(b)]. The analysis of these large
precipitates is complicated by features of the
Cr2Nb particles that have b e e n only briefly
reported by previous investigators [5]. These
features are the crystallographically aligned
faults that appear on the particles (Fig. 5).
The faults are highlighted in contrast only
w h e n their orientations satisfy the Bragg
condition for diffraction in the transmission
electron microscopy. In some particles, these
faults may exhibit several distinct orienta-
(a)
FIG. 3. Bimodal size distribution of Cr2Nb particles in
Cu-8Cr-4Nballoy.Aging was performed for 10h at 773K.
tions. For example, the faults in Fig. 5 are
observed at definite angles of 60 °, 900 or
120 ° [A-C in Fig. 5(a, b)]. To identify these
faults as either microtwins or stacking faults,
considerably more work is required. These
faults are a common feature of large particles,
although free precipitates also display a similar appearance [D in Fig. 5(b)]. The presence of these faults was u s e d for precipitate
identification. In similar Cu-Cr-Nb alloys
with slightly different compositions and
Cr: Nb ratio, Ellis and Michal [5] found both
Cr2Nb and Cr precipitates. In the present
alloy, no Cr precipitates were found. This
conclusion is based on the faulty structure
appearance of the observed precipitates, as
well as on the energy dispersive spectra that
(b)
FIG. 2. Light micrographs ot Cu-8Cr-4Nb alloy in aged 100 h at 773K (a) and as-atomized (b) conditions.
J. R. Groza
136
(a)
indicated only Nb-containing precipitates.
Furthermore, the Cr: Nb ratio of the present
alloy is adequate to form only the Cr2Nb (~)
phase (Fig. 1), in accordance with our microstructural observations. An explanation
of the absence of the Cr precipitates in the
present alloy may be f o u n d in the alloy
chemistry, that is, the Cr: Nb ratio (1.86:1),
which is slightly less t h a n stoichiometric.
In contrast, the alloy in which Ellis a n d Michal observed Cr precipitates contained an
over-stoichiometric Cr: Nb ratio (2.1:1).
The precipitate distribution is s h o w n in
Fig. 6. While the large particles are usually
found at grain boundaries, small precipitates
are observed both in the interior of the grain
a n d at grain boundaries (shown at arrows
in Fig. 6). Another interesting feature of Cu8Cr-4Nb alloys is the clustering of small-size
precipitates. Although not very pronounced,
this clustering is observed as areas featuring
(b)
(a)
(c)
Fig. 4. Differentmorphologiesof large Cr2Nb particles.
Specimens aged 100 h at 1073K (a) and 50 h at 773K
(b). (c) shows the typical SAD pattern of Cr2Nb partide. Magnificationof (b) as (a).
(b)
Fic. 5. Faulty internal structure of Cr2Nb particles.
Specimens aged 5 h at 773K(a) and 50 h at 973K (b).
Magnificationof (b) as (a).
Precipitation-Strengthened Cu-8Cr-4Nb Alloy
(a)
(b)
FIG. 6. Large-particleand small-particle distributions
in Cu-8Cr-4Nb alloys aged 973K for 1 h (a) and 50 h
(b). Magnihcation of (b) as (a).
a high density of small precipitates [Fig. 7(a),
also s h o w n at B in Fig. 3]. At the same time,
some areas with fewer precipitates m a y be
f o u n d [Fig. 7(b)]. We assume this distribution is inherited from the atomization process. The rapidly solidihed powders have a
large range of atomized powder particle sizes
[Fig. 2(b)]. Smaller powder particles have
more Cr and Nb retained in the solid solution
simply because the cooling rate is higher than
for coarse atomized powders. U p o n subsequent heating for extrusion, fine precipitates
within small powder particles dissolve faster
t h a n coarse precipitates. Therefore, clusters
of small precipitates m a y form more easily
in the previously small atomized powder
particles t h a n in their large counterparts.
MATRIX MICROSTRUCTURE
A characteristic matrix microstructure of
137
(a)
(b)
Fxc. 7. Clusteringof hne precipitates(a) and precipitatefree areas (b) in aged specimens. (a) Aged 5 h at 773K,
(b) aged 5 h at 973K. Magnihcation of (b) as (a).
extruded and aged Cu-8Cr-4Nb alloy is
s h o w n in Fig. 8. Grain boundaries are well
distinguished, with some at 120 ° angles
and only few dislocations left in the interior
of the grains.
AGING EFFECT ON MICROSTRUCTURES
The effect of aging on the Cu-8Cr-4Nb alloy
microstructure is s h o w n in Fig. 9. The asextruded specimen [Fig. 9(a)] has fewer small
precipitates (on the order of tens of nanometers) t h a n the extruded and aged specimens [Fig. 9(b-d)]. The heating temperature
for extrusion was sufficiently high to permit
the small secondary precipitates to dissolve
back into copper solid solution. The cooling
rate after extrusion was also sufficiently fast
to maintain a supersaturated copper solid
J. R. Groza
138
Fx(3.8. Matrix structure in Cu-8Cr-4Nb alloys aged
100 h at 1073K.
solution. Therefore, a precipitation process
occurs upon subsequent aging w h e n small
precipitates (in the order of tens of nanometers) are expected to form. All aged specimens indicate that such small precipitates
are formed [Fig. 9(b-d)]. Precipitate structure
does not change markedly with aging for
the times and temperatures used for this
work. For instance, in all aged specimens,
small precipitates in the order of tens of
nanometers remain, even after aging at 973K
[Fig. 9(e)]. The persistence of such small precipitates after long-time aging is an indication of the slow coarsening kinetics of the
Cr2Nb precipitates. A quantitative characterization of precipitate coarsening is in progress and will be reported later.
DISCUSSION
MICROSTRUCTURAL EVOLUTION
Microstructural observations of the extruded
and aged Cu-8Cr-4Nb alloy indicate that the
Cr2Nb particles have two rather distinct microstructures. Large particles display many
orientations with highly irregular morphology [Figs. 4(b) and 5(b)]. In contrast, the
small precipitates exhibit primarily a single
orientation [D in Fig. 5(b)]. Next, there is
a difference in the distribution of large and
small precipitates. In most cases, the large
particles are found at grain boundaries, indicating that growing grains impinged the
already-formed particles [Figs. 4(a), 6, and
8]. The small particles are observed at both
grain boundaries and within the grains
(Fig. 6). To interpret the existence of these
two particle types in Cu-8Cr-4Nb alloys, the
evolution of gas-atomized structures during
extrusion and subsequent aging must be
considered. A reasonable explanation for the
coarse particles is that they are formed from
the liquid state (primary particles). Previous
research indicated that primary particles are
formed during chill block melt spinning of
a similar alloy [5]. In the melt spun material,
the primary particle size was extremely small
(47nm) because of the very high cooling rate
(about 106K/s). Gas atomization used to
produce the present alloy involves signihcantly lower cooling rates (102-103K/s [7])
that allow primary Cr2Nb particles to grow
considerably (around Item-Figs. 2 and 3)
as compared to the melt spun alloy. When
such precipitates grow from the liquid state,
there are no constraints from the matrix, and
particles grow freely with many different
orientations. In this case, the morphology
of primary precipitates may be very irregular, sometimes close to dendritic, as was
experimentally observed [Fig. 4(b)]. In contrast, the small precipitates form from the
highly supersaturated copper solid solution
during cooling below the solidus temperature
(secondary precipitates). Neither Cu-Cr2Nb
nor ternary Cu-Cr-Nb phase diagrams are
available, but both Cu-Cr and Cu-Nb diagrams indicate that the solidification is completed at temperatures very close to the copper melting point. If it is assumed that the
solidus in Cu-Cr-Nb alloys is similarly close
to the copper melting temperature, then the
thermal excursion of the solidihed atomized
powders is large enough to allow secondary
particle precipitation. The size of such a secondary precipitate was calculated considering an undercooling of about 100K below
the solidus and 773K as the lower temperature bound for significant diffusion. This
calculation was based on the available flux
of atoms inside a sphere of radius ~ (D is
diffusivity of the slowest moving species,
and t is cooling time between solidus and
773K) that move to the center to form the
Precipitation-Strengthened Cu-8Cr-4Nb Alloy
139
(a)
(b)
(c)
(d)
FIG. 9. Precipitation sequence of free Cr2Nb compound in Cu-8Cr-4Nb alloys. (a) As-extruded, (b) aged
5 h at 773K, (c) aged 10 h at 773K, (d) aged 50 h at 773K,
(e) aged 50 h at 973K. All magnifications as (a).
(e)
Cr2Nb precipitates [5]. The diffusivity of Nb
was considered to be rate limiting, because
it is lower than the diffusivity of Cr. The results of this calculation for cooling rates of
102-103K/s indicate that the largest secondary
particles formed from supersaturated solid
solution range in size between 24 and 76nm.
This quite large variation of secondary particle size is expected in atomized powders
that have a large powder particle size range
[Fig. 2(b)] and, hence, various powder particle cooling rates. The secondary particle
size values seem in reasonable agreement
with the previous assumption based solely
140
onmicrostructural features that particles on
the order of tens of nanometers are secondary particles. From the above calculation, it
is also clear that the large particles around
l ~ m could not form by precipitation from
solid solution and, therefore, are primary
particles (Figs. 2-4).
The driving force for the precipitation process is expected to be substantial because
the degree of supersaturation of the rapidly
solidified copper solid solution is high. The
observed grain boundary nucleation is expected as in any solid state heterogeneous
nucleation process [Figs. 6(a) and 7(a)]. However, extensive precipitation was noticed inside grains [Figs. 6 and 9(b, d, e)]. This
homogeneous nucleation may indicate that
there may be a certain degree of coherency
between the precipitates and the copper matrix. Indeed, an analysis of the atomic arrangements and spacings suggests a possible
orientation relationship between the {111}
plane of Cu and {1120} plane of CraNb. If
one assumes that the initial precipitation
from supersaturated copper solid solution
may produce coherent precipitates, it is very
likely that this coherency was lost early in
the growth stage such that the precipitates
are completely incoherent after aging for
I h or longer. This assumption is consistent
with previous measurements done by Ellis
and Michal [5], which indicated that the resistivity changes related to the precipitation
process in the solid solution in the Cu-SCr4Nb alloy were completed after 1 h aging
at 773K. Furthermore, the microstructural
analysis of the present material that was aged
for at least I h did not display any coherency
of Cr2Nb precipitates within the copper
matrix. This may be an indication that the
nucleation and growth processes of these
precipitates are completed during this aging
time, in agreement with the resistivity
measurements.
The subsequent extrusion temperature
was sufficiently high (1133K) to permit the
dissolution of some of the fine particles precipitated in the rapidly solidified material.
Evidently, the sizable primary precipitates
did not dissolve during this heating stage.
J. R. Groza
Similar to the atomized material, secondary
Cr2Nb particles precipitate out of the supersaturated solid solution after the extrusion
step. This precipitation process is clearly
shown by comparing the particle size of the
as-extruded alloy [Fig. 9(a)] and the 1-h-aged
alloys [Fig. 9(b)], in which a new generation
of very small particles is formed. The microstructural results of specimens aged for
longer times indicate that the coarsening
process of the Cr2Nb precipitates is very
slow [Fig. 9(b-e)]. These microstructural observations of aged specimens are in good
agreement with the mechanical tests done
by Ellis and Dreshfield [6], which indicate
a high degree of thermal stability of the dispersoids in Cu-Cr-Nb alloys up to 973K. The
assumption for slow coarsening kinetics is
consistent with the low diffusivities [8] and
solubilities of Cr and Nb in copper up to
973K [1, 2]. As noted by Ellis and Dreshheld
[6], coarsening of Cr2Nb becomes signihcant at temperatures higher than about
1100K. According to the Cu-Cr and Cu-Nb
phase diagrams, the solubilities of both Cr
and Nb in copper exhibit a dramatic increase
above 1100K [1, 2]. Previous research on binary Cu-Nb and Cu-Cr alloys has also established that both Nb and Cr precipitates are
quite resistant to particle coarsening in about
the same or lower-temperature range [9, 10].
Clearly, w h e n the stable, high melting point
CraNb compound is formed, the thermal
stability of Cu-Cr-Nb alloys is further improved. As already indicated, a detailed
study of particle coarsening kinetics in Cu8Cr-4Nb alloy and thermal stability is in progress and will be reported later.
In conclusion, the above differences in
the formation s t a g e - f r o m liquid or solid
s o l u t i o n - m a y explain the bimodal size of
the CraNb particles. While the primary particles may grow very fast from the liquid solution, all secondary particles are very small
because of substantial diffusion restriction
in the solid state. Although a distinct size
cutoff between secondary and primary particles is difficult to establish, calculations
have shown that the secondary particles are
in the 24-76nm range in the as-atomized
Precipitation-Strengthened Cu-8Cr-4Nb Alloy
powders. This estimated range is in fairly
good agreement with the experimentally observed values.
141
knowledged. The author is also indebted to Ken
Anderson for secondary particle size calculations.
References
CONCLUSIONS
Cr2Nb particle distribution in as-extruded
and aged Cu-8Cr-4Nb alloys obtained from
atomized powders is bimodal with precipitates formed from both liquid and solid solution. Primary particles solidified from liquid are characterized by large size (around
l~m), numerous orientations of internal
faults and grain boundary distribution. Secondary particles formed from solid solution
are very fine (tens to low hundreds of nanometers), usually have only one orientation,
and are precipitated both at grain boundaries
and within grains.
There are no distinct microstructural
changes by aging the as-extruded Cu-8Cr4Nb alloys. The small precipitates (on the
order of tens of nanometers) are observed
in all aged specimens, thus giving an indication that the coarsening process in the Cu8Cr-4Nb alloy is very slow. This slow coarsening process is in accordance with the good
thermal stability of mechanical properties
in this alloy after long-time exposure at high
temperatures. More quantitative studies in
this respect are definitely required.
The support of the NASA Lewis Research Center
and permission to publish this work are greatly
appreciated. The experimental assistance and
valuable discussions with Drs. D. Ellis, D. Hull,
R. Dickerson, and S. Farmer are gratefully ac-
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Received January 1993; accepted June 1993.