Research Summary
High-Temperature Alloys
The Effect of Solution Heat
Treatment on a Single-Crystal
Ni-Based Superalloy
B.C. Wilson, J.A. Hickman, and G.E. Fuchs
Understanding of the solution heat
treatment response of a second generation, single-crystal nickel-based superalloy, CMSX-4, is necessary before the
process can be shortened in an effort to
save money. The current solution heat
treatment used for CMSX-4 involves high
temperatures for long durations and can
be quite expensive. This investigation
helps to characterize the heat-treatment
process for this alloy. The achieved
microstructure is described for both
the as-cast and fully solution-annealed
conditions. In addition, an alternate, less
expensive heat treatment was selected
for its lower temperatures and shorter
overall duration. Microstructures
and differential thermal analysis are
presented for all three conditions.
INTRODUCTION
Single-crystal nickel-based superalloys are known for their high strength and
creep resistance at high temperatures.
These alloys are used extensively in
the aerospace industry for turbine
blades and vanes in the “hot section”
of today’s gas turbine engines.1 While
these properties depend largely on the
alloy composition, they also rely on
proper heat treatments to bring out these
exceptional properties.2 Traditionally,
two heat treatments are used for nickelbased superalloys. First is the solution
heat treatment, designed to homogenize
the microstructure and reduce the effects
of elemental segregation.3 The second
is one or more aging heat treatments,
designed to develop a cuboidal γ/γ′
microstructure. This investigation
focuses on the effects of solution heat
treatments on a nickel-based superalloy
at lower temperatures for a shorter
duration, which may reduce costs. The
material used for this study is CMSX-4,
a second-generation, single-crystal
2003 March • JOM
nickel-based superalloy.
CMSX-4, widely used in the aerospace industry, is considered a secondgeneration superalloy because it contains
approximately 3 wt.% Re.4 The solution
heat treatment for this alloy is long and
expensive, requiring almost a full
day at elevated temperatures between
1,277°C and 1,318°C. Following this
treatment, the material must be aged for
an additional 26 hours. As furnace time
is costly, any reduction in temperature
or duration of heating will reduce costs
significantly. This is especially true for
high-output operations with thousands
of parts requiring heat treatments. The
goal of this investigation is to determine
how this alloy will react to a modified
solution heat treatment.
The modified heat treatment chosen
for this study is the standard PWA 1480
solution heat treatment, which runs at
a significantly lower temperature and a
much shorter duration. It has only three
steps with slow ramp rates between hold
temperatures, and requires about half the
time of the standard heat treatment. If the
modified heat treatment proves to be close
a
300 µm
to satisfactory, then the possibility exists
of altering the solution heat treatment
and, ultimately, saving money.
To fully evaluate the feasibility of
altering the solution heat treatment,
three test groups were created. For the
as-cast group no heat treatments were
applied. The second group received
the standard solution and aging heat
treatments. The third group received
the modified heat treatment and the
standard aging heat treatment. The
same aging heat treatment was used
for both groups that received solution
heat treatments in order to prevent
differences due to the aging process.
Experimental observation was gained
through metallography, microprobe
analysis, differential thermal analysis,
and creep testing. See sidebar for
experimental procedures.
RESULTS
Microstructure
As-cast CMSX-4 has a two-phase γ/γ′
microstructure. The microstructure is
cored, or segregated, with a dendrite pat-
b
50 µm
Figure 1. Photomicrographs of as-cast CMSX-4. (a) Optical micrograph of dendrites
with secondary and tertiary arms extending from the primary dendrite, as well as
γ/γ′ eutectic areas in between. (b) Scanning-electron micrograph of coarsening γ/γ′
morphology approaching the eutectic region in the center. A dendrite core can be
seen to the left of the image.
35
EXPERIMENTAL PROCEDURE
Material
a
5 µm
This investigation focuses on a second-generation, single-crystal nickel-based superalloy,
CMSX-4. The material composition is given in Table A. The CMSX-4 master alloy
was produced by Canon-Muskegon (Muskegon, OH). The single-crystal test bars were
produced by PCC Airfoils (Minerva, OH) through an investment casting directional
solidification process that yielded 24 test bars, 20 cm long, and of varying diameter. The
test bar diameter distribution is as follows: 12 1.3-cm bars, 6 1.6-cm bars, 4 1.9-cm bars,
and 2 2.5-cm bars. All of the single crystals were grown in the <001> direction and were
inspected by Laue back-reflection. Test bars were also visually inspected to ensure
the absence of any grain defects such as high and low angle boundaries and freckles.
Once the bars were made and inspected, three test groups were established, one for
each of the three heat treat conditions. Four bars were assigned for each group (two
1.3-cm bars and two 1.6-cm bars).
Heat Treatments
b
5 µm
Figure 2. Scanning-electron micrographs
of CMSX-4 following the standard solution
heat treatment. (a) Prior to aging, γ′ is
irregular in size and shape. (b) Following
aging, γ′ has coarsened and become
more regular.
The three solution-heat-treatment conditions used for this research are as-cast, the
standard solution heat treatment for CMSX-4, and a modified lower temperature, shorter
duration, solution heat treatment. As already mentioned, the modified heat treatment
needed to be of lower temperature and shorter duration. For this investigation, the PWA
1480 solution heat treatment, developed by Pratt & Whitney (East Hartford, CT), was
chosen as the alternate heat treatment. PCC Airfoils performed the PWA 1480 heat
treatment and the standard CMSX-4 solution heat treatment. After the solution heat
treatments were completed, both groups underwent the same aging heat treatments.
The standard and modified heat treatment schedules as well as the aging heat treatment
schedules are given in Table B. The 1.6-cm diameter bars from all three groups were
then exposed to one final long-term, high-temperature heat treatment to test the long-term
stability of the microstructure. These six bars were placed in an air furnace and held
at 1,000°C for 1,000 hours (approximately six weeks). During the exposure, the
temperature was verified with two Type K thermocouples in addition to the furnace control
thermocouple. At the end of the exposure, the bars were air-cooled.
Specimen Preparation for Metallography
tern and γ/γ′ eutectic regions throughout
the microstructure (Figure 1a). Scanning
electron microscopy reveals a relatively
fine γ′ distributed within the dendrite
cores that becomes coarse near the
eutectic regions (Figure 1b). The cored
microstructure is due to compositional
gradients throughout the material that
formed upon solidification. The dendrite
cores are enriched in tungsten and
rhenium while the eutectic regions are
depleted of these two elements. The
eutectic regions, however, are enriched
in tantalum, titanium, and aluminum
while the dendrites are depleted of
them.5,6
This behavior is seen in the partitioning coefficients. The partitioning coefficients and average concentrations of
selected elements for the three zones
within the microstructure are given in
Table C. A value of one indicates no
deviation from the average composition
of the alloy and, hence, no significant
segregation.
A value greater than one corresponds
to an element that partitions, or segregates, to the dendrite core, while a
value lower than one corresponds to an
36
Samples were prepared for metallographic examination from each of the heat-treated
conditions discussed previously, including before and after the long-term exposure to
temperature. Disks approximately 0.4-cm thick were cut from the bars that were to be
observed metallographically. These disks were mounted and polished prior to etching.
The etch selected for this investigation was Pratt &Whitney etch #17 (100 mL H2O + 100
mL HNO3 + 100 mL HCl + 3 g MoO3). This etch is a γ′ etch that selectively dissolves
the γ′ in the microstructure. Scanning electron microscopy and light-optical microscopy
were used to observe the microstructure.
Differential Thermal Analysis
Differential thermal analysis (DTA) was also performed for all six conditions. Again,
0.9-cm thick disks were cut from the single crystal test bars and sent to Dirats Laboratories
(Westfield, MA) for analysis. Data was gathered upon heating in a DuPont (Philadelphia,
PA) 9000 series differential thermal analyzer. The liquidus, solidus, and γ′ solvus
temperatures were then determined and reported. The DTA unit was calibrated prior to
testing with high-purity nickel at a scan rate of 20°C/min. A purged high-purity argon
atmosphere with a flow rate of approximately 50 cm3/min. was used in all tests. Samples
were contained in high-purity alumina crucibles during testing.
Specimen Preparation for Creep Testing
Two creep tests were performed on specimens of each of the six different conditions.
Each sample was low-stress ground to make a creep rupture specimen with gauge diameter
element that partitions to the eutectic
region. From these values it can be
seen that tungsten and rhenium heavily
partition to the dendrite, and tantalum,
titanium, and aluminum heavily partition
to the eutectic. The three elements
nickel, cobalt, and chromium do not
heavily partition to either the dendrite or
the eutectic. This behavior can be easily
observed because of the relatively small
compositional changes among the three
regions of the microstructure.
Solution heat treatments, though long
and expensive, are required to achieve a
desirable microstructure. Following the
standard solution heat treatment, the
microstructure becomes very uniform.
The γ/γ′ eutectics have been dissolved
and a fine γ′ (approx. 0.3–0.5 µm) is
apparent throughout the microstructure
(Figure 2a).7 Once the solution heat
treatment is completed, the aging heat
JOM • March 2003
of 0.5 cm and a gauge length of 2.6 cm. The tests were performed on Satec creep frames
at a constant load to produce an initial stress of 310 MPa (45 ksi) and at a temperature of
950°C. The specimens were attached to an extensometer that was connected to an LVDT
to measure displacement. Three type-K thermocouples were attached along the gauge
section to monitor and maintain a constant temperature.
Microprobe Analysis
As-cast CMSX-4 is highly segregated, which is a large contributor to the need for a
solution heat treatment.5,6 One method to analyze the degree of segregation is microprobe
analysis. The analysis was conducted on an as-cast metallography sample. This sample
was observed in the polished, but unetched condition. Three types of microstructural
regions were examined to determine local concentrations of alloying elements: dendrite
cores, eutectic regions, and the interdendritic region. Five different representative sites
for each of the three regions were selected for analysis. From the composition data,
partitioning coefficients of the alloying elements can be determined. The partitioning
coefficient, k′ = (dendritic composition)/(interdendritic composition), indicates how
strongly a given element segregates to either the dendrite or the eutectic regions. The
higher the partitioning coefficient is, the higher the concentration of a given element in the
dendrite core region. Conversely, the lower the partitioning coefficient is, the lower the
concentration of a given element in the dendritic region with respect to the interdendritic
region. For example, the weight percentage of W in a dendrite of as-cast CMSX-4 is 1.69
times as great as the weight percentage of W in the interdendritc region (Table C).
a
b
20 µm
10 µm
Table A. Composition of CMSX-4 (Heat 6V0051) in Weight Percent
Ni
Cr
Co
Mo
W
Ta
Re
Al
Ti
Hf
Bal.
6.4
9.6
0.6
6.4
6.6
2.9
5.64
1.03
0.1
Table B. Heat Treatment Schedules
Heat Treatment
c
Standard 1,277°C/4 h → 1,287°C/2 h → 1,296°C/3 h → 1,304°C/3 h → 1,313°C/2 h → 1,316°C/2
h → 1,318°C/2 h/GFC*
Modified 1,210°C/2 h → ramp 16.7°C/h → 1,285°C/2 h → ramp 1.67°C/h → 1,287°C/0.5 h/GFC*
Aging HT 1,140°C/6 h/AC** → 871°C/20 h/AC**
* Gas furnace quench
** Air cooled
Table C. Average Compositions of CMSX-4
Dendritic Pattern and Partitioning Coefficients (in wt.%)
Average Composition
Dendrite Core
Interdendritic Area
Eutectic Area
Partitioning Coefficient
Ni
W
Re
Al
Ti
Ta
Co
Cr
61.42
60.06
61.94
61.64
0.97
6.40
7.91
5.86
4.67
1.69
2.90
5.82
3.63
2.87
2.03
5.64
4.94
5.71
5.61
0.88
1.03
0.59
0.92
1.18
0.50
6.60
4.23
6.23
6.93
0.61
9.60
10.34
9.79
10.07
1.03
6.40
6.10
5.94
7.03
0.87
treatment is performed. Aging serves
to slightly coarsen the γ′ to 0.5 µm
in size and to form the very uniform,
cuboidal structure often associated with
γ′ (Figure 2b).
The modified heat treatment is the
PWA 1480 heat treatment developed by
Pratt & Whitney. The PWA 1480 treatment utilizes lower temperatures and
a shorter duration. The microstructure
obtained by the modified heat treat2003 March • JOM
ment is considerably different from
the standard solution heat treatment.
The eutectic γ/γ′ regions are not fully
dissolved and large regions of extremely
coarse γ′ are present throughout the
material (Figure 3a). In addition, a high
degree of segregation is still evident
following the modified heat treatment.
The aging heat treatment does manage
to form the proper γ′ structure within the
dendritic regions, however, due to the
10 µm
Figure 3. Scanning-electron micrographs
following PWA 1480 solution heat treatment. (a) Partially dissolved γ/γ′ eutectic
region with coarse γ′ at boundaries. (b)
Coarse structure at dendrite/eutectic
boundary. (c) Following aging, γ′ in
dendrite is the proper size and shape,
however, a distinct boundary is present
between the dendrite and the eutectic.
large undissolved eutectic regions, a fine
γ′ structure is not formed throughout the
microstructure (Figure 3b). Furthermore,
distinct boundaries are present and are
observed between the dendrites and the
eutectic areas. Here, a rapid change
from fine γ′ to exceedingly coarse γ′ is
readily noticeable (Figure 3c).
The samples then undergo longterm, high-temperature exposure to test
microstructural stability. Following the
long-term exposure, metallography was
performed on the test bars. The as-cast
material was found to have significant
coarsening and topologically close
packed (TCP) formation. Some of the
eutectic regions were dissolved to form a
37
a
20 µm
b
20 µm
Figure 4. Scanning-electron micrographs topologically close packed (TCP) phases in
as-cast CMSX-4. (a) Following the 1,000 hour exposure, the γ′ coarsened, the γ/γ′
eutectic partially dissolved, and some TCP phases formed along the dendrite/eutectic
border (light gray, top of micrograph). (b) TCP phase enveloped in γ′ surrounded by
an overly coarsened γ/γ′ matrix.
microstructure similar to that formed by
the PWA 1480 heat treatment yielding
the highly coarse structure that was
present at the borders between the
dendrites and the eutectic areas. The
TCP phases primarily formed in regions
near these dendrite/eutectic borders
(Figure 4a). They were also typically
enveloped in γ′, meaning the continuous
matrix is interrupted by long γ′ structures
with TCP phases inside (Figure 4b).
The material with the PWA 1480
heat treatment produced similar results.
The γ/γ′ matrix coarsened significantly
during the long high-temperature
exposure. In addition, the residual
eutectic areas continued to dissolve into
the microstructure (Figure 5a), possibly
due to the high degree of segregation that
remained in the microstructure following
the solution heat treatment. Furthermore,
TCP phases were frequently observed
throughout the microstructure, though
not to the degree as that observed in the
as-cast microstructure (Figure 5b).
The standard solution heat treatment
produced results considerably different.
There was still γ′ coarsening evident
(Figure 6a). The difference, however,
is in eutectic dissolution and TCP
resistance. First, since there were no
residual eutectics to be dissolved, this
type of behavior was not observed in
the material with the standard solution
heat treatment. Second, the amount of
TCP observed was greatly reduced.
TCP formations were still present, but
to a much smaller scale (Figure 6b). The
uniform microstructure associated with
the standard heat treatment appears to
be much more TCP resistant.
38
DIFFERENTIAL THERMAL
ANALYSIS
Differential thermal analysis was
performed on the materials in all six
conditions [before and after long-term,
high temperature exposure (Table I)].
Prior to exposure, both the standard and
modified heat-treated materials exhibit
almost identical liquidus and solidus
temperatures, the only difference being
the γ′ solvus temperatures. The as-cast
material exhibits a thermal response
very different from either of the two
heat-treated conditions. Although there
is a rather large difference in the
microstructures, only a small difference appears upon differential thermal
analysis prior to the long-term exposure
test. Following the exposure, the
modified and standard heat treatments
exhibited very different DTA responses.
While the liquidus temperatures are the
a
50 µm
same, the solidus temperature of the
material with the PWA 1480 solution
heat treatment dropped slightly. The
γ′ solvus temperatures, though higher,
maintained the same difference as before.
The solidus temperatures, however,
behaved differently. Long-term, hightemperature exposure slightly decreased
the solidus of the modified alloy, while
the solidus of the standard material
experienced an increase. These differences, though, are small and may be
within experimental scatter. The uniform
structure produced during the standard
solution and aging heat treatments
appears to be more stable with only
small changes in the liquidus, solidus,
and solvus temperatures. The irregular
structure observed due to the modified
heat treatment still exhibits significant
segregation, which results in some
amount of diffusion during the longterm heat treatment and a drop in the
solidus temperature.
CREEP TESTING
As seen in Table II, the samples with
the standard solution heat treatment
outperformed the other treatments in all
aspects of creep testing. These samples
exhibited creep lives of 380 hours and
568 hours, which exceeded all other
specimens by at least 83 hours. In
addition, although the aging of 1,000°C
for 1,000 hours proved to degrade
properties in each of the sets of heat
treatments, the standard treatment
samples exhibited longer creep lives
than the other aged specimens.
The samples with the modified solution heat treatment surpassed the as-cast
b
20 µm
Figure 5. Scanning-electron micrographs of long-term, high-temperature exposure effects
on CMSX-4 with the PWA 1480 solution heat treatment. (a) Continued dissolution of γ/γ′
eutectic along with γ′ coarsening within the dendrites. (b) TCP phases in the material with
the modified solution heat treatment.
JOM • March 2003
a
10 µm
b
produced by this heat treatment, however, retained some segregation from the
as-cast stage in its production. This heat
treatment was not able to dissolve the
γ/γ′eutectics and as a result, produced a
very coarse microstructure. Differential
thermal analysis (DTA) of the two
heat-treated conditions, however,
produced similar results. The solidus,
liquidus, and γ′ solvus temperatures
were approximately the same for the
two heat treatments.
Following the solution and aging
heat treatments, the test bars received a
long-term, high-temperature exposure.
The purpose of this exposure is to
reveal the reaction of the material to a
simulated operating environment over
an extended period of time. During
this exposure, typically two responses
will occur. First, the γ′ structure will
continue to coarsen. Rafting of the γ′
is another response associated with
coarsening for long periods of time.
The second possible response is the
formation of TCP phases. Though the
effect of TCP phases on a single crystal
component is not fully understood, two
possibilities do arise. First, though many
morphologies are possible, the needle
or plate-like morphology is common,
which can act as a stress raiser with the
microstructure. Second, TCP phases also
remove the solid-solution strengtheners tungsten and rhenium from the
10 µm
Figure 6. Scanning-electron micrographs of long-term, high-temperature exposure effects
on CMSX-4 with the standard solution heat treatment. (a) Coarsening of γ′ is exhibited in
this alloy, but to a lesser degree. (b) TCP formations were present in the microstructure;
however, they are fewer and smaller in size when compared to the as-cast and PWA
1480 heat-treated conditions.
structure in rupture life even after being
heated for 1,000 hours at 1,000°C.
Therefore, although the PWA 1480
heat treatment does not produce a
CMSX-4 alloy with creep life equivalent
to material with the standard heat treatment, it does improve its properties.
The solution heat treatment also
affected the percent creep at rupture.
The as-cast samples exhibited 21% and
30% creep. For the PWA 1480 solution
heat treatment samples, the as-heattreated condition produced 24% and
28% creep while the standard heat
treatment produced 27% and 28% creep.
Following the exposure heat treatment,
the PWA 1480 heat-treated samples
proved to have the most total plastic
strain of the two solution heat treatments.
Following the exposure, the PWA 1480
heat treatment samples exhibited 35%
and 36% creep while the standard solution heat treatment actually produced
lower elongation values than it recorded
before the long-term exposure. One of
the as-cast samples, after the long-term
exposure, produced the greatest percent
creep with a value of 44%.
leads to the preferred microstructure for
CMSX-4. To get this microstructure,
however, the material spent almost two
full days in a furnace at high temperature.
A reduction in the temperature or time
required for heat treatment, therefore,
could save money.8 However, it is not
desirable to alter the heat treatment
process in such a way that degrades the
material’s performance.
As a result of the need for a more
cost-effective heat treatment, a different
solution heat treatment was selected.
The PWA 1480 solution heat treatment
is a commercial heat treatment that
utilizes lower temperatures and a shorter
overall duration. The microstructure
Table I. DTA Results of CMSX-4 (Temperatures in °C)
Before Long-Term Exposure
Liquidus
Solidus
γ Solvus
As-Cast
PWA 1480
Standard
As-Cast
PWA 1480
Standard
1,383
1,322
N/A
1,386
1,340
1,276
1,386
1,341
1,286
1,384
1,327
N/A
1,385
1,338
1,280
1,386
1,344
1,290
Table II. Creep Results for CMSX-4
DISCUSSION
As-cast CMSX-4 has a highly segregated, two-phase γ/γ′ microstructure.
It is this chemical segregation, or
compositional gradient, that causes
the material to require a solution heat
treatment.2 By solution heat treating
CMSX-4, the effects of segregation can
be drastically reduced. In addition to
the solution heat treatment, one or more
aging heat treatments are utilized to
fully strengthen the material. This step
2003 March • JOM
After Long-Term Exposure
Sample ID
PWA 1480 HT/aging HT
PWA 1480 HT/aging HT/
1,000°C for 1,000 h
STD soln HT/aging HT
STD soln HT/aging HT/
1,000°C for 1,000 h
As Cast
As cast/
1,000°C for 1,000 h
302-1
302-2
308-1
308-2
316-1
316-2
320-1
320-2
305-1
305-2
309-1
309-2
Time to Rupture
Time to 1%
Total % Creep
(h)
Creep Elongation (h)
Elongation
204
251
154
212
568
380
297
200
182
170
141
133
74.50
104.38
49.00
84.19
81.08
170.45
100.18
89.43
53.50
62.05
30.18
35.11
23.95
28.32
36.53
34.78
27.33
28.65
26.68
15.82
30.09
21.26
26.90
44.33
39
surrounding matrix. These two effects
together within the microstructure
raise concerns about the possibility
of premature failure. As a result,
TCP phases are typically considered
undesirable.9–11
The long-term exposure affected
the as-cast material by producing a
coarse microstructure and TCP phases
throughout the material. Topologically
close-packed phases were prevalent in
the as-cast microstructure. The standard
solution heat treatment, however,
produced different results. The uniform
microstructure that was produced after
aging was only slightly coarsened with
a few small TCP formations present.
The PWA 1480 solution heat treatment,
however, resembled the as-cast material
more than the standard solution heat
treatment. The large γ/γ′ eutectic regions
that were still present following the
aging heat treatment were surrounded
by a very coarse microstructure. The
TCP formations were very prevalent
within this microstructure as well.
The retained segregation was still
evident, even after the long-term, hightemperature exposure.
Creep testing is critical in understanding how a material will behave
when subjected to a high-stress, hightemperature environment. This is
especially the case for turbine blade
materials like CMSX-4. Again, as
with microstructure, creep behavior is
directly impacted by the applied heat
treatments. All six conditions were
tested to determine any differences in
the two solution heat treatments and any
differences in the rate at which creep
properties degrade.
Of the three heat-treated conditions,
the standard heat treatment produced
the longest creep life. The PWA 1480
solution heat treatment produced a
definite increase in creep life over the
as-cast material; however, it was still
40
much lower than the standard solution
heat treatment. The same trend can be
seen in the time required to reach 1%
creep. The percent creep at rupture was
similar for the three conditions prior to
the exposure heat treatment. Following
the exposure, however, the as-cast
and PWA 1480 conditions experienced
significant increases in percent creep
at rupture. The standard solution heat
treatment, however, actually experienced
a slight decrease.
CONCLUSIONS
The modified heat treatment proved
to be insufficient to produce an acceptable homogeneous microstructure in
CMSX-4. A high degree of retained
segregation persists after the PWA
1480 heat treatment of CMSX-4. Upon
exposure to 1,000°C for 1,000 hours,
the modified samples resembled the
as-cast microstructure more than the
microstructure produced by the standard
solution heat treatment. Creep testing
produced similar results. The creep
properties of the sample with standard
heat treatments were superior to those
of the as-cast and modified heat treatment specimens. Long-term exposures
resulted in the formation of significant
amounts of γ′ coarsening and TCP
phases in the as-cast and modified
heat treatment samples, which further
degraded the creep properties of these
samples. Only a limited amount of
γ′ coarsening and TCP phases were
observed in samples with the standard
heat treatments, which resulted in a
limited decrease in creep strength. The
modified heat treatment produced an
improvement over the as-cast material;
however, it is not as good as the standard
solution heat treatment, therefore, the
modified heat treatment examined in
this study does not appear to be an
acceptable alternative to the standard
solution heat treatment.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the support of the National Science
Foundation (award number 0072671)
and Tom VanVranken at PCC Airfoils
for supplying the heat treatments of
the samples.
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Structural Materials and Protective Coatings, ed.
A.K. Koul et al. (Ottawa, Canada: NRC of Canada,
1994).
9. W.S. Walston, J.C. Shaeffer, and W.H. Murphy, “A New
Type of Microstructural Instability in Superalloys-SRZ,”
Superalloys 1996, ed. R.D. Kissinger et al. (Warrendale,
PA: TMS, 1992), pp. 9–18.
10. E.W. Ross and K.S. O’Hara, “René N4: A First
Generation Single Crystal Turbine Airfoil with Improved
Oxidation Resistance, Low Angle Boundary Strength
and Superior Long Time Rupture Strength,” Superalloys
1996, R.D. Kissinger et al. (Warrendale, PA: TMS,
1992), pp. 19–25.
11. S.T. Wlodek, “The Stability of Superalloys,” Long
Term Stability of High Temperature Materials, ed.
G.E. Fuchs, K.A. Dannemann, and T.C. Deragon
(Warrendale, PA: TMS, 1996), pp. 3–40.
B.C. Wilson, J.A. Hickman, and G.E. Fuchs are
with the Department of Materials Science and
Engineering at the University of Florida.
For more information, contact G.E. Fuchs, University
of Florida, Department of Materials Science and
Engineering, P.O. Box 116400, Gainesville, FL
32611-6400.
JOM • March 2003