C-M Harris Single crystal alloy
technology (CMSX-4) 1987 US4643782A
Publication
Number: US4643782A
Publication
Date: 1987-02-17
Priority
Number: US1984591023A
Application
Date: 1984-03-19
Title:
Single crystal alloy technology
Inventor
- w/address: Harris Kenneth,Spring
Lake,MI,US | Erickson Gary L.,Muskegon,MI,US
Assignee/Applicant:
Cannon Muskegon Corporation,Muskegon,MI,US
Front Page Drawing:
Abstract:
Single crystal nickel-base superalloy having very fine gamma
prime after heat treatment in a temperature range permitting total gamma prime
solutioning without incipient melting. Component of the heat treated alloy has
exceptional resistance to creep under high temperature and stress, particularly
in that part of the creep curve representing one percent or less elongation.
The alloy exhibits exceptionally low steady-state creep rate.
First Claim:
1. A single crystal
casting to be used under high stress, high temperature conditions characterized
by an increased resistance to creep under such conditions and a heat treat
window of approximately 35° F. cast from an alloy consisting essentially of the
following elements in the following proportions expressed as percentages of
weight:
______________________________________
Co 9.6
Cr 6.6
Mo 0.6
W 6.4
Ta 6.5
Al 5.6
Ti 1.0
Hf 0.10
Re 3.0
Ni Bal
______________________________________
BACKGROUND OF THE INVENTION
This invention relates
to nickel-base, single crystal superalloys for use in casting components for
high stress, high temperature gas turbine applications.
This alloy and alloys
of this type are intended for single crystal casting of gas turbine blades,
particularly such blades used in maximum performance aircraft turbine engines
where high resistance to thermal fatigue, creep and oxidation are essential.
The basic technology
of alloys for the casting of single crystal components is described in U.S.
Pat. Nos. 3,494,709, 4,116,723 and 4,209, 348. An alloy materially improved
over those described in these patents is described in my co-pending application
for U.S. Patent Ser. No. 339, 318, filed Jan. 15, 1982, entitled "Single
Crystal (Single Grain) Alloy" U.S. Pat. No. 4,582,548. The alloys of said
application are marketed under the trademark "CMSX" as CMSX-2 and
CMSX-3 and are hereafter identified as CMSX-2 and CMSX-3. This invention is a
creep strength/creep rate improvement on the alloys described in that
application. It will be marketed under the "CMSX" trademark as
CMSX-4.
Many factors enter
into what constitutes an alloy having not only the desired mechanical and
environmental performance features but also practical single crystal
castability and heat treating characteristics. These two characteristics, i.e.,
component mechanical/environmental properties response and commercial
producibility, normally conflict with one another in that alloy compositions
which result in the desired performance features many times have impractical
casting and heat treatment characteristics. Likewise, the reverse is also true.
Among the problems
encountered is that of creating an alloy having sufficient creep-rupture and
oxidation resistance while also exhibiting a heat treatment temperature range
which permits it to be heat treated to a temperature at which all of the
primary gamma prime goes into solution without the alloy reaching its incipient
melting temperature. In addition, it is desirable that at least some of the
eutectic also be solutioned. This range is called the heat treatment
"window". Failure to provide a practical "window" such as
25° F. or more makes it all but impossible to heat treat the castings without
unacceptable diminishing mechanical properties or scrap rate due to either
incomplete solutioning of the primary gamma prime or incipient melting. An
alloy which cannot be effectively heat treated to total solutioning of the
primary gamma prime without experiencing a significant rejection rate due to
incipient melting is little more than a laboratory curiosity and is without
practical utility. The alloys CMSX-2 and CMSX-3 described in U.S. patent
application Ser. No. 339,318, U.S. Pat. No. 4, 582,548, noted above, are alloys
which overcome this problem. In a variation of the basic alloy (CMSX-2)
described in that application, the coated oxidation/corrosion performance of
the alloy was increased with the addition of hafnium (CMSX-3) while retaining a
practical solution heat treatment window.
The present invention
is the result of research having the objective of retaining the desirable
commercial producibility characteristics typical of the CMSX-2 and CMSX-3
alloys while significantly improving the high temperature, high stress, creep
resistance of the alloy.
Creep in the form of
permanent, plastic deformation of an alloy is normally defined in terms of the
percentage of elongation occurring at a given temperature/stress over a
specific time period before rupture occurs. While increasing the time and
temperature at which creep translates into rupture is important data concerning
alloy characteristics, it is not particularly significant to the designer of
high performance aircraft turbine engines. The designer is more concerned with
the time and temperature range at which a specific percentage of creep will
occur. This is important because it determines when and under what conditions a
specific dimensional change in the casting will cause malfunction such as
results from physical interference between engine components. As the
performance characteristics of engine designs increase the clearance between
components normally must be decreased, reducing the amount of creep which can
be tolerated. At the same time, the increased performance characteristics
normally necessitate higher temperatures and speeds, factors which increase
stress and accelerate creep. In the modern, high performance, gas turbine
engine, designers consider one percent creep the practical limit of
acceptability.
The invention provides
a nickel-base superalloy which retains the practical heat treatment
"window", alloy stability and single crystal castability
characteristics of the CMSX-2 and-3 alloys while very significantly improving
creep resistance characteristics by increasing both the permissible operating
temperature and the time before a one percent creep will occur. This has been
done by increasing the refractory element components of the alloy, including
the addition of rhenium. The invention achieves a very significant increase in
creep resistance without creating alloy instability which can result from the
incorporation of rhenium.
BRIEF DESCRIPTION OF
THE DRAWINGS
FIG. 1 is a graph of
comparative Larson Miller stress-rupture tests on the alloy of this invention
and CMSX-2;
FIG. 2 is a
photomicrograph of one of the alloys developed in the creation of this
invention taken at 1000× after the specimen has been tested at 36.0 ksi/1800°
F./194.3 hours;
FIG. 3 is a
photomicrograph of the alloy of this invention at 1000. times.;
FIG. 4 is a
photomicrograph of the alloy of this invention taken at 1000× near fracture
after testing at 30 ksi/1800° F. /507. 5 hours;
FIG. 5 is a
photomicrograph of the alloy of this invention taken in the thread section of the
test specimen at 1000×;
FIG. 6 is a
photomicrograph of the alloy of this invention taken at the thread section at
1000× after 36 ksi/1800° F./174. 8 hours;
FIG. 7 is a
photomicrograph of the alloy of this invention taken near the fracture at 1000×
after testing at 36 ksi/1800° F. /178. 0 hours;
FIG. 8 is a
photomicrograph of the alloy of this invention taken near the fracture at 1000×
after testing at 75 ksi/1600° F. /232. 6 hours;
FIG. 9 is a
photomicrograph of the alloy of this invention taken at 1000× at the thread
section after testing at 75 ksi/1600° F. /232.6 hrs;
FIG. 10 is a
photomicrograph of the alloy of this invention taken at 1000× of the alloy of
this invention after heat treatment of 2 hours/2350° F. (1288° C.)+2
hours/2360° F. (1293. degree. C.) slow cool+2 hours/2350° F. (1288° C.)+2
hours/2360° F. (1293° C.) GFQ;
FIG. 11 is a graph
comparing the full range of creep characteristics of the CMSX-3 alloys and the
alloy of this invention; and
FIG. 12 is a graph of
the creep characteristics to one percent deformation of the alloy CMSX-3
compared with the creep characteristics of the alloy of this invention.
DESCRIPTION OF THE
PREFERRED EMBODIMENT
The basic alloy from
which this invention was developed was the hafnium containing, single crystal
alloy described in U.S. patent application Ser. No. 339,318, U.S. Pat. No.
4,582,548, hereafter identified as CMSX-3. This alloy has the composition
expressed as percentages of weight: ##TBL1##
Based upon the known
alloy technology relating to the use of rhenium as a refractory element and,
therefore, expected to increase the strength of an alloy, 3% by weight rhenium
was initially substituted for 3% by weight tungsten. Cobalt was substantially
increased and chromium was significantly decreased. The result did not produce
the desired increase in strength. However, the substantial increase in cobalt
did have the desired result of giving good alloy stability.
In a further effort to
develop the desired creep resistance without scarificing corrosion resistance
the chromium content was increased from 6.8% to 7.7%. This also was found not
to produce the expected or desired improvement.
This alloy, like the
first attempt to provide a high creep resistant alloy for use under high stress
and temperature while exhibiting no tendency to form mu or sigma phase
formations failed to produce the desired increase in strength and, thus,
resistance to creep.
In an effort to take a
different approach to the creep problem the chromium content was reduced and
the refractory elements tungsten and tantalum were both increased as
percentages of weight. This results in an alloy having the composition:
##TBL2##
This alloy provided
the desired creep strength but at the price of instability when tested at 36
ksi (248 MPa)/1800° F. (982. degree. C.)/194.3 hours in stress rupture tests.
This alloy under high
temperature stress began to evidence the formation of "needles" or
acicular characteristics of the chrome-rhenide sigma phase (see FIG. 2). This
is evidence of instability, a characteristic of unbalanced rhenium and chromium
content. Thus, while rhenium is a known strengthener of alloys of this type it
also is prone to creating conditions which materially shorten the useful life
of the alloy since the chrome-rhenide acicular formation provides a
creep-strength reducing and crack propagating condition.
In an effort to try to
avoid the problem of instability and yet develop the desired creep resistance,
the composition of the alloy was again changed to increase the nickel content
and reduce the chromium and cobalt while retaining the increased level of
refractory elements.
The resulting alloy
had the preferred composition as percentages of weight: ##TBL3## and an
operative composition in the range of: ##TBL4##
At the same time
unwanted trace elements were restricted to as little as possible: ##TBL5##
The result was a
sudden and substantial change in the alloy characteristics. Heat VF552 was
prepared having the composition of Table III and single crystal specimens were
cast using a high thermal gradient withdrawal process. Stress-rupture test
specimens 0.089 inch (2.26 mm)/0. 070 inch (1.78 mm) diameter were machined
from the longitudinal (001) direction with all being oriented within 10° of the
primary axis. Also RT Tensile and Creep Rupture 0.188 inch (4.78 mm) diameter
specimens were machined from the longitudinal (001) direction, all being
oriented within 10° of the primary axis. The specimen heat treat was 2 hours at
2350° F. (1288° C.)+2 hours at 2360° F. (1293° C.) slow cool+2 hours at 2350°
F. (1288° C.)+ 2 hours at 2360° F. (1293° C.) GFQ by Vac Hyde Processing
Corporation. In addition the specimens were Pseudo coated 5 hours at 1800.
degree. F. (982° C.) air cooled and then aged 20 hours at 1600. degree. F.
(871° C.) then air cooled. The machining and testing was performed by Joliet
Metallurgical Laboratories. The results of comparative stress-rupture tests
conducted with the alloy of this invention and the alloys CMSX-2 and CMSX-3 are
set out in the following table: ##TBL6##
In addition
comparative creep rupture tests with CMSX-3 after Onera heat treatment were
conducted and the results are set out in the following table: ##TBL7##
RT Tensile tests were
conducted on the alloy of this invention with the results set out in the
following table: ##TBL8##
Comparative Larson
Miller Stress Rupture tests were conducted on MFB 0. 070" and 0.089"
diameter machined within 10° of (001) on specimens cast as single crystals from
the alloy of Table IV (heat VF 552) and of CMSX-2, heats VF267, 312, 438, 466
and 472. The results appear in the graph, FIG. 1. The photomicrographs, FIGS.
3-9, clearly show the uniformity of the microstructure of the alloy and its
stability under high temperature and stress.
FIGS. 11 and 12
graphically illustrate the very substantial increase in creep resistance. FIG.
11 illustrates the fact that the time required to rupture at 36.0 ksi at 1800°
F. is increased over 100%. This is a very substantial change. FIG. 12, however,
is even more significant in that it graphically illustrates the even greater
change the invention accomplishes in that portion of the creep curve which is
1% or less which is the range of primary interest to gas turbine design
engineers. It will be seen that the initial part of the curve has been
substantially flattened. Thus, the period required to produce a 1% creep under
the high temperature/stress condition of 36 ksi/1800° F. has been increased
from approximately 31 hours to 80 hours, an increase of about 158%. This is a
severe test and its significance is that the major portion of the change in
creep resistance occurs in the time required to produce the first 1.0% of
elongation. Therefore, this change is functionally significant because it
occurs in that phase of the alloy's characteristics which determine its
functionality and practical utility. The reduction in chromium content has
reduced the N.sub.V3 B number of the alloy with the alloy having an N.sub.V3 B
number of 2.25 max. The reduction in chromium and anticipated lowering of
environmental properties is considered acceptable in view of the major change
in creep characteristics.
The photomicrographs,
FIGS. 4 and 5, clearly show the much finer gamma prime grain structure
following stress-rupture testing resulting from the addition of rhenium. The
change in the morphology of the gamma prime resulting from the addition of
rhenium was discovered when the post-treat microstructure of test pieces made
from the alloy were observed. It was also found that rhenium not only restricts
initial growth or directional coarsening of the gamma prime phase during
stress/temperature exposure, it continues to perform this function in the alloy
as the time/temperature/stress load applied to the alloy is increased. It is
observed from FIGS. 4 and 7 that even near the fracture and under the same
temperature/time/stress conditions as produced the photomicrographs of FIGS. 5
and 6, the "rafting" (directional coarsening) of the gamma prime has
progressed only to a small degree in comparison with that which would have been
observed under identical conditions had the alloy been heretofore known
nickel-based superalloys developed for single crystal casting technology, such
as either CMSX-2 or CMSX-3. This has been found to be true despite the fact
that rhenium, being a refractory element, while valuable to increase alloy
strength, is also known in the art to destabilize alloys when they are
subjected to high stress at high temperatures as clearly appears in the
formation of acicular chrome-rhenide needles shown in FIG. 2, when a higher
chromium content was utilized. The amenability of the alloy of this invention
to full solutioning of the ascast gamma prime to produce a fully solutioned
microstructure containing 65-68 volume percent of re-precipitated fine,
coherent gamma prime with an average size of 0.45 micron or less is believed to
be a contributing reason the alloy displays its ability to maximize its creep
resistance, especially in the initial stages of creep resulting in the
especially important changes in the creep resistance occurring below one
percent elongation. It is of further significance that the alloy has
accomplished this change in creep resistance without excessive narrowing of the
heat treatment window and without introducing microstructural instability. It
is also clear from the composition of the alloy as set out in Table IV that it
is an alloy of very precise composition with only a very small permissible
variation in any one element, if the combined characteristics of creep control,
high temperature and stress microstructural stability and a practical heat
treat window are to be maintained. Any adverse change in any one of these
characteristics, even a minor change, would materially lessen and adversely
affect the alloy's characteristics.
The heat treatment
window is particularly important because without an adequate temperature spread
between the solvus temperature for the gamma prime and the incipient melting
point, it is not possible to totally solution the primary gamma prime without
incurring a high scrap rate due to incipient melting. A spread of less than 20°
F. is impractical, except in the laboratory, because production heat treat equipment
cannot be accurately controlled to maintain a temperature within the range,
especially in view of the high temperatures and long heat treat periods
required. Because of the higher refractory element content of this alloy the
heat treat periods should be increased from four to six to eight hours. The
fact that this alloy has a solution heat treat temperature spread of 30° F.-40°
F. assures complete solutioning of the primary gamma prime without risk of
incipient melting. Tests have indicated that it is practical to stay within the
35° F. window using available commercial vacuum solution heat treatment
equipment.
While this alloy can
be used for any purpose requiring high strength castings incorporating a single
crystal, its particular use is in the casting of single crystal blades and
vanes for gas turbine engines. While its primary use is in aircraft turbine
engines, there are stationary engine applications requiring the specialized
high performance characteristics of this alloy. Particularly is this the case
in turbine engines, the performance characteristics of which require very
restricted clearances thereby materially limiting the amount of permissible
creep.
This alloy has
particular application in small turbine engines designed to develop high
performance characteristics. These engines, to obtain high performance, are
normally operated at higher component temperatures than larger engines.
Therefore, the problem of creep is increased. The alloy also has application in
small engines designed for short term performance at maximum possible
temperature and stress to maximize their performance characteristics. In both
cases, creep in excess of one percent is considered unacceptable. The creep
characteristics of heretofore known state of the art alloys have limited
operating temperatures and, thus, maximum performance capability. Tests which
have been conducted on the alloy of this invention indicate a 32. degree. F.
(17.8° C.) stress-rupture temperature capability advantage at the 35 ksi (241
MPa) stress level compared with the most advanced state-of-the-art high nickel
super-alloys such as CMSX-2 and CMSX-3. On the basis of time to one percent
creep the temperature capability advantage is 45° F. (25. 0° C.). Tests have
also indicated capability of high temperature-stressed operation as high as
2000° F. (1093° C.), in short-life very high performance gas turbine engines.
The alloy has exhibited excellent oxidation resistance in an oxidizing
environment under these maximum operating conditions.
The invention, with a
very precise alloy composition, provides an alloy capable of high
temperature-stress performance with a significant advantage in creep
resistance/creep rate especially in that portion of the range which is
practical for engine design.
Claims: We claim:
1. A single crystal
casting to be used under high stress, high temperature conditions characterized
by an increased resistance to creep under such conditions and a heat treat
window of approximately 35° F. cast from an alloy consisting essentially of the
following elements in the following proportions expressed as percentages of
weight:
______________________________________
Co 9.6
Cr 6.6
Mo 0.6
W 6.4
Ta 6.5
Al 5.6
Ti 1.0
Hf 0.10
Re 3.0
Ni Bal
______________________________________
2. A single crystal
casting to be used under high stress, high temperature conditions characterized
by an increased resistance to creep under such conditions and a heat treat
window of approximately 35° F. cast from an alloy consisting essentially of the
following elements in the following proportions expressed as percentages of
weight:
______________________________________
Co 9.3-10.0
Cr 6.4-6.8
Mo 0.5-0.7
W 6.2-6.6
Ta 6.3-6.7
Al 5.45-5.75
Ti 0.8-1.2
Hf 0.07-0.12
Re 2.8-3.2
Ni Bal
______________________________________
3. The single crystal
casting claimed in either claims 1 or 2 wherein the alloy from which it was
cast is characterized by the capability of complete solutioning of the primary
gamma prime without incipient melting.
4. The single crystal
casting as claimed in claim 3 further characterized by the capability of the
casting to withstanding 36 ksi at 1800° F. for an average 80 hours before
sustaining a creep of one percent.
5. A gas turbine blade
cast as claimed in either claims 1 or 2.
6. A gas turbine vane
cast as claimed in either claims 1 or 2.
7. The casting as
claimed in claim 3 wherein the casting is a gas turbine engine blade.
8. The casting as
claimed in claim 3 wherein the casting is a gas turbine engine vane.
9. The single crystal
casting claimed in claim 3 further characterized by the absence of phase
instability after testing under temperature and stress sufficient to rupture
the casting.
10. A component for
use in a gas turbine engine, cast as a single crystal from an alloy having an
incipient melting temperature at least 30° F. above its gamma prime solvus
temperature and consisting essentially of the elements Cr 6.4-6.8, Co 9.3-10.0,
Mo 0.5-0.7, W 6.2-6.6, Ta 6.3-6.7, Al 5.45-5.75, Ti 0.8-1.2, Hf 0.07-0.12, Re
2.8-3.2, balance Ni, said component after heat treatment at a temperature and
for a time period sufficient to solution all its primary gamma prime having the
capability of withstanding 36 ksi at 1800° F. (982.2° C.) for 80 hours before
sustaining a creep of one percent.
11. A component as
claimed in claim 10 wherein the heat treatment included 2 hours at 2350° F.
(1288° C.) plus 2 hours at 2360° F. (1293° C.) then slow cooled, then 2 hours
at 2350° F. (1288° C.) plus 2 hours at 2360° F. (1293° C.) followed by a gas
fan quench.
12. A component as
claimed in claim 10 wherein the heat treatment included 4 hours at 2350° F.
(1288° C.) plus 4 hours at 2360° F. (1293° C.) followed by a gas fan quench.
13. A component as
claimed in claim 12 wherein the component is aged 10 to 30 hours at 1600° F.
(871° C.) and then air cooled.
14. A component for
use in a gas turbine engine, cast as a single crystal from an alloy having an
incipient melting temperature at least 30° F. above its gamma prime solvus
temperature and consisting essentially of the elements Cr 6.4-6.8, Co 9.3-10.0,
Mo 0.5-0.7, W 6.2-6.6, Ta 6.3-6.7, Al 5.45-5.75, Ti 0.8-1.2, Hf 0.07-0.12, Re
2.8-3.2, balance Ni and being capable of complete primary gamma prime
solutioning with alternate heat treatment cycles.