Nickle base superalloy articles and method for making

Methods for increasing the forgeability of cast superalloy materials are described. An extremely overaged microstructure is developed by solutionizing the material and slow cooling in the vicinity of the solvus temperature to cause a precipitation of extremely coarse gamma prime material. Subsequently the material can be isothermally forged.

DESCRIPTION 
1. Technical Field 
This invention relates to the forging of gamma prime strengthened nickel 
base superalloy material, especially in cast form, and, in particular, to 
a heat treatment which improves the forgeability of such materials. 
2. Background Art 
Nickel base superalloys are widely used in gas turbine engines. One 
application is for turbine disks. The property requirements for disk 
materials have increased with the general progression in engine 
performance. Early engines used steel and steel derivative alloys for disk 
materials. These were soon supplanted by the first generation nickel base 
superalloys such as Waspaloy which were capable of being forged, albeit 
often with some difficulty. 
Nickel base superalloys derive much of their strength from the gamma prime 
phase. The trend in nickel base superalloy development has been towards 
increasing the gamma prime volume fraction for increased strength. The 
Waspaloy alloy used in the early engine disks contains about 25% by volume 
of the gamma prime phase whereas more recently developed disk alloys 
contain about 40-70% of this phase. The increase in the volume fraction of 
gamma prime phase reduces the forgeability of the alloy. Waspaloy material 
can be forged from cast ingot starting stock but the later developed 
stronger disk materials cannot be reliably forged and require the use of 
more expensive powder metallurgy techniques to produce a shaped disk 
preform which can be economically machined to the final dimensions. One 
such powder metallurgy process which has met with substantial success for 
the production of engine disks is that described in U.S. Pat. Nos. 
3,519,503 and 4,081,295. This process has proved highly successful with 
powder metallurgy starting materials but less successful with cast 
starting materials. 
Other patents relating to the forging of disk material include U.S. Pat. 
Nos. 3,802,938; 3,975,219 and 4,110,131. 
In summary, therefore, the trend towards high strength disk materials has 
resulted in processing difficulties which have been resolved only through 
recourse to expensive powder metallurgy techniques. 
It is an object of the present invention to describe a method through which 
cast high strength superalloy materials may be readily forged. 
It is another object of the present invention to describe a heat treatment 
method which substantially increases the forgeability of nickel base 
superalloy materials. 
Yet another object of the present invention is to provide a method for 
forging cast superalloy materials containing in excess of about 40% by 
volume of the gamma prime phase and which would otherwise be unforgeable. 
A further object is to disclose a combined heat treatment and forging 
process which will produce a fully recrystallized microstructure having a 
uniform fine grain size and which will substantially reduce forging 
stresses. 
It is yet another object of the invention to provide a highly forgeable 
nickel base superalloy article having super overaged gamma prime 
morphology with an average gamma prime size in excess of about 3 microns. 
DISCLOSURE OF INVENTION 
Nickel base superalloys derive most of their strength from the presence of 
a distribution of gamma prime particles in the gamma matrix. This phase is 
based on the compound Ni.sub.3 Al where various alloying elements such as 
Ti and Cb may partially substitute for Al. Refractory elements such as Mo, 
W, Ta and Cb strengthen the gamma matrix phase and additions of Cr and Co 
are usually present along with the minor elements such as C, B and Zr. 
Table I presents nominal compositions for a variety of superalloys which 
are used in the hot worked condition. Waspaloy can be conventionally 
forged from cast stock. The remaining alloys are usually formed from 
powder, either by direct HIP consolidation or by forging of consolidated 
powder preforms; forging of cast preforms of these compositions is usually 
impractical because of the high gamma prime content although Astroloy is 
sometimes forged without resort to powder techniques. 
A composition range which encompasses the alloys of Table I, as well as 
other alloys which appear to be processable by the present invention, is 
(in weight percent) 5-25% Co, 8-20% Cr, 1-6% Al, 1-5% Ti, 0-6% Mo, 0-7% W, 
0-5% Ta, 0-5% Cb, 0-5% Re, 0-2% Hf, 0-2% V, balance essentially Ni along 
with the minor elements C, B and Zr in the usual amounts. The sum of the 
Al and Ti contents will usually range from 4-10% and the sum of Mo+W+Ta+Cb 
will usually range from 2.5-12%. The invention is broadly applicable to 
nickel base superalloys having gamma prime contents ranging up to 75% by 
volume but is particularly useful in connection with alloys which contain 
more than 40% and preferably more than 50% by volume of the gamma prime 
phase and are therefore otherwise unforgeable by conventional (nonpowder 
metallurgical) techniques. 
In a cast nickel base superalloy the gamma prime phase occurs in two forms: 
eutectic and noneutectic. Eutectic gamma prime forms solidification 
process while noneutectic gamma forms by solid state precipitation during 
cooling after solidification. Eutectic gamma prime material is found 
mainly at grain boundaries and has particle sizes which are generally 
quite large, up to perhaps 100 microns. The noneutectic gamma prime phase 
which provides most of the strengthening in the alloy, is found within the 
grains and has a typical size of 0.3-0.5 micron. 
TABLE I 
______________________________________ 
Waspa- Astro- RENE RCM 82.sup.(3) 
IN 
loy loy 95 AF 115.sup.(2) 
MERL 76 100.sup.(1) 
______________________________________ 
Co 13.5 17 8 15 18 15 
Cr 19.5 15 13 10.7 12 10 
Al 1.3 4 3.5 3.8 5.0 4.5 
Ti 3.0 3.5 2.5 3.9 4.35 4.7 
Mo 4.3 5.25 3.5 3.0 3.2 3 
W -- -- 3.5 6.0 -- -- 
Cb -- -- 3.5 1.7 1.3 -- 
C .08 .06 .07 .05 .025 .18 
B .006 .03 .010 .02 .02 .014 
Zr .06 -- .05 .05 .06 .06 
Ni Bal Bal Bal Bal Bal Bal 
% .gamma.'.sup.(4) 
25 40 50 55 65 65 
______________________________________ 
.sup.(1) Also contains 1.0% V 
.sup.(2) Also contains .75% Hf 
.sup.(3) MERL 76 contains .4% Hf 
.sup.(4) Volume percent 
The gamma prime phase can be taken into solution by heating the material to 
an elevated temperature. The temperature at which a phase goes into 
solution is its solvus temperature. The solutioning (or precipitation) of 
the gamma prime occurs over a temperature range. In this disclosure, the 
term solvus start will be used to describe the temperature at which 
observable solutioning starts (defined as an optical metallographic 
determination of the temperature at which 5% by volume of the gamma prime 
phase, present upon slow cooling to room temperature, has been taken into 
solution) and the term solvus finish refers to the temperature at which 
solutioning is essentially complete (again determined by optical 
metallography). Reference to the gamma prime solvus temperature without 
the adjective low/high will be understood to mean the high solvus 
temperature. 
The eutectic and noneutectic types of gamma prime form in different 
fashions and have different compositions and solvus temperatures. The 
noneutectic low and high gamma prime solvus temperatures will typically be 
on the order of 50.degree.-150.degree. F. less than the eutectic gamma 
prime solvus temperatures. In the MERL 76 composition the noneutectic 
gamma prime solvus start temperature is about 2050.degree. F. and the 
solvus finish temperature is about 2185.degree. F. The eutectic gamma 
prime solvus start temperature is about 2170.degree. F. and the gamma 
prime solvus finish temperature is about 2225.degree. F. (since the 
incipient melting temperature is about 2185.degree. F., the eutectic gamma 
prime cannot be fully solutioned without partial melting). 
Forging is a metal working process in which metal is deformed, usually in 
compression, at a temperature which is usually above its recrystallization 
temperature. In most forging processes there are three attributes desired 
of the process and the product. They are (1) that the finished product 
have a desirable microstructure, preferably a uniform recrystallized 
structure, (2) that the product be essentially crack-free, and (3) that 
the process require a relatively low stress. Naturally the relative 
importance of these three will vary with the particular situation. 
In its broadest form the present invention comprises developing a severely 
overaged (super overaged) gamma prime morphology in a superalloy material. 
The mechanical properties of precipitation strengthened materials, such as 
nickel base superalloys, vary as a function of gamma prime precipitate 
size. Peak mechanical properties are obtained with gamma prime sizes on 
the order of 0.1-0.5 microns. Aging under conditions which produce 
particle sizes in excess of that which provides peak properties produce 
what are referred to as overaged structures. A super overaged structure is 
defined as one in which the average noneutectic gamma prime size is at 
least three times (and preferably at least five times) as large (in 
diameter) as the gamma prime size which produces peak properties. Because 
forgeability is the objective, the gamma prime sizes referred to are those 
which exist at the forging temperature. The provision of such a coarse 
gamma prime morphology dramatically enhances the forgeability of the 
material. It also appears that the gamma prime size required for improved 
forgeability is somewhat related to the fraction of gamma prime present in 
the material. For lower fraction gamma prime materials a smaller particle 
size will produce the desired result. For example we believe that a 1 
micron gamma prime size will suffice for material having a 40% (by volume) 
gamma prime content but that a 2.5 micron gamma prime size is needed in 
material containing 70% (by volume) of the gamma prime phase. 
For a constant gamma prime content, as the gamma prime particle size 
increases the interparticle spacing (the thickness of the intervening 
gamma matrix phase layer) also increases. 
According to a preferred form of the invention the cast starting material 
is heated to a temperature between the gamma prime start and finish 
temperatures (or within the solvus range). At this temperature a portion 
of the noneutectic gamma prime will go into solution. 
By using a slow, cooling schedule the noneutectic gamma prime will 
reprecipitate in a coarse form, with the particle sizes on the order of 5 
or even 10 microns. This coarse gamma prime particle size substantially 
improves the forgeability of the material. The slow cooling step starts at 
a heat treatment temperature between the two solvus temperatures and 
finishes at a temperature near and preferably below the noneutectic gamma 
prime low solvus at a rate of less than 10.degree. F. per hour. This 
process can also be described as a super overage treatment. 
FIG. 2 illustrates the relationship between the cooling rate and the gamma 
prime particle size for the RCM 82 alloy described in Table I. It can be 
seen that the slower the cooling the larger the gamma prime particle size. 
A similar relationship will exist for the other superalloys but with 
variations in the slope and position of the curve. FIGS. 3A, 3B and 3C 
illustrate the microstructure of RCM 82 alloy which has been cooled at 
2.degree. F., 5.degree. F. and 10.degree. F. per hour from a temperature 
between the eutectic gamma prime solvus and the noneutectic gamma prime 
solvus (2200.degree. F.) to a temperature (1900.degree. F.) below the 
gamma prime solvus start. The difference in gamma prime particle size is 
apparent. FIG. 4 shows the flow stress for a particular forging operation 
as a function of the cooling rate for the RCM 82 alloy; reducing the 
cooling rate from 10.degree. per hour to 2.degree. per hour reduces the 
required forging flow stress by about 20%. FIG. 5 shows the flow stress 
versus flow strain for an upset forging operation performed on materials 
processed according to the present invention and material processed 
according to the prior art. The conventionally processed material shows a 
steady state flow stress of about 14.0 ksi and cracks at a strain of about 
0.27 (27% reduction in height). Material processed according to the 
invention shows a steady state flow stress of about 6.5 ksi and no 
cracking was observed through a reduction of 0.9 (90% reduction in 
height). 
A particular benefit of the invention process is that a uniform fine grain 
recrystallized microstructure results from a relatively low amount of 
deformation. In the case of a cylindrical preform upset into a pancake the 
invention process produces such a microstructure with less than about 50% 
reduction in height; with conventional processes more than 90% reduction 
in height is required. 
Following the forging step, the forging will usually be heat treated to 
produce maximum mechanical properties. Such a treatment will include a 
solution treatment (typically at or above the forging temperature) to at 
least partially dissolve the gamma prime phase followed by aging at lower 
temperatures to reprecipitate the dissolved gamma prime phase in a desired 
(fine) morphology. Those skilled in the art appreciate that variations in 
these steps permit optimization of various mechanical properties. 
Turning now to other aspects of the invention, the starting material is 
preferably fine grained at least in its surface regions. All cracking 
encountered during development of the invention process has originated at 
the surface and is associated with large surface grains. 
We have successfully forged material having surface grain sizes in the 
order of 1/16-1/8" diameter with only minor surface cracking. This was 
accomplished in a severe forging operation, the upsetting of a cylindrical 
billet to form a pancake shape. This type of forging places the 
cylindrical outer surface in a substantial and unrestrained tensile 
condition. It appears that in other less severe forging applications 
material having a larger surface grain size (e.g. 1/4") could be forged. 
We believe that the interior grain size, the grain size more than about 
one-half inch below the surface of the casting can be substantially 
coarser than the surface grains. The limiting grain size may well be 
related to the chemical inhomogeneities and segregation of which occur in 
extremely coarse grain castings. Equally important is the retention of 
grain size during the forging process. Processing conditions which lead to 
substantial grain growth are not desirable since increased grain size is 
associated with diminished forgeability. 
The as cast starting material will usually (and preferably) be given a HIP 
(hot isostatic pressing) treatment which consists of exposure to a highly 
pressurized gas at a temperature sufficient for the metal to deform by 
creep. Typical conditions are 15 ksi applied pressure at a temperature 
below but within 150.degree. of the gamma prime solvus for a period of 
time of 4 hours. The result obtained by this treatment is the closure of 
internal voids and porosity which may be present. The HIP treatment would 
not be required if a casting technique could be developed which would 
insure freedom from porosity in the cast product and might not be required 
if the finished product was to be used in a nondemanding application. 
The gamma prime size in the material is then increased as previously 
described. The material is heated to a temperature at which a substantial 
quantity (i.e. at least about 40% by volume and preferably at least about 
60% by volume) of the noneutectic gamma prime is taken into solution and 
then slowly cooled to cause a substantial portion of the solutionized 
noneutectic gamma prime material to reprecipitate as coarse particles. The 
material will usually be cooled to at least 50.degree. F. below the solvus 
start temperature and will most usually be cooled to a temperature which 
approximates the forging temperature. 
The cooling rate should be less than about 10.degree. F. and preferably 
less than about 5.degree. F. per minute. With reference to FIG. 1 any 
straight line starting at point 0 and falling between 0.degree. F./min and 
10.degree. F./min will produce the desired result. It appears however that 
fluctuating cooling rates may not be satisfactory. See for example line 1 
which has a portion A in which the cooling rate excedes 10.degree. F./hr. 
This would probably be unsatisfactory. We believe that the process will 
tolerate cooling rates somewhat in excess of 10.degree. F./hr., e.g. 
20.degree. F./hr. over short portions of the cooling cycle but this is not 
preferred. Cooling cycles performed in a furnace with an erratic 
temperature controller did not produce the desired microstructure even 
though the overall cooling rate was substantially less than 10.degree. 
F./hr. Of course, cooling in a furnace with a conventional on/off 
controller occurs as a series of very small steps but the thermal inertia 
of the furnace smooths out these fluctuations. 
As a further observation, consider curves 2 and 3 which are both curves no 
part of which has a slope in excess of 10.degree. F./hr. Even though both 
terminate at point X, preliminary indications are that the results 
produced by curve 3 (relatively rapid cooling followed by slower cooling) 
will be preferred to the results from curve 2 (slow cooling followed by 
faster cooling). The benefits of such a modification would be economic 
rather than technical in nature. 
It is highly desired that the grain size not increase during the previously 
described gamma prime growth heat treatment. One method for preventing 
grain growth is to process the material below temperatures where all of 
the gamma prime phase is taken into solution. By maintaining a small but 
significant (e.g. 5-30% by volume) amount of the gamma prime phase out of 
solution grain growth will be retarded. This will normally be achieved by 
exploiting the differences in solvus temperature between the eutectic and 
noneutectic gamma prime forms. In certain alloys having relatively high 
carbon contents the (essentially insoluble) carbide phase will suffice to 
prevent grain growth. Application of this invention to such alloys will 
relax the temperature constraints which would need to be observed if 
retained gamma prime material were relied upon for grain boundary 
stabilization. A combination of retained gamma prime phase and carbide 
phase can also be utilized. It is also possible that a certain amount of 
grain growth may be acceptable especially in forging processes where 
excessive tensile strains are not encountered and/or in the forging of 
relatively forgeable alloys. 
Retention of sufficient gamma prime material to prevent grain growth can be 
achieved by using a processing temperature between the eutectic and 
noneutectic gamma prime solvus temperatures so that retained eutectic 
gamma prime phase prevents grain growth. We appreciate, however, that it 
is possible in some alloys to solution heat treat the alloy so as to 
substantially eliminate the eutectic gamma prime phase by completely 
solutionizing the eutectic gamma prime followed by reprecipitation. The 
invention process is still applicable in this event; it is merely 
necessary to select a processing temperature at which a small but 
significant amount of the gamma prime phase is retained, an amount 
sufficient to prevent significant grain growth. 
The forging operation will be conducted isothermally (using heated dies) 
and in a vacuum or inert atmosphere. In this context "isothermal" includes 
those processes in which minor (i.e. .+-.50.degree. F.) temperature 
changes occur during forging. The die temperature will preferably be 
.+-.100.degree. F. of the workpiece temperature but any die condition 
which does not chill the workpiece sufficiently to interfere with the 
process will be satisfactory. The forging temperature will usually be 
below but within 200.degree. F. of the noneutectic gamma solvus start 
temperature, although forging in the lower end of the range between the 
noneutectic solvus start and finish temperature is also possible. 
The forging temperature will usually be near the noneutectic gamma prime 
low solvus. Forging is conducted at a low strain rate, typically on the 
order of 0.1-1 in/in/min. The dual strain rate process of U.S. Pat. No. 
4,081,295 may be employed. The required forging conditions will vary with 
alloy, workpiece geometry and forging equipment capabilities and the 
skilled artisan will be readily able to select the required conditions. 
In normal circumstances the invention heat treatment will permit forging of 
cast nickel base materials to final configuration in a single operation 
although geometric considerations may dictate the use of multiple forging 
steps with different shaped dies (without intervening processing being 
required). One sequence involves use of flat dies to upset a cast preform 
to a pancake followed by use of shaped dies to achieve a complex final 
shape. 
In unusual circumstances the present invention process might be repeated, 
i.e. multiple invention heat treatments along with forging operations, but 
this will not normally be required. 
Other features and advantages will be apparent from the specification and 
claims and from the accompanying drawings which illustrate an embodiment 
of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
An alloy having a nominal composition of the RCM 82 alloy in Table I was 
cast into a cylinder 6" in diameter and 8" high having a grain size of 
ASTM 2-3 (0.125-0.18 mm avg. dia.). This material contains about 60-65% 
(by volume) of the gamma prime phase. The noneutectic gamma prime solvus 
temperature range is about 2050.degree.-2185.degree. F. and the eutectic 
gamma prime solvus temperature range is about 2150.degree.-2220.degree. F. 
This casting was produced by Special Metals Corporation, apparently using 
the teaching of U.S. Pat. No. 4,261,412. 
This casting was HIP treated (2165.degree. F., 15 ksi for 3 hours) to close 
residual porosity (sufficient gamma prime particles are present at 
2165.degree. F. to prevent grain growth). The casting was then heat 
treated at 2165.degree. F. for 2 hours and cooled to 2000.degree. F. at 
2.degree. F./hr. (again grain growth did not occur). The resultant 
noneutectic gamma prime particle size was about 8.5 microns. This material 
was then forged at 2050.degree. F. at 0.1 in/in/min to a reduction of 76% 
(producing a 2" high.times.12" diameter pancake) without cracking. 
In the absence of the invention heat treatment, this amount of reduction 
would not be achieved without extensive cracking and the required forging 
forces would be greater than those observed with the invention process. 
Even where cracking did not occur the structure would be undesirable in 
that it would only be partially recrystallized. 
Certain microstructural features are illustrated in FIGS. 6A, 6B, 7A and 
7B. FIG. 6A illustrates the microstructure of cast material. This material 
has not been given the invention heat treatment. Visible in FIG. 6A are 
grain boundaries which contain large amounts of eutectic gamma prime 
material. In the center of the grains can be seen fine gamma prime 
particles whose size is less than about 0.5 micron. 
FIG. 6B illustrates the microstructure of the material after conventional 
forging. Visible in FIG. 6B are fine recrystallized grains at the original 
grain boundaries which surround material which is essentially 
nonrecrystallized. This nonuniform (necklace) microstructure is believed 
not to provide optimum mechanical properties. 
FIG. 7A shows the same alloy composition after the heat treatment of the 
present invention but prior to forging. The original grain boundaries are 
seen to contain areas of eutectic gamma prime. Also, significantly, the 
interior of the grains contain gamma prime particles whose size can be 
seen to be much larger than the corresponding particles in FIG. 6A. In 
FIG. 7A the gamma prime particles have a size on the order of 8.5 microns. 
After forging the microstructure can be seen to be substantially 
recrystallized and uniform in FIG. 7B. The FIG. 7B material is believed to 
have superior mechanical properties to the FIG. 6B material. 
Thus, in summary, the present invention process achieves the three goals in 
forging an otherwise unforgeable material without penalty. The reduction 
at which cracking occurs is dramatically increased (FIG. 5); the final 
product has an improved microstructure (FIG. 7B); and the flow stress 
required for forging is substantially reduced (FIG. 4). 
It should be understood that the invention is not limited to the particular 
embodiments shown and described herein, but that various changes and 
modifications may be made without departing from the spirit and scope of 
this novel concept as defined by the following claims.