Superalloy properties through stress modified gamma prime morphology

A high strength nickel base superalloy having a novel microstructure is described. The article consists of a gamma matrix containing a oriented gamma prime second phase having a plate or rod form. This noncuboidal gamma prime phase is produced by aging solution treated material under an applied stress. When this process is applied to a certain class of superalloys which are rich in refractory elements, the resultant structure provides exceptional resistance to creep at elevated temperatures.

DESCRIPTION 
1. Technical Field 
The present invention relates to the production of high strength nickel 
base superalloy articles through the application of stress during aging of 
solution treated material. 
2. Background Art 
It has been previously known that the application of stress to an alloy 
which forms precipitate particles can modify the morphology of the 
particles. This is described in the following articles. 
1. "The Effect Of Uniaxial Stress On The Periodic Morphology Of Coherent 
Gamma Prime Precipitates In Nickel-Base Superalloy Crystals," 
Metallurgical Transactions, Vol. 2, January 1971, pgs. 215 to 217. 
2. "The Effect of Orientation And Sense Of Applied Uniaxial Stress On The 
Morphology Of Coherent Gamma Prime Precipitates In Stress Annealed 
Nickel-Base Superalloy Crystals," Metallurgical Transactions, Vol. 2, 
February 1971, pgs. 543 to 553. 
3. "Effects Of Stress Coarsening On Coherent Particle Strengthening," 
Metallurgical Transactions, Vol. 3, August 1972, pgs. 2157 to 2162. 
4. "Influence Of Uniaxial Stress On The Morphology Of Coherent Precipitates 
During Coarsening--Elastic Energy Considerations," Acta Metallurgica, Vol. 
24, 1976, pgs. 559-564. 
These articles deal with the effect of stress on the morphology of 
precipitate particles but there is no discussion in these articles of the 
effect of the resultant structure on elevated temperature creep 
properties. 
DISCLOSURE OF INVENTION 
The present invention relates to a process which can be applied to a class 
of nickel base superalloys in single crystal form, to provide unique 
microstructures and exceptional properties at elevated temperatures. A 
class of alloys is described in which the gamma prime phase is saturated 
with an element selected from the group consisting of tantalum, columbium, 
vanadium and mixtures thereof and in which the gamma matrix phase is 
saturated with an element selected from the group consisting of tungsten, 
chromium, molybdenum and rhenium. 
The alloy is formulated so that the gamma prime phase lattice parameter is 
smaller than the gamma phase lattice parameter (a negative misfit). The 
alloy is also formulated so that it contains from 30 to 75 volume percent 
of the gamma prime phase. The alloy is then directionally cast, as a 
single crystal, and homogenized by appropriate heat treatment where the 
gamma prime phase is solutionized. 
The solutionized material is then quenched to room temperature at a rate 
sufficient to suppress significant growth of the gamma prime phase and 
aged with a stress applied. When the applied stress is in the &lt;100&gt; 
direction, and the articles thus treated will have exceptional properties 
in the &lt;100&gt; direction. The effect of these combinations of steps is to 
produce an oriented noncuboidal gamma prime phase with the gamma prime 
being preferably present in the form of continuous platelets whose major 
dimensions are transverse to the applied stress axis. 
The present invention relates to superalloy articles having remarkable 
properties at elevated temperatures. Two broad aspects are important, the 
specific alloy composition and the unique heat treatment. 
The alloy must meet certain criteria which are in some respect contrary to 
the teachings of the prior art. In particular, it is desired so that both 
the gamma prime phase and the gamma phase be saturated with refractory 
elements. As used herein, the term "refractory elements" includes the 
elements tantalum, columbium, vanadium, tungsten, chromium, molybdenum and 
rhenium. 
The elements tantalum, columbium and vanadium partition or segregate to the 
gamma prime phase while the elements tungsten, chromium, molybdenum and 
rhenium essentially partition to the gamma phase upon solidification. 
For the achievement of optimum properties it is required that both of these 
phases be saturated with these elements. By "saturated" we mean that the 
elements are present in an amount great enough to just cause the formation 
of equilibrium phases which are rich in these refractory elements. Such 
phases are referred to in the superalloy art as Topologically Closed 
Packed (TCP) phases such as .alpha.Mo, .alpha.Cr, .alpha.W or ta phases. 
In the prior art, the formation of such phases was avoided since these 
phases were deemed to be deleterious to the properties of nickel base 
superalloys, based on the nickel-aluminum binary system or gamma-gamma 
prime system. 
The previously enumerated elements partition either to the gamma phase or 
the gamma prime phase. Since the elements have different atomic diameters, 
the additions of various elements and the partitioning of these elements 
to the various phases will effect the relative lattice parameters of the 
phases. 
In the prior art, there have been definite attempts made to control the 
alloying additions in a fashion such that the lattice parameters of the 
gamma and the gamma prime phases are equalized so that there is negligible 
lattice parameter mismatch between the gamma and gamma prime phases and 
the gamma matrix. This matching of lattice parameters was believed 
necessary for long creep lives. Virtually all commercial superalloys 
contain gamma prime phase with a lattice parameter which is slightly 
greater than that of the gamma phase. 
It is, however, somewhat difficult to accurately assess the mismatch in 
commercial superalloys because of the variations in measurement techniques 
and because measurements are usually conducted at room temperature 
although the effects are really only apparent at elevated temperatures. 
It appears, that it is essential for the success of the present invention 
that the gamma prime phase lattice parameter be at least 0.1% smaller than 
the gamma lattice parameter when measured at room temperature using X-ray 
techniques. The relative coefficient of thermal expansion of the phases 
are such that this negative misfit is probably even larger at elevated 
temperatures. 
The third constraint is that the alloy must contain from about 30 to 75 
volume percent of the gamma prime phase and preferably from about 45 to 
about 65 volume percent of the gamma prime phase. 
The fourth constraint refers to the amount of chromium which is present. 
Chromium in amounts of greater than about 5% appears to lower the 
properties of the alloy and amounts greater than about 8% are not desired. 
The previous restrictions define the class of nickel base superalloys in a 
somewhat unconventional fashion. However, we believe that these 
restrictions are essential to the successful practice of the invention, 
and that the previous description is the best method of describing these 
alloys. In the subsequent illustrative examples, several representative 
compositions will be described. 
The heat treatment required to achieve the exceptional properties will now 
be described. As an initial step, the alloy must be solutionized so that 
it is compositionally homogeneous throughout. Homogeneity is defined as 
less than 1% variation in refractory content throughout the alloy. This is 
essential if the desired results are to be achieved. This homogenization 
will be achieved by annealing at elevated temperatures at or above the 
gamma prime solvus but below the incipient melting temperature. The high 
refractory metal content necessitates holding at this temperature for a 
substantial amount of time to achieve homogenization. Having placed the 
alloy in an homogeneous condition, it is necessary to cool the alloy at a 
rapid rate to room temperature for parts with thicknesses of less than 
about one half inch air cooling is sufficiently rapid. The rapid cooling 
retards the growth of the gamma prime phase which would occur at a slower 
cooling rate so that the cooled article contains a high number of very 
fine gamma prime particles. 
The alloy is then aged at a temperature of between about 1200.degree. F. 
and the gamma prime solvus temperature to cause the controlled growth of 
the gamma prime phase. Most importantly, the aging step is conducted with 
a stress applied to the article and the applied stress causes the gamma 
prime phase to nucleate and grow in the form of fine platelets or 
optionally rods depending upon the stress conditions. An applied tensile 
stress produces platelets while on applied compressive stress produces 
rods. The platelet morphology is believed to be stronger and more stable 
under use conditions in which tensile stresses will be encountered. The 
spacing between adjacent gamma prime platelets or lamellae will be less 
than about 0.5 microns. 
The stress required to produce the oriented morphology is not great, it 
appears that a stress greater than 10% of the yield strength, at the 
temperatures in questions, will be sufficient. In tensile creep specimens 
oriented morphologies are observed even in the regions of the samples in 
the grips where a relatively low stress condition exists. 
The time required to produce the oriented morphology will vary with 
temperature, with a lesser time being required at the higher temperatures. 
As an alternative to this stress aging step the article may be placed in 
service as solution heat treated so that the oriented gamma prime phase is 
formed in service. 
FIG. 1 is a transmission electron micrograph of a Ni-Al-Mo-Ta alloy 
(described below with reference to FIG. 3) which has been conventionally 
treated so that it contains the equilibrium cuboidal gamma prime 
morphology. FIG. 2 is a replica which illustrates the (platelet) gamma 
prime morphology of the invention. The sample from which FIG. 2 was taken 
had been aged under stress to produce the platelet morphology.

BEST MODE FOR CARRYING OUT THE INVENTION 
The advantages of the present invention may be better appreciated through 
reference to the Figures which are described below. All of the data in the 
Figures was obtained from an alloy containing Ni-5.8%Al-14.3%Mo-6%Ta 
(weight pct.). 
This alloy meets the criteria previously set forth, namely it is a 
gamma-gamma prime nickel base superalloy in which both the gamma and gamma 
prime phases are saturated in refractory elements. Also, the gamma prime 
phase has a lattice parameter which is about 0.6% smaller than the gamma 
lattice parameter (measured at room temperature) and the gamma prime phase 
is present in an amount of about 65 volume percent. The test samples from 
which the information in the Figures was developed were all cast single 
crystals (in the 100 orientation) which were homogenized at 2250.degree. 
F. for 16 hours and then air cooled to room temperature. In this treatment 
the homogenization and solutionization treatment were combined. Using air 
cooling on the tensile samples in question provided a sufficiently fast 
cooling rate to effectively suppress significant growth of the gamma prime 
phase. In all of the graphs a curve is presented which is referred to as 
the "Standard Heat Treatment". This heat treatment consists of a four hour 
heat treatment at 1975.degree. F. followed by a 16 hour treatment at 
1600.degree. F. in the absence of applied stress. This standard heat 
treatment is typical of the treatment which is applied to conventional 
turbine blades in connection with the coating process which is the last 
step in the processing sequence. The effect of the standard heat treatment 
is to cause the gamma prime phase particles to assume the equilibrium 
cuboidal morphology. 
Referring now to FIG. 3, the effect of the invention heat treatment 
relative to the standard heat treatment can readily be seen. The curve in 
FIG. 3 labelled "Invention" represents the creep characteristics of the 
alloy described above which had received only the solution heat treatment 
prior to being placed in test at a temperature 1400.degree. F. and at an 
applied stress of 120 ksi. In this test the alloy sample is aged under 
load while in the test apparatus. The effect of this invention treatment, 
on the creep properties, relative to the standard heat treatment is 
apparent. The sample which was given the standard heat treatment shows a 
stress rupture life of slightly less than 500 hours while the sample 
treated according to the invention shows a stress rupture life of somewhat 
more than 1400 hours. Both samples have comparable ductilities, these 
ductilities are somewhat low as a result of the precipitation of 
extraneous phases in the alloy under these test conditions. Under these 
test conditions the invention heat treatment provides an advantage of 
about 3.times. in stress rupture life. Also shown in FIG. 3 is a line 
showing the creep life, under these conditions, of an alloy known as MAR 
M-200, (Ni-9%Cr-10%Co-12.5%W-1%Cb-2%Ti-5%Al-0.15%C), this is a standard 
alloy which is used, in directionally solidified form, in a currently 
produced commercial turbine engine. The advantage of the present invention 
can readily be seen. 
FIG. 4 shows the sample type of data for material tested at 1650.degree. F. 
and an applied stress of 60 ksi. Three curves are shown; the one labelled 
"Standard Heat Treatment" shows the behavior of material which was 
solution treated and then given the standard heat treatment. This material 
shows a stress rupture life of about 120 hours. The curve labelled 
"Invention" shows the behavior of material which was solution treated and 
then air cooled and placed in test, again in this instance the material 
formed the oriented gamma prime phase during the initial steps of the 
testing sequence. This sample shows a stress rupture life of about 300 
hours. The third curve labelled "Pretreated Invention" shows the behavior 
of material which was solution treated, air cooled and then treated at 
1900.degree. F. and an applied load of 30 ksi for a period of 15 hours 
prior to being tested at 1650.degree. F. with an applied load of 60 ksi. 
The effect of the pretreatment at 1900.degree. F. was to form the oriented 
gamma prime phase prior to testing at 1650.degree. F. The effect can be 
seen that the pretreating treatment increases the stress rupture life to 
about 450 hours. Again the creep life of MAR M200 100 single crystals are 
indicated and the invention articles can be seen to have substantially 
superior properties. 
FIG. 4 shows data developed after testing at 1900.degree. F. with an 
applied load of 30 ksi. The curve labelled "Standard Heat Treatment" shows 
the behavior of material after the standard heat treatment and under these 
test conditions this material has a stress rupture life of about 100 
hours. Samples treated according to the invention (placed in test 
subsequent to air cooling from the solution treatment temperature) shows a 
substantial life improvement of almost 5.times.. Once again the creep life 
of MAR M200 single crystals is seen to be markedly inferior to the life of 
the invention. 
The reasons for the superior mechanical properties obtained in articles of 
the present invention are complex and not fully understood. The following 
explanation is based on what is currently known and is believed accurate. 
However, we do not wish to be bound by this explanation especially with 
regard to observations and theories that may arise in the future. 
In conventional superalloys the microstructure consists of (equiaxed) 
cuboidal gamma prime particles in a gamma matrix. Deformation occurs by 
the movement of linear crystalline defects known as dislocations. In 
conventional superalloys dislocation movement is impeded by the inherent 
properties by the gamma and gamma prime phases and a further significant 
resistance to dislocation motion is provided by the interface between 
gamma and gamma prime phase. When a dislocation moving in the gamma phase 
(matrix) encounters a gamma prime particle it is often observed that the 
dislocation bends to avoid passing through the gamma prime particle. In 
conventional high strength superalloys the gamma prime phase is provided 
in an amount of about 60% (by volume) and this high concentration of fine 
particles (&lt;1 micron) provides good mechanical properties. 
In the article of the present invention several factors work in concert to 
provide the observed strength improvements. The inherent resistance to 
dislocation movement within both the gamma and gamma prime phase is 
significantly increased by saturating these phases with the refractory 
elements. The significant mismatch in lattice parameters between the gamma 
and gamma prime phases substantially increases the resistance of the phase 
interfaces to the passage of dislocations. Finally, the oriented 
morphology of the gamma prime phase makes it quite difficult for 
dislocation to bend and avoid passing through the gamma prime phase. 
Another important effect is that the refractory elements, particularly the 
molybdenum retards the growth of the gamma prime phase thus permitting the 
formation of the oriented morphology while annealing under conditions of 
applied stress. 
Other alloys are tested in an effort to determine the significance of 
refractory elements on alloy behavior. Two of these alloys contain 8% 
molybdenum and are substantially inferior to the other two alloys which 
contain 9% molybdenum. This demonstrates the significant effect of 
molybdenum on the alloys. Of the two samples containing 8% molybdenum, one 
sample contained 2% tantalum and 13% aluminum while the other sample 
contained 4% tantalum and 11% aluminum. The aluminum content was reduced 
as the tantalum content was increased so as to maintain an approximate 
consistent fraction of the gamma prime phase. The same effect from 
different tantalum levels was evident in the two samples containing 9% 
molybdenum. 
The conclusion which can be drawn from this comparison is that while 
increasing the tantalum increases creep life of the samples, tantalum is 
not nearly as effective as molybdenum in this regard. For this reason a 
molybdenum content of at least 5% is preferred. 
Another alloy was tested which contained 3.9% cobalt, 2.1% chromium, 2% 
rhenium, 4.1% vanadium, 2% tantalum and 14.6% aluminum balance nickel. 
This is a nickel based superalloy saturated in refractory elements which 
contains about 60% of the gamma prime phase. Because the misfit between 
the gamma and gamma prime phases is quite low, (less than 0.1%) this alloy 
demonstrated some degree of stress induced oriented gamma prime morphology 
but the microstructure was quite coarse and the alloy exhibited very 
little benefit from the treatment of the present invention. In a similar 
vein an alloy containing 6% tungsten, 3% tantalum and 12% aluminum balance 
nickel was tested and again there was no benefit from the process of the 
present invention; again, this was probably as a result of very low misfit 
in lattice parameter. 
Samples were tested which contained chromium additions as a partial 
substantial for molybdenum. The chromium was added for purposes of 
improving the oxidation resistance of the alloy. The results are shown in 
FIG. 7 for testing conducted at 1900.degree. F. with an applied stress of 
25 ksi. The results showed a substantial improvement over the conventional 
superalloy. However, they are inferior to chromium free alloys. 
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.