Combinations of the nickel-base superalloys MERL 76 and low carbon Astroloy, Rene 95 and low carbon Astroloy, Rene 95 and NASA-TRW-VIA were consolidated from prealloyed powders by hot isostatic pressing. These combinations were exposed to 870 C for 1500 hours, and in a gradient furnance to temperatures up to incipient melting and subjected to microstructural examinations. The only phases found at the interfaces were those which were also present in the component alloys

Harf, F. H.

English

NASA Technical Reports Server

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NASA Technical Memorandum 8269 
NASA-TM-82698 19830003039 
Microstructural Behavior of Interfaces in Hot 
Isostatically Pressed, Dual Alloy Combinations 
Fredric H. Hi 
Lewis Research Center 
Cleveland, Ohio 
September 1981 
NI\S/\ 
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https://ntrs.nasa.gov/search.jsp?R=19830003039 2020-03-21T06:49:24+00:00Z
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SUMMARY 
Dual alloy combinations with potential use for turbine engine components 
were evaluated for microstructural stability. The combinations were MERL 76 
and Low Carbon Astroloy, Rene 95 and Low Carbon Astroloy, Rene 95 and 
NASA-TRW-VIA. These nickel-base superalloys, produced from prealloyed 
powders, were combined by hot isostatic pressing. Samples were exposed at 
870° C for 1500 hours in an accelerated test to simulate turbine engine 
service conditions. Other samples were exposed in a gradient furnace to 
temperatures reaching 1300° C. Neither treatment caused the formation of new 
metallurgical phases near the interfaces which were not present in the 
alloys. Gamma prime coarsening was noted after the prolonged exposure to 
870° C • 
INTRODUCTION 
New technology has demonstrated that it is possible to combine, by hot 
isostatic pressing (HIP), two prealloyed powder -metallurgy nickel-base 
superalloys (1). This allows the production of advanced dual alloy turbine 
disks in which the rim can withstand high rupture stresses up to 760° C, with 
a simultaneous improvement in the resistance to tensile and fatigue loading in 
the hub at temperatures of 400°_500° C. Dual alloy disk technology also 
offers an opportunity to conserve strategic materials not only by savings 
through powder metallurgy, but also by the use of alloys with low strategic 
element contents in those portions of the disks where their properties are 
adequate. 
However, before dual alloy combinations can be used, it must be confirmed 
that the combining of two different compositions does not result in 
microstructural instabilities at their interface. This could manifest itself 
by the appearance of new phases during the HIP cycle, subsequent heat 
treatments or during engine operation. The research reported here addresses 
itself to this problem. Four alloys are included in the investigation: 
IliER L 76 and Rene 95 are candidates for use in the hubs of dual alloy turbine 
disks, while Low Carbon Astroloy and NASA-TRW-VlA are suitable for the rims. 
The combinations evaluated were MERL 76 with Low Carbon Astroloy, Rene 95 with 
Low Carbon Astroloy, and Rene 95 with NASA-TKW-VlA. 
Since new phases might result during the HIP cycle and solution 
treatments, exposure to a wide range of temperatures was effected in a 
gradient furnace. Prolonged use of some superalloys at elevated temperatures 
has been known to promote the formation of brittle, acicular phases, 
designated as sigma and mu. The alloy combinations were therefore exposed to 
870 0 C for 1500 hours. The effect of this accelerated exposure treatment on 
the microstructure was determined at the interface of two alloy compositions 
and also in mixed powders, since powder mixtures are likely to occur and may 
even be desirable in actual parts. 
MATERIALS AND PROCEDURES 
/ The four alloys, MERL 76, Rene 95, Low Carbon Astroloy, and NASA-TRW-VIA, 
used in this work had been vacuum induction melted, cast, remelted and argon 
atomized. Analyzed compositions and sieve analyses are presented in table 1. 
-- - - -----------
-, 
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p 
For some of the experiments mixtures of powders were prepared by blending 
equal quantities of MERL 76 and Low Carbon Astroloy, Rene 95 and Low Carbon 
Astroloy, Rene 95 and NASA-TRW-VIA. 
Stainless steel cans with a rectangular base, measuring about 110x15 mm 
were filled with layers, about 15 mm thick, of powders as shown in figure l. 
Each two-layer can contained two different alloys. In each four-layer can two 
alloys contacted each other and also a layer of their prepared mixture. The 
cans were closed with a stainless steel cover, evacuated and sealed by 
welding. The cans were hot pressed isostatically to 140 MPa at 1120° C for 
4 hours . 
After pressing, specimens were cut from all cans for examination of the 
microstructures in the as-HIP condition. Portions of the four-layer cans were 
exposed at 870° C for 1500 hours. Cans with two layers had the canning 
materials removed by grinding. These cans were heated to a maximum of 1300° C 
in a gradient furnace with the gradient parallel to the layers (see fig. 1). 
Readings taken from thermocouples along the gradient showed that at distances 
of 15,40 and 60 mm from the hot end the temperatures averaged 1285°, 1220° 
and 1140° C, respectively. 
MICROSTRUCTURAL EXAMINATION 
Microstructures of as-HIP and heat-treated specimens were examined by 
optical and scanning electron microscopy. Ground and polished specimens were 
etched with Kalling's reagent for optical examination and with a solution of 
33 percent hydrochloric acid, 33 percent acetic acid, 33 percent water, and 
1 percent hydrofluoric acid for electron microscropy. X-ray diffraction was 
performed at 40 kV and 40 mA using copper radiation. 
RESULTS 
The as-HIP microstructures of the MERL 76- Low Carbon Astroloy combination 
are shown in figure 2. The interfaces between the alloys are plainly visible 
and the individual particles in the mixed powders retain their microstructural 
identities. Similar observations were noted for the other alloy combinations 
and are therefore not shown in figures. 
The prolonged exposure of 1500 hours at 870° C did not cause any 
unexpected changes in the microstructure of any alloy combinations (compare 
figs. 3 and 2). Metallographic examination at still higher magnification, 
revealed no new phases in any of the alloy combinations (see fig. 4). As 
expected, the fine gamma prime particles had coarsened somewhat. 
X-ray diffraction analyses after the prolonged exposure showed very weak 
indications of sigma phase in all of the base alloys, except NASA-TRW-VIA. 
Even weaker indications of sigma phase were detected in the mixed alloys. 
Even after exposure in a thermal gradient which exceeded the incipient melting 
temperature of one component alloy, the alloy combinations were stable. The 
microstructures shown for the MERL 76-Low Carbon Astroloy combination in 
figure 5 after a one hour exposure to approximately 1245° C and 1185° Care 
typical for all the combinations tested. Metallographic examination at the 
magnification shown in figure 5 and also at greater magnifications indicated 
that no new metallurgical phases were formed at the interfaces or in the 
diffusion zones. The MERL 76 - Low Carbon Astroloy combination was also 
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evaluated in a parallel contractual program (ref. 1) aimed at developing the 
technology for dual disks; here, no detrimental phases were detected at the 
sample interfaces during a variety of solutioning and aging heat treatments. 
CONCLUDING REMARKS 
The four alloys used in these combinations are not generally regarded as 
sigma-prone. Electron vacancy (Nv) calculations for mixed compositions predicted no tendency toward sigma formation and the Nv numbers were 
• intermediate to those of the base alloys. But, X-ray diffraction showed that 
in 3 of the 4 alloys some sigma phase did form, although the indications were 
never more than weak. Since even examinations at high magnifications revealed 
• no acicular phases it can be assumed that sigma is present only as small, 
discrete particles or a very fine discontinuous grain boundary precipitate. 
• 
In this form its effect on properties is small (2). Furthermore, this 
stability test was purposely conducted at a temperature known to accelerate 
sigma formation (3), but well above that projected for even the most advanced 
turbine disks. The tests give considerable assurance that, using sigma-free 
alloys, deleterious phases will not appear in combinations of those powders in 
dual alloy turbine disks during HIP, heat-treatment nor at operating 
temperatures. This has been demonstrated for interfaces as well as for 
intimately mixed powders. It should be emphasized, however, that while these 
microstructures appear sound it should not be implied therefrom that the 
mechanical properties will be satisfactory. Actual mechanical tests of 
candidate alloy combinations should be performed. 
SUMMARY OF RESULTS 
The microstructures of hot isostatically pressed combinations of 
prealloyed powders of MERL 76 and Low Carbon Astroloy, Rene 95 and Low Carbon 
Astroloy, Rene 95 and NASA- TRW-VIA were examined in various conditions: 
as-HIP, after exposure at 780 0 C for 1500 hours and after heating up to 
1300 0 C in a thermal gradient. 
It was found that all alloy combinations: 
1. Remained free of unique phases at the interfaces and diffusion zones , 
2. Underwent coarsening of the gamma prime phase during long time 
exposure to 870 0 C, and 
3. Retained distinguishable structures in contacting particles of 
different compositions. 
REFERENCES 
1. Kortovich, C. S.; and Marder, J. (vI.: Development of Materia ls and Process 
Technology for Dual Alloy Disks. (TRW, Inc.; NASA Contract NAS3-21351) 
NASA CR-165224, 1981 . 
2. Dreshfield, R. L.; and Ashbrook, R. L.: Further Observations on the 
Formation of Sigma Phase in a Nickel-Base Superalloy (IN-100). NASA TN 
D-6015, 1970. 
3. Mihalisin, J. R.; Bieber, C. G.; and Grant R. T.: Sigma - Its Occurrence, 
Effect and Control in Nickel - Base Superalloys. AIME Trans., vol. 242, 
1968, pp. 2399-2414. 
3 
TABLE 1. - ANALYZED COMPOSITIONS AND SIEVE ANALYSES OF ALLOYS 
Low Carbon ;-
As tro loy MERL 76 Rene 95 NASA-TRW-VIA 
Weight Percent 
Ni Balance Balance Balance Balance 
.. C 0.047 0.028 0.059 0.13 
Cr 14.99 11.95 l3.49 5.92 
IV10 5.00 3.04 3.42 2.05 
• Ti 3.45 4.16 2.57 1. 01 
Al 4.05 5.13 3.65 5.27 
Co 17.09 18.11 7.90 7.46 
Cb 1.45 3.70 0.47 
W 3.38 5.92 
Ta 8.97 
B 0.028 0.018 0.009 0.020 
Zr 0.01 0.06 0.058 
Re 0.32 
Hf 0.43 0.38 
Fe 0.12 0.14 0.33 0.13 
Percent Powder Fraction 
- 80 + 100 10.7 12.7 7.7 
-100 + 140 17.0 22.7 l3.1 1.0 
-140 + 200 17 .7 20.8 16.7 17.1 
-200 + 230 11.6 5.8 9.9 13.6 
-230 + 325 14.5 15.5 15.5 27.1 
-325 28.5 22.5 37.1 40.8 
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4 layer cans 2 layer cans 
As 1500 h Gradient heating 
HIP 87(jl C 1 hour 
M76+ LCA 0 15 40 60 80mm m 
M76 Di stance from hot end, mm 
:~;6--' I: : : I 
1300 1285 1220 1140 
Average temperatures, °c 
; 
:~;+ LCA 
-LCA--- .... ~ _____ ---I 
-R95---- _ 
; 
~~;+ VIA 
-VIA---... • I 
-R95 ---... t-L-_ ::::::::::::::::::::::::::::::::::::::::::::: 
Alloy codes : 
M76 MERL 76 
R95 Rene 95 
LeA Low carbon Astroloy 
VI A NASA-TRW-VI A 
+ combination of 2 comJX)sitions 
mixed in equal proportions 
Figure 1. - Alloy combinations and test layout. 
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MERL 76 
--- Interface 
Mixed 
MERL 76 
+ 
low ca rbon 
Astroloy 
--- Interface 
Low carbon 
Astroloy 
--- Interface 
MERL 76 
Figure 2. - Micros tructures of as-HIP interfaces of the success ive laye rs in a 4-laye r can. 
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Microstructural behavior of interfaces in hot isostatically pressed, dual alloy combinations

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