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I.

INTRODUCTION

THE typical microstructure of nickel-base superalloys consists of y' precipitates in a matrix of y. In many modem single crystal superalloys, the initially discrete y' particles directionally coarsen during elevated temperature creep testing to form continuous y' lamellae, or rafts. These rafts are believed to improve the creep resistance of the material by providing effective barriers to dislocation motion.[~-5] However, unanswered questions still remain regarding the detailed rate controlling mechanisms during creep and the interrelationships among microstructure, composition, and creep resistance of alloys which undergo directional coarsening. The initial y-y' microstructure can significantly influence the high temperature creep properties of nickel-base superalloys t~-3'6] by affecting the subsequent raft development. Specifically, it has been shown that many alloys exhibit a peak in creep strength as a function of initial y' size. ~ In NASAIR 100-type alloys, this peak corresponded to an initial microstructure consisting of cuboidal precipitates which were strongly aligned along (001) directions. During the early stages of creep, the aligned particles directionally coarsened into relatively perfect lamellae, whereas the more irregularly shaped and distributed y' particles in both under and overaged conditions formed more irregular lameUae. Alloys with small magnitudes of lattice mismatch were less sensitive to changes irl initial y' size and shape. This w o r k [1'3] suggests that the optimum initial microstructures for each alloy must be obtained before any compositional effects can be determined. R.A. M a c K A Y , Materials Scientist, and M.V. N A T H A L , Deputy Branch Chief, are with the Materials Division, NASA-Lewis Research Center, Cleveland, OH 44135. D.D. PEARSON, Senior Materials Scientist, is with United Technologies Research Center, East Hartford, C T 06108. Manuscript submitted June 12, 1989. METALLURGICAL TRANSACTIONS A

It is recognized that refractory metal additions can also have significant effects on the mechanical properties of superalloys, t3] Alloying additions have the potential of influencing a number of properties, such as solid solution hardening, y-y' lattice mismatch, y' volume fraction, diffusion rates, and antiphase boundary and stacking fault energies. However, knowledge of the relative effectiveness of different refractory elements and the mechanisms responsible for their effects is lacking. It has been shown that the creep lives at 1038 ~ improved when the Mo content was increased from 12.8 to 14.3 wt pct Mo in the model alloy MMT-143. tT] While Ta additions to the same alloy were beneficial, they did not have as strong of an influence on the creep properties. MacKay and Ebert t2] have shown in the same alloy system that the creep life at 982 ~ was decreased when the Mo content was increased further to 14.6 wt pct. The degradation in creep properties at the 14.6 Mo level was attributed to the precipitation of a deleterious third phase. Thus, it appeared from