The effects of replacing the refractory elements W, Nb, and Ta with Mo in nickel-base superalloys on microstructural, mi
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I.
INTRODUCTION
REFRACTORY elements, because of their high melting temperatures and Pauling parameter differences from nickel, are important alloying elements for nickel-base superalloys used in turbine engines. Historically, in order to achieve high temperature and strength capabilities, elements such as molybdenum, tungsten, tantalum, and niobium have been progressively added to these alloys to strengthen the nickel-cobalt-chromium matrix, the coherent Y' precipitates, and the grain boundary strengthening carbides.~'2 In addition, refractory elements such as Ta and Nb were thought to improve the hot corrosion resistance of superalloys. 3 As a result the United States has become a significant user of these elements. However, the United States must rely heavily on imports of Nb, Ta, and, to a lesser extent, W. 4 Previous research strategies have concentrated on studying the effects on properties due to a mutual substitution between Ni and various refractory elements in superalloys. 1 This approach appears to be capable of yielding the characteristic effects of each refractory element in the alloy. However, further insight reveals that the inferred conclusions may be misleading, since this strategy does not consider any synergistic effects between refractory elements in these complex alloy systems nor their mutual substitutability. The purpose of this three-year study is to investigate the effects of such interactions. The initial research strategy was to choose representative alloy systems and to replace the refractory elements Nb, Ta, and W with Mo, using an atomfor-atom substitution in the alloy. By following this strategy, we hope to begin to isolate and compare the effects of individual elements on each alloy system. The alloy systems under study are UDIMET* 710, B 1900,** and INCONEL***738. UDtMET 710 allows for
a simple systematic study between Mo and W, while B 1900 permits a similar study between Mo and Ta. INCONEL 738 is unique in that it contains all four refractory elements, albeit in dilute amounts. In this alloy we practiced singular mutual substitutions between Mo and Ta, Nb, and W. In each of the alloys, Mo remained the substitutor because of its relative abundance in the United States. In what follows the initial results from this study are reported and discussed. Attention is given to the effect on the basic mechanical properties, i.e., ambient and high temperature tensile behavior, elevated temperature creep and stress rupture behavior, and elevated temperature high mean stress, low cycle fatigue behavior. Of particular importance is the correlation between the microstructure, microchemistry, and mechanical properties, since it is hoped this will yield a more general understanding of how different refractory elements strengthen nickel-base superalloys. Furthermore, preliminary results 5 on the hot corrosion of the standard and modified alloys will briefly be discussed.
II.
PROCEDURE
A. Materials In this study eight alloys were examined: UDIMET 710 and one modified UDIMET 710 composition, BI9
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