High-temperature creep of the intermetallic alloy Ni 3 Al
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I.
INTRODUCTION
IT is well known that the critical resolved shear stress of Ni3A1 increases with temperature up to a peak temperature (Tp) and then decreases rapidly with temperature above that point. It has been shown that the change from the "anomalous" behavior b e l o w Tr to the more "normal" behavior above Tp is associated with a transition from octahedral to primary cube glide, tj] It is generally accepted that this effect arises because dislocation motion on the (111) octahedral plane is inhibited by the thermally activated cross slip o f screw dislocations, while dislocation motion on the (001) plane is a thermally assisted Peierls-type process. This change in the dislocation mechanism for yielding in Ni3A1 suggests that the intermediate- and hightemperature creep properties o f Ni3A1 could also be significantly different. Nicholls and Rawlings t21 have reported a change in the stress dependence o f the steadystate creep rate o f Ni3A1 at Tp. They suggested that this observation was due to a change in dislocation mechanisms, but they did not identify the mechanisms that control creep in the two temperature regimes. Recently published work o f the current authors,[31 on the intermediate-temperature creep o f Ni3A1 (Hf, B), has shown that octahedral glide is exhausted during the early stages o f primary creep, and that dislocation motion on the (010) cube cross-slip plane controls the c r e e p behavior at intermediate temperatures. In the work presented in this article, constant stress creep tests have been conducted in an effort to compare the high-temperature creep behavior o f Ni3A1 (Hf, B) with the intermediatetemperature results. In contrast to the previous work, the test temperatures for the present study exceeded Tp and were in the temperature regime where primary cube glide is known to control the yielding behavior. A limited n u m b e r o f studies on the high-temperature creep mechanisms can be found in the Ni3A1 literature. K.J. HEMKER, formerly Graduate Student, Department of Materials Science and Engineering, Stanford University is Postdoctoral Assistant, Institut de G r n i e Atomique, l~cole Polytechnique F6drrale, 1015 Lausanne, Switzerland. W.D. NIX, Professor, is with the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted April 1 , 1992.
METALLURGICAL TRANSACTIONS A
S h a h 141 measured the high-temperature creep response o f Ni-rich Ni3A1 single crystals f o r a number o f different orientations and reported that steady-state c r e e p was observed f o r all orientations. The creep curves in his article indicate that steady-state creep was attained almost immediately a f t e r loading, and that this steady-state behavior was maintained throughout the duration o f the creep test. Although Shah t4] did not measure an activation energy for steady-state c r e e p , he did report a steady-state creep stress exponent o f n = 3.5. He suggested that this value indicates that creep o f Ni3AI is similar to many face-centered cubic (fcc)
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