Characteristics of a new creep regime in polycrystalline NiAl
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INTRODUCTION
A L T H O U G H unalloyed NiA1 possesses an attractive combination of oxidation, physical, and thermal prop erties for use in gas-turbine aircraft engines, it has poor high-temperature creep strength and low ductility at room temperatures. In addition to its potential technological importance as the base material for alloy development, binary NiA1 is a model-system for studying creep mechanisms in B2 intermetallic compounds, in part due to the presence of a large solid solubility field in the AI-Ni phase diagram and in part due to it being ordered up to its melting point. The creep properties of NiA1 have been studied extensively by a number of investigators for nearly three decades l~-~91 and reviewed recently by several authors.t2~ The first detailed investigation of the elevatedtemperature deformation mechanisms in nearstoichiometric NiAI was conducted by Vandervoort et al.,[~} who reported the dominance of a viscous creep mechanism with a creep stress exponent, n, of about 3.5 and a creep activation energy, Qc, of about 290 kJ molabove 1525 K and the occurrence of an unknown mechanism exhibiting a stress:dependent activation energy with n > 3.5 below this temperature. Values of n > 3.5 have been reported in other investigations, and a close examination of the data reveals that they fall into two broad categories: (a) n ~ to 4 to 7, [6'7'9'11'13'15'161and (b) n >> 7,14'6] depending on stress, temperature, and processing history. The reported values of Q,: from most of these investigations, for which n ~- 4 to 7, are generally in reasonable agreement with the activation energy for lattice self-diffusion of Ni, QN~, in NiAI, which is about S.V. RAJ and S.C. FARMER, Materials Research Engineers, are with the NASA Lewis Research Center, Cleveland, OH 44135. Manuscript submitted April 11, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
305 kJ mol -j for Ni-50 (at. pct) A1, .1231 although somewhat higher values of Q, ~ 325 to 365 kJ mol-~ have also been reported for polycrystalline XD-processed** *All compositions are given in atomic percent in this article. **XD is a trademark of the Martin Marietta Corporation, Bethesda, MD.
NiAI. tlSl Based on these results and observations of subgrains (e.g., Reference 11), it is generally believed that dislocation climb is the dominant creep mechanism under conditions where n ~ 4 to 7 and Q,, ~ Qt ~ Qni, where Q~ is the activation energy for lattice diffusion. In addition, values of Q,, have been observed to be stress, and temperature dependent for near-stoichiometric and Ni-rich alloys. IL6] However, the observations reported in these two studies differ from each other in some aspects. For example, Vandervoort et al. I~l observed that Q,. was stress dependent for Ni-50.4 AI below 1450 K for applied stresses, o-, greater than 11.0 MPa, primarily corresponding to the n ~ 4 to 7 region; however, Qc was constant and equal to about Q~ above 1450 K for o- < 11.0 MPa corresponding to the n ~ 3.5 region. In contrast, Yang and Dodd [61 observed that Q,. is stress dep
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