Deformation Structures in Oriented NiAl and CoAl Single Crystals Deformed at Elevated Temperature
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DEFORMATION STRUCTURES IN ORIENTED NiAI AND CoAl SINGLE CRYSTALS DEFORMED AT ELEVATED TEMPERATURE Y. Zhang, S.C. Tonn and M.A. Crimp Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824-1226 ABSTRACT The deformation characteristics of the B2 intermetallic alloys NiAl and CoAl have been examined using single crystals deformed at 673 K in the orientation. Slip trace analysis and transmission electron microscopy was used to characterize the deformation process. While many aspects of the deformation are similar, some distinct differences were observed. All of the alloys were found to deform by slip. NiAl was found to deform by slip on a combination of (110) and (100) slip planes with the predominant slip plane being a function of alloy stoichiometry. In contrast, CoAl was found to deform by slip only on (100) planes. INTRODUCTION In the search for new high temperature structural materials, the B2 aluminides have received considerable attention due to their high melting temperatures, moderate densities and potential for excellent oxidation and corrosion resistance. Of these B2 alloys, NiAl has received the most attention (for review see [1]). FeAl has also received a considerable amount of research attention. However, CoAl has been the focus of very few investigations. Presumably, this is due to the reputation CoAl has for extreme brittleness. In many respects CoAl and NiAl are very similar. Both of these materials form the B2 structure via a congruent melting point and have a wide range of solubility. The resulting structures are almost identical in terms of melting temperature (1932 K for NiAl and 1921 K for CoAl) and lattice parameter (0.288 nm and 0.286 nm respectively [2,3]). Consequently, the densities are also similar, 5.92 g/cc for NiAI versus 6.08 g/cc for CoAl [2,3] for the stoichiometric alloys. Despite these similarities, significant differences are noted between the mechanical behavior of NiAl and CoAl. These differences were first noted by Westbrook [4] who measured the hardness of single crystals of B2 aluminides. While NiAl and CoAl both display increases in hardness with deviation in alloy stoichiometry from 50 at% Al, Westbrook found that CoAl is significantly harder than NiAl at temperatures ranging from room temperature to 1073 K. Additionally, the hardness of CoAl was found to increase at a greater rate with deviations from perfect stoichiometry. The higher hardness of CoAl is reflected in yield strength measurements recently performed by Fleisher [5] on a wide range of polycrystalline Co-Al alloys tested at temperatures up to 1223 K. When compared with similar tests for NiAl polycrystals [6,7], it is clear that the yield strength of CoAl is much greater than NiAl. In addition to being harder and stronger than NiAl at low to moderate temperatures, CoAl also displays much greater creep strengths. Working with single crystals, Hocking, Strutt and Dodd [8] found that, depending on stress level, creep strain rates at 1323 K were at least 1.5 orders of magnitud
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