Slip System Modification in NiAl
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SLIP SYSTEM MODIFICATION IN NiAl D. B. MIRACLE *, S. RUSSELL AND C. C. LAW** *AFWAL/MLLM, Wright-Patterson AFB, OH 45433 "**Prattand Whitney Aircraft, East Hartford, CT 06108 ABSTRACT An effort to modify the slip vector in the B2 compound NiAl was undertaken to overcome the brittle failure associated with the slip vector which typically operates in this compound. Alloying additions were made to reduce the ordering energy of NiAl, and hence to promote slip. Preliminary indications showed that Cr and Mn were effective in producing slip in polycrystalline NiAl tested at room temperature. Two-surface slip trace analyses and TEM g.b analyses performed on aligned crystals of NiAl showed that dislocations operate on either ( 110) or (112) slip planes. However, the slip vector at 660"C was seen to revert to , which operates on both [100) and (110) planes. INTRODUCTION The primary slip system in the B2 compound NiAl is (l 10), which results in only three independent slip systems [1]. This lack of five independent slip systems is generally accepted as the reason that NiAl fails in a brittle mode at room temperature. Other B2 intermetallic compounds such as CuZn (beta brass) and AgMg deform by the motion of dislocations [2]. Dislocations with the Burger's vector have also been seen to operate on [110) and (112) planes in NiAl under special loading conditions such as loading along a cube axis in single crystals, so that the Schmid factor for the primary slip systems is zero [3]. However, under general loading conditions, dislocations are not observed to operate, since the resolved shear stress required to activate dislocations at room temperature is about an order of magnitude higher than the resolved shear stress required for the activation of dislocations [4,51. Since operation of dislocations satisfies the Von Mises criterion for a general shape change of a polycrystalline aggregate, an attempt to modify the slip vector in NiAl was initiated. Rachinger and Cottrell have suggested that alloying additions which reduce the ordering energy in NiAl could promote slip by decreasing the APB energy between a/2 partial dislocations [2]. The transition elements Co, Fe, and Mn all form B2 compounds with Al, and each of these compounds has an ordering energy less than NiAl. Further, a metastable B2 compound CrAl has been reported in the literature [6]. Therefore, ternary additions of Cr, Mn, Fe, and Co were made in an attempt to reduce the ordering energy of NiAl. After preliminary observations in polycrystalline alloys, two surface slip trace analyses and TEM g.b analyses were performed on single crystals of alloys exhibiting slip. EXPERIMENTAL Ternary alloys containing 5.2, 10.3, and 20.6% (all compositions are given in atomic percent) of Cr and Mn were made, as well as alloys containing 10.3, 20.6, and 30.9% of Fe and Co. Each alloy contained 48.5% Al, and the balance was Ni. Preliminary investigations were done on arc cast material, and single crystals were prepared for more detailed analyses of selected compositions. Single crysta
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