A Comparative First Principles Study of Phase Stability in Ni-Ti And Ni-Al Alloys Around Equiatomic Composition
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A COMPARATIVE FIRST PRINCIPLES STUDY OF PHASE STABILITY IN Ni-Ti AND Ni-Al ALLOYS AROUND EQUIATOMIC COMPOSITION P.E.A. TURCHI*, M. SLUITER*, F.J. PINSKI** and D.D. JOHNSON*** *Lawrence Livermore National Laboratory, Chemistry and Materials Science Department, L-268, P.O. Box 808, Livermore, CA 94550
"**Universityof Cincinnati, Physics Department, Cincinnati,
OH 45221 ***Sandia National Laboratory, 8341 East Ave., Livermore, CA 94550
ABSTRACT Electronic structure and phase stability properties of Ni-Ti and Ni-Al around equiatomic composition are investigated with a first principles approach. The study is based upon the generalized purturbation method applied to the Korringa-Kohn-Rostoker multiple scattering formulation of the coherent potential approximation. Within this framework, effective cluster interactions which build up the configuration-dependent part of the internal energy are calculated. The strength of ordering tendencies in both bcc-based substitutional alloys is compared and contrasted in terms of hybridisation effects, in relation with experimental evidences. INTRODUCTION AND APPROACH Materials based on Ni-Ti and Ni-Al are highly promising for practical applications. Around equiatomic composition, Ni-Ti alloys exhibit interesting shape memory properties associated with a first order displacive, (or martensitic) transformation upon cooling from a high temperature B2 ordered (of Cs Cl type) phase [1]. Although bcc-based Ni-Al alloys also show a stable B2 ordered phase around equiatomic composition, the martensitic transformation occurs in a very narrow range of concentration around 62.5 atomic pct. Ni [2]. To complete the similarities, Ni 3 Ti and Ni3 Al exhibit the same generic local order on different crystalline structures, i.e. DO 24 (of Ni 3 Ti type) on a double hexagonal close-packed structure and LI 2 of Cu 3 Au type) on an fcc structure, respectively. In the Ti or Al rich side of the phase diagrams, both alloys form line compounds [3]. In this preliminary study, we will concentrate on the description of order-disorder phenomena in Ni-Ti and Ni-Al bcc-based alloys around equiatomic composition and examine the hybridisation effects which could explain the difference in ordering tendencies between these two alloys. The study is based on the Generalized perturbation Method (GPM) [4] applied to the Korringa-KohnRostoker multiple scattering formulation of the Coherent Potential Approximation (KKR-CPA) [5]. Within this framework, the difference in energy between a completely ordered state and the random medium is expressed as an expansion in terms of pair and multi-body interactions as follows: Mat. Res. Soc. Symp. Proc. Vol. 186. 01991 Materials Research Society
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AE )=1 oErd (Pn}=J
vnm n&
1 Vn a
+11 3 1 Vvnm =,8C n8c m& 1+...
where (Pn) specifies a configuration (Pn equals 0 or 1 depending on the occupancy of site n by an A or B species), &cn= Pn - c , where c is the concentration in B species, and the V are the effective interactions. For example, Vnm is an effective pair interaction (EPI)
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