Phase stability and transformations in NiTi from density

Abstract

We used density functional theory to characterize various crystalline phases of NiTi alloys: (i) high-temperature austenite phase B2; (ii) orthorhombic B19; (iii) the monoclinic martensite phase B190; and (iv) a body-centered orthorhombic phase (BCO), theoretically predicted to be the ground state. We also investigated possible transition pathways between the various phases and the energetics involved. We found B19 to be metastable with a 1 meV energy barrier separating it from B190. Interestingly, we predicted a new phase of NiTi, denoted B190 0, that is involved in the transition between B190 and BCO. B190 0 is monoclinic and can exhibit shape memory; furthermore, its presence reduces the internal stress required to stabilize the experimentally observed B190 structure, and it consequently plays a key role in NiTi’s properties. 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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