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We present the structural evolution mechanism during the NiTi martensitic transformation and show the origins of this behavior in electronic and phononic anomalies. By employing highly precise all-electron density-functional theory calculations, we establish a barrierless transformation path for equiatomic NiTi consisting of a basal shear composed of bilayer ͗100͕͘011͖ stacking faults to the B2 phase followed by another basal shear which causes a relaxation of the structure's monoclinic angle and results in the B19Ј phase. This path is traced to evolving Fermi-surface nesting regions, which drive the structural transformation between the austenitic and martensitic phases.The importance of thermomechanical and piezomechanical engineering and design has generated considerable interest in equiatomic NiTi ͑nitinol͒ for its exhibition of a reversible martensitic transformation near room temperature and shape memory effect. With an abundance of possible applications as medical devices and materials for aerospace, industrial, and commercial use, a full understanding of why and how this crystal undergoes a structural transformation is crucial. However, despite great progress toward understanding this material both theoretically and experimentally, the atomistic model for this transformation path is not well established. Here we present a mechanism for this transformation and uncover the underlying cause of its behavior.In total, there are four different phases that have been observed throughout the martensitic transformation of NiTi: B2, R, B19, and B19Ј. The high-temperature austenite B2 phase transforms to the low-temperature martensite B19Ј phase upon cooling either directly or through intermediate B19 or R phases if alloyed with ternary elements such as Cu and Fe, respectively. Here we seek an understanding of the direct equiatomic NiTi transformation path from B2 to B19Ј, which is still not clearly established. Otsuka and Ren 1 proposed that this transformation occurs by the application of a ͗110͕͘110͖ basal shear/shuffle followed by a ͗110͕͘001͖ nonbasal shear and that, in the absence of the nonbasal shear, NiTi evolves to the B19 structure similar to that of many other Ti alloys ͑e.g., TiPd, TiNiPd, and TiAu͒. More recently, Huang et al. 2 proposed that the martensitic phase was not the B19Ј structure that was established by experiment and previous calculations 3-8 but rather a base-centered-orthorhombic ͑BCO͒ structure and that B19Ј is instead effectively stabilized by internal stresses. Subsequently, Morris et al. 9 calculated a transformation from B2 to this BCO phase, which they identified as B33. The transformation involves shuffling pairs of ͕011͖ planes via stacking faults which leads to the orthorhombic B33 structure. To date there is no established atomistic transformation path. In this Rapid Communication we resolve these issues by first investigating those previously proposed paths and then clearly establish a direct transformation path for NiTi.To fully understand the structural evolution of the phases...

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Formation of the Ni 4 Ti 3 precipitate has a strong effect on the shape memory properties of NiTi alloys. In this work, growth of this precipitate is studied using phase-field modelling and density functional theory (DFT) calculations. Using first-principles calculations, the composition-dependent stability and elastic properties of the B2 phase are obtained. Composition-dependent elastic constants are incorporated into our phase-field model to investigate the interplay between stress and concentration fields around the precipitate. The model introduces a source of diffusion due to mechanical relaxation which is accompanied by local softening/hardening of the B2 phase. The results are discussed in light of previous experimental and simulation studies.