An atomistic study of self-accommodation martensite morphologies and microstructure evolution during forward and reverse martensitic transformations in single crystal and bicrystal NiTi alloys

Abstract

The forward and reverse martensitic transformations in NiTi alloys with different crystallography conditions, in terms of [100]-oriented and [110]-oriented single crystal models as well as a bicrystal model possessing twist grain boundary, are studied deeply using molecular dynamics simulations. An atomic tracing method is proposed to identify the specific numbers of B19′ martensite variants, hence the self-accommodation martensite morphologies and microstructure evolution during cooling and heating are clearly demonstrated at the atomic scale. The triangular self-accommodation morphology consisted of three correspondence variants (CVs) possessing {1¯1¯1}M type I twin relationships is formed in the NiTi single crystals at the beginning of the martensitic transformation, and stays settled till the end of the transformation. Whereas three types of self-accommodation morphologies including triangular, “herring-bone” and mixed morphologies are formed in the bicrystal, due to the geometrical constraint induced by the grain boundary. The mixed self-accommodation morphology is unstable, which occurs briefly during martensitic transformation and thus evolves into “herring-bone” morphology ultimately. During reverse transformation, B2 austenite phase nucleates preferentially at the triple-junctions of the triangular self-accommodation morphology in NiTi single crystals, and propagates along the junction plane rather than the (001)M compound twin boundaries in the bicrystal alloy.