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 {111}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.
NSTL
Elsevier