Atomic scale modeling of the coherent strain field surrounding Ni4Ti3 precipitate and its effects on thermally-induced martensitic transformation in a NiTi alloy

Abstract

Precipitation of Ni4Ti3 in NiTi alloy profoundly affects material properties, while the coherency strain fields and their effects on the thermally-induced martensitic transformation are not known in detail, especially for the B19’ variants and morphologies. Therefore, molecular dynamics simulations, as well as an Eshelby solution and phase-field microelasticity theory were applied to investigate the strain fields caused by the Ni4Ti3 precipitates with different aspect ratios. The maximum strain (along the central axis of the precipitate) in the matrix and its relative position are formulated as function relationships. Ms (martensitic transformation start temperature) and Af (austenitic transformation finish temperature) during martensitic transformation were determined and analyzed. Two previously published self-accommodation B19’ structures, the triangular and “herring-bone’’ morphologies are investigated in detail. An intermediate state between the triangular and “herring-bone’’ morphologies, termed mixed self-accommodation, is observed for the first time and proved to be the most unstable structure, providing a potential design route for low hysteresis microactuators. The formation processes of these self-accommodation morphologies are analyzed and discussed. Our simulations are the first time to reveal a variety of twinned B19’ morphologies selected by precipitates and their effects on thermally-induced martensitic transformation at the atomic scale.