An atomistic investigation of the kinetics of detwinning

This paper presents a “first principles” atomistic study of the dynamics of detwinning in a shape-memory alloy. In order to describe the macroscopic motion of twin boundaries, the continuum theory of twinning must be provided with a “kinetic relation”, i.e. a relation between the driving force and the propagation speed. This kinetic relation is a macroscopic characterization of the underlying atomistic processes. The goal of the present atomistic study is to provide the continuum theory with this kinetic relation by extracting the essential macroscopic features of the dynamics of the atoms. It also aims to elucidate the mechanism underlying the process of detwinning.The material studied is stoichiometric nickel–manganese, and interatomic interactions are described using three physically motivated Lennard-Jones potentials. The effect of temperature and shear stress on detwinning — specifically on the rate of transformation from one variant of martensite to the other — is examined using molecular dynamics. An explicit formula for this (kinetic) relation is obtained by fitting an analytic expression to the simulation results. The numerical experiments also verify that transverse ledge propagation is the mechanism underlying twin-boundary motion. All calculations are carried out in a two-dimensional setting.