Atomistic simulation of porous nanocrystalline NiTi shape memory alloy considering coupling effects of transformation and plasticity

Abstract

Nanocrystalline NiTi alloy exhibits recoverable martensitic transformation and irrecoverable plastic deformation when subjected to sufficiently high stresses. Mechanical behaviors including superelasticity and plasticity of NiTi alter drastically with the change in its initial microstructure such as void and grain size. This work mainly focuses on investigating the coupling effects of transformation and plasticity as well as the grain size and porosity dependent superelastic-plastic responses of porous nanocrystalline NiTi by using molecular dynamics simulations. Results demonstrate that plastic deformation exerts a potent inhibitory influence on the reverse phase transformation from martensite to austenite. The mechanisms of the transformation-plasticity coupling and the generation of residual strain after unloading are revealed at the atomic level. The evolution of the microstructure manifests that the residual martensite due to partial reverse transformation and the dislocation inside the grains as well as the disordered structures at the interfaces make the contribution to the total residual strain. The concentration of plastic shear strain around the voids and on the grain boundaries indicates that grain-boundary sliding and stress concentration around the voids contribute significantly to the plastic deformation. Simulated results indicate that porosity and grain size exert significant effects on the superelastic-plastic behaviors.