Martensitic phase transformation in NiTi bi-crystals with symmetric Σ25 twist and tilt grain boundaries

Abstract

NiTi alloys attract a lot of attention of researchers for a number of reasons; among them are their practical importance and challenges for theoretical understanding. The most exciting feature of these alloys is the shape memory effect due to the martensitic transformation at temperatures close to the room temperature. There exist many factors affecting the transition temperatures in such materials, such as a deviation from stoichiometric composition, dislocation density, grain size, and the type of grain boundaries. The latter factor is one of the less explored, and we are aware of just a few studies in this direction. In the present work, molecular dynamics simulations are carried out to reveal the effect of symmetric tilt and twist grain boundaries in bi-crystals with nanosized grains on the forward and reversed martensitic transformations during cooling down from the austenite B2 phase and subsequent heating up from the martensite B19' phase. Phase composition, elastic strain components, relative change of volume, potential energy per atom, and shear stresses are calculated and analyzed as the functions of temperature. It is found that the type of grain boundaries in the bi-crystals strongly affects the transition temperatures. Start and finish temperatures of the forward and reverse martensitic transformations are much lower in the bi-crystal with twist grain boundaries as compared to that having tilt grain boundaries. Overall, the simulation results of this study are in a good qualitative agreement with the available experimental data.