On the effect of superimposed external stresses on the nucleation and growth of Ni4Ti3 particles: A parametric phase field study

The effect of a superimposed stress on the coarsening of interacting Ni4Ti3 particles is studied using the multi-phase field method. It is found that the thickness/diameter ratio of a Ni4Ti3 particle in a (1 1 1)B2 plane increases with an increasing [1 1 1]B2 stress component. The particle shape can change from a disk to a sphere with increasing applied stress. It is also found that diffusional and mechanical interactions between two Ni4Ti3 particles can promote the nucleation of new particles. This provides an explanation for the autocatalytic nature of nucleation reported previously. Compressive stresses along [1 1 1]B2 increase the volume fraction and growth velocity of the Ni4Ti3 particles of the (1 1 1)B2 plane. Misoriented particles disappear during particle growth. The simulation results are discussed in the light of previous experimental results.

Research highlights
► Nucleation and growth of Ni4Ti3 precipitates in NiTi shape memory alloys is studied by multi-phase field simulations.
► A model of for thermodynamically consistent treatment of stoichiometric phases is proposed and applied in the present study.
► External compressive stress is predicted to change the morphology of the precipitates and to favor variants whose axis is parallel to the direction of stress.
► Autocatalytic nucleation of a chain of precipitates is explained by the trade of between solutal and strain related deviation for thermodynamic equilibrium.