Phase field modeling of Ni-concentration distribution behavior around Ni4Ti3 precipitates in NiTi alloys

Abstract

Ni-concentration distribution behavior surrounding Ni4Ti3 precipitates in the initial supersaturated B2 matrix with Ni-concentration of 51.8at.% has been extensively investigated by means of three-dimensional (3D) phase field modeling which naturally takes into account multiple precipitate variants. The simulation results show that increasing the precipitate diameter from 70 to 112nm leads to a decrease of 0.08at.% for the lowest Ni-concentration along the broad face of the precipitate, and a decrease of 0.06at.% for that along the rim face of the precipitate. When the precipitate spacing increases from 154nm to 240nm and the precipitate distance increases from 122nm to 202nm, the length of undepleted zone of Ni-concentration changes from 50nm to 120nm and from 40nm to 100nm, respectively. A change in the interfacial energy from 50 to 100mJ/m2 influences not only the precipitate shape by decreasing the aspect ratio of precipitate diameter to thickness from 3.98 to 2.52, but also the lowest Ni-concentration with an increase of 0.28at.%. In addition, variation of the strain energy contribution leads to a shift of the equilibrium Ni-concentration of the B2 matrix phase, and an increase of the elastic strain energy contribution by scaling factor from 4 to 16 without subsequent concentration relaxation would lead to a significant decrease of 0.70at.% for the equilibrium Ni-concentration of the matrix, the decrease can be offset to much less than 0.10at.% through concentration relaxation.