Effects of B2/B19′ phase boundary on thermally induced phase transition in NiTi: An atomistic study

Molecular dynamics simulations are performed to study the thermally induced phase transition in NiTi B19′ single crystal at high temperatures (400, 500, 600 and 700 K), B2 single crystal at low temperatures (50, 100, 150 and 200 K) and the system containing a B2/B19′ phase boundary at high, medium (300, 320 and 340 K) and low temperatures. Homogeneous nucleation and growth of the B2 and the B19′ phases are observed in B19′ and B2 single crystal systems, respectively. In B2/B19′ system, heterogeneous nucleation and growth of the B2 phase are observed at high temperatures, which results in forming a B2/B2 grain boundary and a new B2/B19′ phase boundary; the B2 phase transforms homogeneously to B19′ phase at low temperatures; the B2 and B19′ phases coexist without heterogeneous nucleation of any phase at medium temperatures. Compared with the single crystal, the B2/B19′ phase boundary provides the sites for heterogeneous B2 nucleation at high temperatures and has little effect on the phase transition at low temperatures.