Self-diffusion and interface diffusion in crystalline and amorphous Ni/Ti multilayer: A molecular dynamics study

Diffusion phenomena in crystalline and amorphous Ni/Ti binary multilayer systems have been simulated by the molecular dynamics (MD) technique in a wide temperature range of 473–1600 K and compared with the experimental results. Evolution of the diffusion process and crystalline phase formation has been investigated as a function of annealing temperature. Formation of the B2-phase (austenite) of the NiTi alloy has been noticed at the interface for an amorphous multilayer system at a relatively lower temperature compared to the crystalline multilayer system, which corroborates well with our earlier experimental findings on this system. MD simulation has also helped to study the diffusion of Ni and Ti separately and it is observed that the diffusion of Ni and Ti in an amorphous system is higher than the corresponding diffusion in a crystalline system. A very interesting difference has been noticed with self-diffusion being dominant in Ti and interface diffusion in Ni. As a result, the formation of new FCC and BCC Ti phases has been observed in the amorphous Ti layer. Individual diffusion coefficients of Ni and Ti are estimated for both crystalline and amorphous systems, and it is observed that the variation of diffusion coefficients with temperature follows the Arrhenius-type relation, with lower activation energies obtained for the amorphous system over the crystalline system. Furthermore, the diffusion mechanism for both the systems is discussed. The above study gives important insight into the interface diffusion process of a technologically important multilayer system and would be immensely helpful in designing applications of such multilayers.