The linear relationship between diffusivity and crystallization kinetics in a deeply supercooled liquid Ni50Ti50 alloy

The NiTi amorphous alloys are heated to melting (just above T_g) using pulsed laser irradiation to manufacture NiTi shape memory alloys (SMAs). It has been found that the crystallization of these deeply supercooled Ni₅₀Ti₅₀ liquids is unexpectedly fast. Despite the technological importance, the origin of this fast crystallization behavior, however, still remains elusive. In this work, we investigate the crystallization process of deeply supercooled Ni₅₀Ti₅₀ liquids (700–825 K) via molecular dynamics simulation. Through the extensive simulations, we reveal the ultrafast crystallization behavior and, more importantly, establish the strong non-Arrhenius temperature dependence of the kinetic coefficient u_kin for the crystal growth. The linear relationship between u_kin and the diffusion coefficient D obtained from this work provides compelling evidence that the fast crystallization behavior at large supercoolings is rooted in the atomic diffusivity rather than the viscosity, which is the consequence of the breakdown in the Stokes–Einstein relation.