From first to second order nonequilibrium phase transition in crystal-amorphous interface: Effects of spatial and kinetic constraints

Crystalline materials undergo phase transitions to disordered state at the melting point when being heated; but the transition can also occur from crystal to amorphous solids when subject to various other stimuli such as chemical mixing, irradiation, or hydrogen permeation, etc. The amorphization transition observed in experiments is often first order, but theories predict a possibility of the second order. Here, we demonstrate that the first order transition can indeed become continuous in a diffusion couple using molecular dynamics simulation. The amorphization transition happens anisotropically at the interface between a crystal and an amorphous alloy with a steady concentration gradient. Under the non-equilibrium condition and the spatial constraint, the amorphization of the crystalline phase can proceed without the abrupt changes in thermodynamic properties often seen in the first order transition. A homogeneous model and theoretical analysis also confirm that an isentropic transition can appear when sufficient defects or impurity atoms are introduced under certain kinetic constraints. Our study provides a reasonable explanation for the inconsistency existing in the experiments and simulations or theories on the thermodynamic understanding of the disordering process in the crystal-amorphous transitions.