Kinetics of Order–Disorder Transformation in Alloys under Electron Irradiation

A simple kinetic model is presented which describes the order–disorder reaction under irradiation. The model is based on the Bragg-Williams approximation. It is assumed that radiation-induced disordering is opposed by radiation-enhanced reordering. The ordering process is modeled by a chemical reaction whose elementary step occurs via vacancy jumps. The activation energy of such jumps is a linear function of the degree of order. Compared to the value in disordered material it is higher for a jump contributing to disordering and lower in the opposite case. A set of rate equations is derived which allow to calculate the steady-state vacancy concentrations as a function of the degree of order. The treatment is given in general form applicable to alloys with B2, LI2, and D1a, structure. Details are worked out numerically for B2 and D1a alloys. The results for D1a are compared with those of recent experiments on Ni4Mo.