Unusual kinetics of strain-induced diffusional phase transformations in Cu-Cr-Zr alloy

Abstract

This paper reports experimental results demonstrating nonmonotonic changes of the solid solution concentration in the process of high-pressure torsion of the preliminary aged Cu-Cr-Zr alloy. The solid solution concentration which is very low in the initial state passes through a maximum before it finally stabilizes at a lower value. Such a behavior is, strictly speaking, impossible for a purely diffusion-controlled process under stationary conditions. Observations on the evolution of the second phases particles in the course of deformation suggest a possible mechanism behind this phenomenon. Severe deformation causes refinement of the particles initially present in the alloy by, most probably, quasi-brittle fracture, what creates fragments with sharp edges and makes possible their partial dissolution by Gibbs-Thomson mechanism. The morphology and sizes of the partially dissolved fragments as well as of newly precipitated particles make them less susceptible to fracture than those formed by the preliminary aging. So, under severe deformation, unlike the usually considered models, a “dissolving” subset of particles evolves not only due to diffusion; in the other words, the deformation creates a difference between “dissolving” and “precipitating” subsets of particles. As combined fracture and dissolution transform the initial ensemble of particles, the dissolution gradually slows down unlike the precipitation, which rate is controlled by the solution concentration and density of precipitation sites. As a result, these processes first reach a transitional balance, corresponding to the maximum concentration, and later a stable dynamic equilibrium on its lower level.