The role of phase transitions in the evolution of dispersion particles in chromium bronzes upon the equal channel angular pressing

Abstract

Samples of a diluted Cu-Cr-Zr alloy were processed with a combination of severe plastic deformation (SPD) methods. The average sizes and distribution density of particles with different chemical compositions undergo an evolution that indicates that phase transformations occur even during the cold deformation. A decomposition of the solid solution due to the deformation-induced acceleration of diffusion contributes to this evolution, but the whole complex of observations cannot be explained only by this phenomenon and implies the presence of the deformation-induced dissolution of particles. A possible mechanism of the dissolution linking this process to a mechanical fragmentation of the particles upon the SPD is discussed. A dependence of the dissolution efficiency on the composition of the particles was noted. A conclusion about the simultaneously occurring kinetically opposite phase transitions is also confirmed by an observation of changes in the lattice constant of copper that are non-monotonous versus the accumulated strain. The crystalline structure refinement upon the SPD proceeds through self-organization of dislocation pile-ups into grain boundaries what implies an influence of the particles –obstacles to dislocation motion – on the structural transformations. Thus, there exists an interdependence between strain mechanisms and phase transitions such as the second phase particles precipitation and their dissolution in the copper matrix that influences the structure refinement, the dispersion particles distribution and, consequently, the strength and other properties of the material. This influence may be especially complicated in alloys that like the one under study have a second phase consisted of particles of many different compositions.