Effect of severe plastic deformation on the precipitation kinetics and the properties of CuCoNiBe alloys

Abstract

The effect of the precipitation hardening by means of the conventional heat treatment and the combined thermomechanical treatment (i.e., equal channel angular pressing (ECAP) with subsequent aging) on the electrical conductivity, mechanical properties and precipitation kinetics of precipitation hardening, quadruple CuCo1Ni1Be alloy was investigated. No experimental studies on the effect of ECAP on the electrical conductivity and precipitation kinetics of CuCoNiBe alloys have been reported. Systematic microstructural and mechanical investigations show that both ECAP processing and post-ECAP aging result in a significant increase in the hardness and strength of the alloy due to the grain boundary strengthening by means of the grain refinement (and the formation of subgrains) and due to the precipitation hardening by means of the formed precipitates (primary and secondary beryllides) in the fine-grained structure, respectively. Moreover, the electrical conductivity of both solution treated and ECAPed alloys increases with increasing aging time due to the precipitation of the solute atoms in alpha-Cu matrix leading to decreasing electrical resistivity. However, the CuCo1Ni1Be alloys subjected to ECAP exhibit a lower conductivity than the solution treated ones, which can be attributed to an increase in resistivity caused by increased probability of electrons scattering by severe plastic deformation. The linear relationship between electrical conductivity and volume fraction of the precipitates according to the Martition's law and the empirical Avrami equation are used to investigate the precipitation kinetics of the CuCo1Ni1Be alloy. The S-shaped volume fraction of precipitates vs. aging time curves clearly indicate that the precipitation kinetics of the CuCoNiBe alloys can be significantly promoted by ECAP processing. The results of the present study provide new insights into the interaction of the severe plastic deformation, the kinetics of precipitation, and the microstructural, mechanical and physical properties of the CuCo1Ni1Be alloy.