Defect Microstructure Evolution in an Immiscible Composite Cu43%Cr Alloy after High-Pressure Torsion and Annealing using Positron Annihilation Spectroscopy.

Abstract

The microstructure of a Cu43%Cr alloy after high-pressure torsion (HPT) processing and annealing for 1 h was analyzed using Doppler broadening – variable energy PAS (DB-VEPAS) and conventional positron annihilation lifetime spectroscopy (cPALS). DB-VEPAS analysis of the near-surface defects reveals the existence of a nanosized oxide layer whose thickness increases from 43 to 103 nm with temperature (210–850 °C) while the diffusion length is unaffected around 20 nm. cPALS analysis revealed two lifetime components of the bulk defects, namely the components related to either vacancies or dislocations, for the as-received material with annealing at 925 °C. After HPT processing, the alloy showed two components which correspond to positrons trapped and annihilated at dislocations (lifetime ̴ 160 ps) in Cu and Cr and at clusters of vacancies (about 13–10 vacancies). The intensity of the first component decreases with increasing annealing temperatures from 210 to 850 °C, thereby implying a partial annihilation of dislocations due to microstructure recovery. The variation of the second component depends on the variation of vacancy cluster size (from about 13 and 10 to about 4 vacancies) resulting from different annealing temperatures. Additionally, Vickers microhardness measurements show that the alloy is substantially hardened after processing by HPT for N = 20 turns. After annealing for 1 h at 210, 550 and 850 °C, the HPT-processed alloy after 5 turns demonstrated a gradual softening by microstructural recovery. Annealing-induced hardening is observed after HPT for 20 turns followed by heating up to 550 °C while softening is observed after annealing at 850 °C.