Evidence for two-stage hardening in an Al-Zn-Mg-Cu alloy processed by high-pressure torsion

Experiments were conducted to assess the effect of torsional straining on the microstructural evolution of an Al-5.7Zn-1.8Mg-2.1Cu alloy containing dispersed second phase particles prior to deformation. The results show that the material exhibits two distinct and consecutive hardening stages when processed through 1/8, 1, 5, 30, 50 and 100 turns in high-pressure torsion (HPT). There is an initial increase in the Vickers hardness from-120 Hv in the unprocessed state to-240 Hv after 1 turn. In this first stage, hardening occurs pri-marily through the accumulation and rearrangement of dislocations into fine cells/subgrains. A quasi-sta-tionary condition is achieved with further straining where the microhardness remains nearly constant and the grain size is-140 nm up to 30 revolutions. Thereafter, the microstructure consists of ultrafine and slightly elongated grains but the second phases are continuously fragmented. This fragmentation produces a new hardening stage and the development of a uniform array of highly elongated grains with an average size of-93 nm after 100 HPT turns. It is shown that this two-stage hardening is consistent with conven-tional Hall-Petch strengthening and the development of the second stage is attributed to a reduction in grain boundary mobility due to the segregation of solutes. (c) 2023 Elsevier B.V. All rights reserved.

Automated summary

Experiments were conducted to investigate the microstructural evolution of an Al-5.7Zn-1.8Mg-2.1Cu alloy subjected to torsional straining. The material exhibited two consecutive hardening stages during high-pressure torsion (HPT) processing. The first stage involved the accumulation and rearrangement of dislocations, resulting in the formation of fine cells/subgrains. The second stage involved the fragmentation of second phase particles and the development of highly elongated grains.