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

Abstract

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 primarily through the accumulation and rearrangement of dislocations into fine cells/subgrains. A quasi-stationary 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 conventional 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.

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ORCID for Terence G. Langdon: orcid.org/0000-0003-3541-9250

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