Evolution of microstructures in an Al-Zn-Mg-Cu alloy processed by multi-stage high pressure torsion and annealing

Abstract

Severe plastic deformation (SPD) can enhance aluminium alloy, via producing ultrafine grains (UFG). However, the UFG materials are susceptible to grain growth at high temperature. It is important to understand the evolution of microstructures in UFG Al alloys processed by SPD and their response to post-SPD annealing. This thesis investigates the evolution of microstructures in a commercial AA7075 processed by onestage and two-stage high pressure torsion (HPT) as well as annealing. The temperature dependence of the composition of the η phase was investigated. The high temperature η phase was formed in the 1st-stage 200 ℃-HPT processed sample. This η phase become chemically unstable during a 2ndstage 100 ℃-HPT and 2nd-stage room temperature (RT) HPT processing, and its chemical composition evolved towards low-temperature values. The study of the UFG AA7075 thermal stability revealed that the 1st-stage and 2nd-stage RT-HPTprocessed samples exhibit a different microstructural evolution under annealing. RT-precipitated η in the 1st-stage RT-HPT-processed sample and RT-transformed η in the 2nd-stage RT-HPT-processed sample transform into a coarser form of the high-temperature η during the annealing. The coarsening of the RT-transformed η particles is kinetically faster than that of the RT-precipitated η. The 1st-stage RT-HPT-processed sample with weak GB segregation exhibits better microstructural stability than the 2nd-stage RT-HPT-processed sample with stronger GB segregation. This phenomenon is due to restrained recovery and slow transformation kinetics of the GB RT-precipitated η precipitates. The Mg-rich Al-Mg particles with composition close to Al12Mg17 were also observed. It was revealed that these particles were by-product of the thermodynamic-driven compositional modification of the high-temperature η during low temperature 2nd-stage HPT processing. The Mg-rich Al-Mg particle was not stable during post-HPT annealing, transforming to an Al-rich Al-Mg phase.