Temperature-dependent-composition of η phase in an Al-Zn-Mg-Cu alloy under high pressure torsion: Kinetics and thermodynamics

This research addresses fundamentals regarding the temperature-dependent composition of η phase in Al-Zn-Mg-Cu alloys under high pressure torsion (HPT) to uncover kinetics and thermodynamics governing the η phase composition evolution. Two-stage HPT experiments are deployed to study the stability of the high-temperature η under low-temperature processing. The first-stage HPT on a solid-solution-treated Al-Zn-Mg-Cu alloy (AA7075) at 200 ℃ for 10-revolutions is to get the high-temperature η, and the second-stage HPT is performed at either 100 ℃ or RT. Atom probe tomography reveals that the high-temperature η phase becomes chemically unstable during the low-temperature processing and its chemistry evolves towards low-temperature values. Surprisingly, a high-Mg Al-Mg phase is, for the first time, observed to form in the alloy under the 2nd-stage HPT processing, in correlation with the decomposition of the high-temperature η phase under the low-temperature processing. Unlike the 1st-stage HPT processed alloy, the 2nd-stage HPT processed alloy develops the segregation of Zn with other elements at grain boundary, accompanied with the chemical modification of the high-temperature η phase. The thermodynamics driving η phase composition variation with temperature provides the Al alloys with temperature variable for multi-stage treatments to engineer precipitation for optimised properties.