Effect of vacancy-mediated precipitation on the microstructure and mechanical properties of 7075 aluminum alloy

Abstract

The non-isothermal aging (NIA, 125–300 °C) response of 7075 Al alloy was tracked to assess vacancy-solute interactions on precipitation kinetics and their resulting impact on the mechanical properties. Positron Annihilation Spectroscopy (PAS) revealed that solution heat treatment (SHT)-induced monovacancies enhance solute diffusion, shifting positron trapping from V-Mg to V-Zn, and ultimately forming V-Mg/Zn-promoting complexes that facilitate precipitate nucleation at lower aging temperatures (125–200 °C). At higher temperatures (225–300 °C), the effects of vacancies weakened as precipitates absorbed vacancies, thereby reducing diffusion and yielding finer precipitates. Microstructural analyses (XRD, SEM, and TEM) revealed the following precipitation sequence: the formation of fine Al₂Cu, GP zones, and η’ precipitates in the early aging stages and then subsequent coarsening of these precipitates between 125 and 200 °C; and the dissolution of GP zones, slight refinement of η’, and the formation of stable η precipitates at 225–300 °C. These microstructural transformations had a profound impact on mechanical performance, with peak strength observed during early-stage aging (YS: 310 MPa, HV: 244), followed by a decline to its lowest point at 225 °C (YS: 143 MPa, HV: 147) and a partial recovery at the final aging temperature (YS: 201 MPa, HV: 181). This study helps optimize heat treatment and thereby enhance the performance of 7075 aluminum alloy for aerospace applications by exploring vacancy and precipitation dynamics.