Microstructure evolution and precipitation strengthening behaviors of non-isothermal aged SiC/7xxxAl composite

Abstract

Non-isothermal aging (NIA) treatments have presented significant advantages in improving the comprehensive performance and aging hardening efficiency of the 7000 series aluminum alloys, but there is no attention paid to their composites. This study takes a linear heating aging process as an example to reveal the precipitation behaviors of a 15 vol.% SiC/7085Al composite as well as its impact on mechanical properties using differential scanning calorimetry, transmission electron microscopy, small-angle neutron scattering, hardness measurements, and tensile testing. The results indicated the formation of GP (I, II) zones, η' and η precipitates in sequence, leading to the hardness and strength initially increasing and then decreasing with rising NIA temperatures. The maximums were reached at 183 °C, corresponding to the appearance of η' precipitates in large quantities. Owing to the rapid temperature rise during the NIA process, the precipitates entered the coarsening and redissolution stage before they were entirely formed, resulting in reduced peak strength compared to the T6 treatment. The composite exhibited a more significant reduction in strength than the 7085Al alloy because: (i) the annihilation of vacancies suppressed the formation of GPII zones, thereby weakening their transition to η' precipitates; (ii) quenching dislocations promoted the coarsening of precipitates. An improved NIA process, incorporating both heating and cooling aging treatments, was effectively designed with the assistance of in-situ SANS technology to address this issue, which allows for achieving strength comparable to that after the T6 treatment with only 15% of the aging time consumption. This research fills the gap in investigating the NIA precipitation behaviors of aluminum matrix composites, providing guidance for the formulation of NIA schedules.