Thermal stability, microstructure evolution and grain growth kinetics of ultrafine grained Al 7075 alloy processed by cryogenic temperature extrusion machining

Abstract

In this study, an investigation was conducted over the effect of annealing on microstructure and precipitation evolution, microhardness, thermal stability, as well as grain growth kinetics of Al 7075 alloy processed by cryogenic temperature extrusion machining (CT-EM). A severely deformed microstructure with refined grains and high dislocation density was obtained in the initial CT-EM sample. There was an improvement in the hardness of the chip sample due to the precipitation strengthening in the case of sample annealed at 150 °C, when the dislocation density decreased. With the increase of the annealing temperature, more dislocation-free equiaxed grains were present in the microstructure. The recrystallization process started at 150 °C and got completed at 300 °C. The ultrafine grained (UFG) Al 7075 alloy fabricated by CT-EM was thermally stable up to 350 °C, which could be attributed to the pinning effect of precipitates. The differential scanning calorimeter (DSC) results indicated the efficiency of CT-EM treatment in facilitating precipitate kinetics. The activation energy of grain growth in the Al 7075 alloy subjected to CT-EM was higher than that of materials processed by other severe plastic deformation (SPD) processes.