Effects of homogenization time on aging behavior and mechanical properties of AZ91 alloy

This study demonstrates that the holding time of homogenization treatment conducted before aging strongly affects the precipitation behavior of AZ91 alloy during aging and its mechanical properties after aging. When homogenization is conducted for a relatively short time of 8 h, obvious Al solute segregation is still observed near the grain boundaries and in the interdendritic region because of insufficient diffusion of the solute atoms. This locally high Al concentration accelerates discontinuous precipitation at the grain boundaries and continuous precipitation in the interdendritic region in the early stage of aging treatment. However, the growth of formed discontinuous precipitates (DPs) is prematurely terminated on account of a decrease in the driving force for the growth of DPs owing to the presence of alpha-Mg dendrites with a low Al concentration and promotion of the formation of continuous precipitates (CPs). As a result, the microstructures of the AZ91 samples homogenized for 8 h and 96 h are obviously different after aging for 2.h, but they become similar after peak aging for 24 h. The hardness and tensile strength of the two samples with different homogenization times are nearly identical in the homogenized and peak-aged states. However, in the 2-h-aged state, the hardness and strength of the sample homogenized for 8 h are higher than those of the sample homogenized for 96 h, which is mainly attributed to the presence of a larger amount of hard DPs in the former sample. The tensile strength and ductility of the two samples under the homogenized, 2-h-aged, and peak-aged conditions and the causes of their differences under these conditions are discussed in detail.