Effect of Room Temperature Ultrasonic Vibration Compression on the Microstructure Evolution and Mechanical Properties of AZ91 Alloy

To investigate the potential of direct ultrasonic vibration on improving the performance of magnesium alloys, this study first employed the ultrasonic vibration compression (UVC) on the solid solution treated AZ91 alloy, and explored its microstructure evolution and mechanical properties under UVC. Within only two seconds, the UVC alloys showed large deformation strains of 34.8–54.4%, and sudden increase of sample temperature to 243 °C. Microstructure characterizations proved that UVC promoted the formation of abundant shear bands, fine grains, and the bimodal distribution of Mg₁₇Al₁₂ precipitates consisting of submicron particles located within the shear bands and nano-sized ones within the matrix. Owing to the unique microstructure, the micro-hardness (and nano-hardness) value of UVC alloy was increased by 37.7% (35%) when compared with the solution-treated alloy. Moreover, the nano-modulus of the developed AZ91 alloy was also significantly increased to 62 GPa by statistical nanoindentation tests, which could be ascribed to increased Mg₁₇Al₁₂ precipitates and decreased c/a value to some extent. In general, this work provides a new insight into the design and preparation of high-performance magnesium alloys by UVC at room temperature.