Enhancement of surface hardening of β phase Mg-Li alloy through nanocrystallization and amorphization by warm ultrasonic shot peening

This paper presents a warm ultrasonic shot peening (WUSP) strengthening technique for the β-phase Mg-Li alloy. The impacts of ultrasonic shot peening (USP) and WUSP on the surface microstructure and microhardness of the β-phase Mg-Li alloy were investigated and contrasted through the utilization of X-ray diffraction, optical microscopy, transmission electron microscopy, and microhardness testing.The experimental results demonstrated that, in the USP treatment, when the peening duration was set at 320 seconds, the grains were refined to approximately 91.6 nm, and the surface hardness reached 129.37 HV, which was 104.37% higher than that of the original sample. After the WUSP treatment, when the temperature was maintained at 100 °C, the grains were refined to around 10.65 nm, with an amorphization degree of 6.35%. Meanwhile, the surface hardness attained 154.41 HV, which was 19.35% higher than that at room temperature and 143.93% of the original sample's hardness.Furthermore, the mechanism underlying the surface grain nanocrystallization and partial amorphization of the β-phase Mg-Li alloy during USP and WUSP was explored, and the influence of temperature on the microstructure evolution mechanism of the β-phase Mg-Li alloy surface was analyzed. It was found that when the temperature is 100 °C or below, the “Temperature's Refinement Effect” prevails, whereas when the temperature is 150 °C or above, the “Temperature's Coarsening Effect” dominates.