Enhanced low-cycle fatigue behavior LZ91 Mg-Li alloy with ultrasonic nanocrystal surface modification

Ultrasonic nanocrystal surface modification (UNSM) is widely used to improve the high-cycle fatigue properties of metals. However, research on the effect of UNSM on the low-cycle fatigue behavior of metals, particularly magnesium (Mg) alloys, is limited. In this study, the LZ91 Mg-Li alloy is strengthened using UNSM. The microstructures, surface roughness, tensile properties, and low-cycle fatigue properties of the strengthened LZ91 alloy are measured. The results reveal that the surface roughness decreases and the hardness increases significantly after UNSM treatment, particularly the hardness of the alpha phase. Furthermore, the alpha phase is elongated and aligned along the direction of processing rotation. When fatigue cracks encounter the hardened alpha phase, they follow an easier expansion direction. Therefore, the crack extension mode changes to increase the fatigue life of the LZ91 alloy.