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

Abstract

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 α phase. Furthermore, the α phase is elongated and aligned along the direction of processing rotation. When fatigue cracks encounter the hardened α phase, they follow an easier expansion direction. Therefore, the crack extension mode changes to increase the fatigue life of the LZ91 alloy. K E Y W O R D S fatigue cracking, low-cycle fatigue, Mg-Li alloy, ultrasonic nanocrystal surface modification Highlights • The UNSM treatment improves the mechanical properties of LZ91 Mg-Li alloy. • The hardened α phase changed the crack expansion pattern after UNSM treatment. • The UNSM treatment improves the fatigue life of LZ91 Mg-Li alloy. | INTRODUCTIONMg-Li alloys are green engineering materials in the 21st century and have attracted considerable attention in various fields because of their low density, high specific strength, stiffness, good processing, and recycling properties. 1,2 The LZ91 Mg-Li alloy has multiple applications in various fields. In the aerospace industry, it can be used to construct rocket cabin bodies and lunar landing platform components. In the field of weapons, it can be utilized as missile tails, shells, armor, and bulletproof material. 3,4 However, these components are subjected to cyclic deformation or vibration caused by factors such as airflow and weather. 5 This can lead to low-cycle fatigue damage. Owing to their low strength and hardness, poor formability, and difficulty in obtaining high-quality surface properties, the widespread use of Mg-Li alloys is limited. Hence, the surface integrity

Microstructures and properties of ultrasonically surface-modified Cu–0.2Be–1.0Co alloy

“…The tip can rapidly strike to the surface of a metal material with extra external static load controlled by a CNC machine [20]. The high frequency impact can induce plastic deformation and compress residual stress in the metal surface layer, which will generate gradient grain refinement microstructures as well as dense dislocations and consequently lead to surface strengthening [21,22]. Compared with other surface plastic deformation methods, such as high-pressure torsion [23], singleroll angular-rolling [24], laser shock peening and ultrasonic shot peening [25][26][27], USM exhibits precise controllability, complex geometric processing capability, less surface deterioration, high work efficiency, low cost and easy operability [28][29][30].…”