Ultrasonic fatigue tests are performed on a magnesium alloy with and without ultrasonic peening treatment (UPT). Surface enhancement layer leads to the complete change of crack initiation sites. However, crack initiation mechanism keeps the same and results in a single-faceted morphology at crack initiation site. Microcracks initiate as Mode II crack within the original grain, but deflect to Mode I crack outside of the original cracked grain. A threshold SIF value is proposed to evaluate the retarding effect of grain boundary on microcrack propagation. Outside of the original cracked grain, Mode I crack propagation below the threshold ΔK_σ-th is responsible for the formation of fine granular area (FGA, a nano-grain layer). Based on the Numerous Cyclic Pressing (NCP) model, it is proposed that crack type should be another necessary condition for the formation of FGA.

对镁合金进行超声疲劳测试，无论是否采用超声喷丸处理 (UPT)。表面增强层导致裂纹萌生位置的完全改变。然而，裂纹萌生机制保持不变，并导致裂纹萌生部位的单面形态。微裂纹在原始晶粒内以模式 II 裂纹开始，但在原始裂纹晶粒外偏转为模式 I 裂纹。提出了一个阈值 SIF 来评估晶界对微裂纹扩展的阻滞作用。在原始裂纹晶粒之外，模式 I 裂纹扩展低于阈值ΔK _σ-th负责细颗粒区（FGA，一种纳米颗粒层）的形成。基于多次循环压制（NCP）模型，提出裂纹类型应该是形成FGA的另一个必要条件。