Experimental study of low-cycle fatigue behavior in a Mg-Y-Zn alloy with initial LPSO phase

Asymmetric cyclic stress-controlled low-cycle fatigue (LCF) tests were performed on Mg-Y-Zn alloys with different initial long-period stacking-ordered structure (LPSO) phases at room temperature. The influences of the initial LPSO phase content on the LCF life of the tested alloys were investigated, and an improved Basquin model for predicting the LCF life was proposed, considering the influence of the initial LPSO phase content. The coupling effects of the LPSO phase content and morphology on the LCF behavior were studied through scanning electron microscopy (SEM) experiments on the fatigue fractures of three alloys. The results show that the hindrance degree of LPSO phase to fatigue cracks is affected by the morphology of LPSO phase and the relative position of the LPSO phase with respect to fatigue cracks. When fatigue cracks are perpendicular to the main LPSO phase surface, the hindering effect is more pronounced, and the hindrance degree of the LPSO with different morphologies to fatigue crack propagation predominantly follows this sequence: lamellar > granular > rod > block. When fatigue cracks are relatively parallel to the main LPSO phase surface, the hindrance degree of LPSO relative fatigue cracks with different shapes is mainly in the following order: granular > rod > lamellar > block. The variation of fatigue crack path through the LSPO phase is primarily due to the properties of the LSPO phase itself, followed by morphology and spatial relative position. In addition, under high-stress conditions, fatigue cracks primarily originate from the LPSO phase, whereas under low-stress conditions, fatigue cracks predominantly arise from the matrix. Consequently, the Mg-9Y-1.8Zn alloy exhibits the best fatigue performance under high-stress conditions with an approximate LPSO phase content of 23.7%. Meanwhile, the Mg-9Y-3.1Zn alloy, with an LPSO phase content of around 34.9% under low-stress conditions, demonstrates the best fatigue performance.