Investigation of surface fatigue microcrack growth behavior of cast Mg–Al alloy

To understand the fatigue microcrack growth mechanisms of a cast AM50 alloy, fatigue microcrack growth experiments were carried out with online observation using a scanning electron microscope (SEM). The results concerning the fatigue microcrack propagation under higher stress levels (σmax=0.86−1.00σ0.2) displayed that microcracks propagate mainly along grain boundaries or interdendritic regions of α-Mg grains. Fatigue crack initiation also occurs on the grain boundaries near the root of a notch because of stress concentration, and fatigue crack initiation rarely occurs at Mg17Al12 (β-phase) regions within the α-grains. The fatigue crack propagates microscopically in a zigzag manner. A method for estimating the fatigue microcrack growth rate was proposed in terns of the crack tip opening displacement (CTOD).