Fatigue Crack Propagation Behavior and Mechanism of AZ91HP Magnesium Alloy

The fatigue crack propagation behavior of AZ91HP alloy in as die-cast(F), solution treatment(T₄), and solution plus aging(T₆) heat treatment states was investigated. The characteristics of fatigue fracture structure and the mechanism of crack propagation for the alloy were discussed. The results show that the fatigue crack propagation behavior of the alloy is strongly affected by the value of stress intensity factor(ΔK). With ΔK decreasing, the fatigue fracture becomes smoother and smoother. But with the increase of ΔK, the fracture becomes rougher. The plasticity-induced crack closeness effect was observed in the process of fatigue crack propagation. This effect makes the value of crack-opening stress intensity factor (K_op) larger, and effective stress intensity factor (ΔK_eff) smaller. The crack propagation rate was inversely proportional to the plasticity of AZ91HP alloy. Because the plasticity of the alloy was increased in T₆,F,T₄ sequential order, the crack propagation rate was decreased in T₆,F,T₄ sequential order.