Fracture-based modeling of heat-treated AZ91/AZE911 magnesium alloys for estimating high-cycle fatigue lifetime by Paris crack growth rate and striations spacing

In the present research, the fracture behavior of the AZ91 magnesium alloy is analyzed based on the striations spacing on the fracture surface for predicting the fatigue High-Cycle Fatigue lifetime. At first, equations and relations were extracted based on the Paris law. Then, striations spacing was measured using ImageJ software and field emission scanning electron microscope images of fracture surfaces of heat-treated Mg–Al–Zn alloys, containing and non-containing 1% rare earth elements (1% RE). Finally, constants of the Paris law were calculated and calibrated. Results showed that a 1% RE addition decreased the striations spacing and enhanced the fatigue resistance (between 14 and 40%). In addition, the lifetime scatter band and mean error decreased from ± 2.7X to ± 1.5X and from 150 to 33%, respectively, as the accuracy of the recommended model. Heat-treating transformed the continuous precipitations to blade-shaped precipitations on the Mg-matrix and decreased the grain size remarkably. The addition of 1% RE formed the new Al 11 RE 3 phase and created a better distribution between the cast defects. In addition, fatigue striations in AZ91 alloy had more curvature and discontinuity and were more significant and coarser than those in AZE911 + 1% RE (AZE911) alloy. Graphical abstract