This article investigates the influences of heat-treating on the microstructures and the high cycle fatigue (HCF) properties of AZ91 and AZE911 (AZ91 + 1%RE) magnesium alloys. For such an objective, AZ91 and AZE911 alloys were used after solution treatment at 415°C for 5 hours aged at 215°C for 3 hours and at 215°C for 5 hours, respectively. To investigate the HCF behavior, a rotational bending fatigue test was performed with stress ratio (R) −1 and frequency of 100 Hz at room temperature. Optical microscopy (OM) study demonstrates that heat treatment leads to a reduction in grain size and transformation of continuous and discontinuous precipitations into needle-shaped precipitations, which are located on the grains of the alpha phase. Scanning electron microscopy (SEM) showed both marks of quasicleavage and cleavage on the fracture surface of specimens. These planes indicated the brittle behavior of the fracture. Moreover, in heat-treated specimens, the size of cleavage patterns was smaller, and microcracks were shorter. These behaviors affected the strength of the material and the fatigue lifetime. The results of mechanical tests show a negligible influence of heat-treating on the HCF behaviors of AZ91-T6 and AZE911-T6 alloys. Stress-lifetime curves (S-N) show an increase in fatigue strength in 3.8 × 105 fatigue cycles, from 95 MPa to 125 MPa for the AZ91-T6 alloy and from 125 MPa to 155 MPa for the AZE911-T6 alloy, after heat treatment.

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 Al11RE3 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

The fractographic analysis of the fracture surface is one solution to determine the kind of failures and predict the fatigue lifetime and resistance of mechanical components under cyclic loading. In the present research, the fracture behavior of the AZ91 magnesium alloy, under stress-controlled high-cycle fatigue loading, was analyzed based on the fatigue striations on the fracture surface. At first, equations and relations were extracted with the Paris crack growth law and the space of the fatigue striations. Then, striation spacing was measured by the experimental results of the high-cycle fatigue (HCF) testing for heat-treated Mg-Al-Zn alloys, containing and non-containing rare earth elements. Finally, constants of the Paris law were calculated and calibrated. Results showed that rare earth elements addition decreased the space between the striations, and subsequently, the fatigue resistance of the AZ91 alloy increased. The reasons are extreme grain size reduction by heat-treating and formation of new Al RE phase by rare earth elements addition. The obtained results of the predicted fatigue lifetime, in comparison to the experimental ones, the scatter band of ±1.5X demonstrated the accuracy of the recommended model by a mean error of 33%.