Effect of grain size on fatigue behaviors of Mg-2.6Nd-0.35Zn-xZr alloys

High-performance magnesium alloys are in great demand to meet the lightweight design requirements of aircraft. Grain size has long been recognized as a key factor influencing the mechanical properties of alloys. This study investigates the effect of grain size, controlled by Zr addition, on the fatigue behavior of a recently developed low-cost Mg-2.6Nd-0.35Zn alloy, through systematic characterization and analysis of stress-life (S-N) curves, fatigue crack propagation, fracture surface morphology, stress intensity factor, and crack propagation threshold. The results show that after heat treatment (solution at 525±5 °C for 8 h and water quenching at 60–80 °C, followed by aging at 250±5 °C for 14 h and then air cooling), coarse-grained specimens (average grain size approximately 596 µm) containing 0.12wt.% Zr exhibit greater resistance to fatigue crack propagation than fine-grained specimens (average grain size approximately 94 µm) containing 0.46wt.% Zr. Coarse grains promote intergranular fracture, while fine grains favor transgranular fracture. In addition, coarse grains reduce the sensitivity of the crack tip to stress concentration. Furthermore, fine-grained samples demonstrate a longer total fatigue life, owing to their superior resistance to crack initiation, which significantly prolongs the crack initiation stage. These findings highlight the importance of optimizing grain size to achieve the best possible fatigue resistance in Mg-Nd-Zn-Zr alloys for practical engineering applications.