Influence of neodymium on high cycle fatigue behavior of die cast AZ91D magnesium alloy

High cycle fatigue behavior of die cast AZ91D magnesium alloy with different Nd contents was investigated. Axial mechanical fatigue tests were conducted at the stress ratio R=0.1 and the fatigue strength was evaluated using up-to-down load method on specimens of AZ91D with different Nd contents. The results showed that the grain of AZ91D alloy was refined, the size and amount of β-Mg17Al12 phase decreased and distributed uniformly with increasing Nd content. At the number of cycles to failure, Nf=107, the fatigue strength of AZ91D evaluated by up-to-down load method increased from 75 MPa (0.5%Nd) up to 85 MPa (1.0%Nd) and then decreased to 79 MPa (1.5%Nd), respectively. The fatigue crack of AZ91D alloy initiated at the surface defect or porosities and inclusions which existed near the surface, and propagated along the grain boundary. The fatigue crack propagation region was composed of some small facets. The clear fatigue striations appeared at the alloy of containing 1.0%Nd. The fatigue fracture surfaces of test specimens showed the mixed-fracture characteristics of quasi-cleavage and dimple. The fatigue step, trans-granular and along-granular cracks existed in the local fracture surface.