Fatigue behaviour and fracture mechanism of a rolled AZ31 magnesium alloy

Abstract

Axial fatigue tests have been performed using smooth specimens and CT specimens of a rolled AZ31 magnesium alloy in laboratory air at ambient temperature. Fatigue strength and fatigue crack propagation (FCP) characteristic were evaluated and fracture mechanism was discussed on the basis of crack initiation, small crack growth and fracture surface analysis. The relationship between FCP rate and stress intensity factor range for large cracks consisted of two sections with different slopes due to the transition of the operative micromechanisms of fracture. The FCP resistance was significantly lower than that of aluminium alloys and pure titanium. The fatigue strength at 107 cycles was 50 MPa that led to a considerably low fatigue ratio of 0.23. There existed two different modes of crack initiation depending on applied stress level. Above 70 MPa, cracks initiated at the specimen surface in transgranular or intergranular manner due to cyclic slip deformation, while below that stress subsurface crack initiation took place. The growth of small cracks initiated at the surface coincided with the FCP characteristic after allowing for crack closure for large cracks, but the operative fracture mechanisms were different between small and large cracks. At the subsurface crack initiation site, smooth facets were always present regardless of applied stress level. The features of the facets were very similar to those observed in titanium alloy that have the same crystal structure. The facet sizes were smaller than, or nearly the same as, the average grain size, suggesting a crystallographic nature of the facets. Furthermore, there was no correlation between the maximum stress intensity factor for facet and fatigue life.