Very high cycle fatigue behaviour of as-extruded AZ31, AZ80, and ZK60 magnesium alloys

摘要

The very high cycle fatigue properties of extruded AZ31, AZ80, and ZK60 magnesium alloys were investigated. Fatigue tests were performed at ultrasonic cyclic frequency and at a load ratio of R = -1 at ambient temperature using smooth electropolished specimens. Fatigue failures were observed at lifetimes above 10{sup}9 cycles. The fatigue life was found to increase with decreasing stress amplitude. The fracture surfaces and fracture profiles of selected specimens cycled until failure were examined. The purpose of the study was to determine the role of the micro structure on the fatigue crack nucleation and growth. Furthermore, the fatigue properties were discussed on the basis of micro-structure and the presence of inclusions which are known as crack initiation sites. In AZ31 and AZ80 alloys only surface-induced fatigue cracks were observed. On the other hand, in the ZK60 alloy both surface- and interior-induced fatigue cracks were observed. Both mechanisms operate in the ZK60 also at a lifetime of around 10{sup}10 cycles. Interior-induced fatigue cracks were accompanied by clear fish-eye marks on the fracture surfaces of the ZK60 alloy.