Influence of Humid Environment on Fatigue Behaviour of Cast AM60B Magnesium

Abstract

Low-cycle fatigue (LCF) tests were performed on cast AM60B Mg at room temperature in a high vacuum, at ambient conditions, and in 20 Torr pure water vapor (100 % relative humidity at room temperature). Surprisingly, fatigue life increased in the case of cycling in a humid atmosphere as compared to loading in vacuum. This behavior is attributed to the surface film formed on magnesium by water vapor, which suppresses slip band formation, but favors crack initiation in interdendritic regions. Consequently, only few fatigue cracks are formed early in fatigue life in water vapor, and link-up of these cracks is retarded. The number of cycles to crack initiation is much larger in vacuum environment. However, a high number of slip bands is formed early in fatigue life throughout the specimen surface. Many small cracks nucleate at these slip bands and then rapidly link-up to form the fatal crack that causes premature failure. To rationalize the effect of a humid environment, a plasticity-enhanced hydration mechanism is proposed. Hydration of the surface film on magnesium is intensified for aluminum contents below 4 wt%, which is the case for the primary α-Mg within the dendrite cells of AM60B. Plasticity-induced enhancement of the hydration prevents crack initiation at slip bands within dendrite cells in the LCF range.