Environmental Influence on Fatigue Mechanisms in Cast AM60B Magnesium

Abstract

In this paper, in-situ scanning electron microscopy (SEM) observations are presented focusing on the formation and propagation of small cracks in a cast magnesium alloy under low-cycle fatigue conditions. Using an environmental SEM, observations were made under both high vacuum and 20 Torr of water vapour. It was observed that fatigue damage mechanisms are substantially different in vacuum and water vapour, respectively. Fatigue cracks form more rapidly in a humid atmosphere whereas crack density was simultaneously reduced. Crack nucleation frequently occurred along slip bands in fatigue tests conducted in vacuum whereas the cracks nucleated preferentially in the interdendritic regions in case of water vapour environment. The differences are attributed to the surface layer formed on magnesium in a humid atmosphere. Experimental evidence indicates that hydrogen embrittlement has also to be taken into account.