In-Situ Observations of Low-Cycle Fatigue Damage in Cast AM60B Magnesium in an Environmental Scanning Electron Microscope

Abstract

We present in-situ observations of low-cycle fatigue damage in cast AM60B magnesium. The in-situ fatigue tests were conducted in an environmental scanning electron microscope under both high vacuum and 20 Torr of water vapor. In both environments, fatigue cracks were observed to form and grow within the dendrite cells and through the interdendritic regions. Crack formation and growth through the dendrite cells proceeded along persistent slip bands. The persistent slip bands were typically oriented at about 45 deg with respect to the loading axis and were more frequently observed in relatively large dendrite cells. Crack formation and growth through the Mg interdendritic regions, laden with Al-Mg intermetallic particles, was facilitated by slip incompatibilities in adjacent dendrite cells, microporosity, and damaged second-phase particles. The detectable “crack-formation” size at slip bands and within interdendritic regions was typically equivalent to the dendrite cell size (DCS), since cracks rapidly spanned this distance once nucleated. Cracks formed during cycling in vacuum were more uniformly distributed and showed a lack of complete closure upon unloading, in contrast to cracks formed during cycling in water vapor. The cracks formed in water vapor were much more isolated and showed indication of significant environmental attack and associated embrittlement at the crack tip, as evidenced by the near-perfect mating of crack faces upon unloading. Final fracture occurred by the coalescence of numerous cracks throughout the microstructure, distributed differently depending on the testing environment. The water-vapor environment accelerated the formation of selected, isolated cracks, leading to more localized damage compared to the highly distributed damage growth and coalescence observed in the material cycled in vacuum.