Three-dimensional in situ observations of short fatigue crack growth in magnesium

Three-dimensional (3D) short fatigue crack growth behaviour in a cast magnesium alloy, Elektron 21, was studied using a combination of X-ray diffraction contrast tomography (DCT) and microtomography at the European Synchrotron Radiation Facility. A 3D map of grain shapes and crystallographic orientations was produced in a miniature fatigue specimen using DCT. A focused ion beam instrument was used to introduce small notches in selected grains. Synchrotron microtomography was used to study the evolution of fatigue cracks from these notches during interrupted in situ fatigue cycling. Stage I crack growth occurred preferentially on the basal plane of the magnesium hexagonal close packed crystal, with local crack growth rates between 4 and 40 nm cycle 1. Retardations in the local crack growth rate were observed in certain grains and at certain grain boundaries. The observed interactions between the crack and the polycrystalline microstructure can be explained using Schmid factors and a development of the tilt-twist model of Zhai and Wilkinson (Zhai T, Wilkinson AJ, Martin JW. Acta Mater 2000;48;4917).