In-situ studies on deformation and fracture characteristics of AZ91 Mg alloy

In this work, the influence of the second phases on the tensile properties of a direct chill cast AZ91 alloy in as-cast and solutionized conditions has been evaluated. Solutionization heat-treatment resulted in a decrease in the volume fraction of the eutectic β-Mg17Al12 phase, and an increase in the grain size of the α-Mg was observed. Increasing the solutionizing time resulted in lowering the yield strength and increased overall ductility of the alloy. The process of crack initiation and propagation were elucidated using in-situ tensile testing in the scanning electron microscope (SEM) in assistance with electron back-scattered diffraction (EBSD) and fractography. The crack initiation in the as-cast and partially solutionized conditions occurred predominantly in the β-Mg17Al12 eutectic, whereas Al-Mn inclusions did not contribute significantly to the damage process. While, in the fully solutionized condition, the absence of the β-Mg17Al12 eutectic, the crack was observed to initiate by the pile-up of dislocation at the grain boundary in the α-Mg matrix and by the fracturing of Al-Mn inclusions. In all the conditions, different types of twins formed during deformation; however, these twins did not contribute to fracture initiation.