Void growth and coalescence in a magnesium alloy studied by synchrotron radiation laminography

Damage accumulation is studied in a rolled, partially annealed magnesium alloy by means of synchrotron tomography. To avoid failure by plastic instability and generate an effective damage process zone, round notched bars are deformed up to crack initiation then sheet-like samples are cut out for subsequent tomography observations. The effect of the hydrostatic stress is explored by considering two notch radii. In a given specimen, advantage is taken of the naturally occurring gradients in plastic strain and stress triaxiality to gain insight into the various stages of damage progression, damage-to-fracture transition, as well as crack propagation. The key finding is that damage is quite diffuse in both types of specimens. The observations document extensive void nucleation, growth and coalescence, reminiscent of the most ductile and tough metallic materials. Crack growth is found to be inherently anisotropic. Crack initiation is found to occur predominately at the center of the specimen with a blunt notch, and near the notch root in the specimen with a sharp notch. Quantitative measurements based on about 1000 microvoids and blunted microcracks enable values of void volume fraction and microcrack blunting to be fairly estimated, at macroscopic crack initiation.