4D X-ray tomography characterization of void nucleation and growth during deformation of strontium-added AZ31 alloys

We report the influence of strontium on the evolution of microstructural damage during tensile deformation of hot-rolled AZ31 alloys tested parallel to the rolling, transverse, and diagonal directions using a 4D (3D X-ray tomography plus time) technique. X-ray computed tomography images were collected at several strains during each tensile test, and again after the fracture had occurred, to quantify the nucleation, growth, and coalescence of voids. Adding strontium leads to the formation of Al4Sr particles in the microstructure. Cracking of these particles provides the primary micromechanism of damage nucleation. Increasing strontium content increases the size and number of Al4Sr particles, thereby increasing the number of potential sites for void nucleation. Despite this, the ductility of AZ31, as well as its strength, when tested in all three directions, is enhanced by adding Sr. Nanoindentation tests confirm that this is accomplished by reducing the matrix strength through a decreasing role of aluminum solid solution hardening, which is more than compensated by an increase in precipitation strengthening. Fracture occurs through the interactions of shear bands with second-phase particles. Shear fracture is dominant on the fracture surfaces of the strontium-free alloys, while void growth induced dimples are found in much greater number on the fracture surfaces of the strontium-added alloys. Void growth is accurately described by optimization of the Rice-Tracey model using regression analysis. This study explains why strontium additions to AZ31 can be used to improve both strength and ductility by controlling the evolution of damage during deformation.