Effect of gradient microstructures on strengthening and toughening of AZ31

A designed gradient microstructure has the potential to enhance mechanical performance of materials. Severe shot peening was used to develop gradient microstructure In twin-roll cast AZ31. The resulting microstructure has four layers including recrystallized ultra-fine grains, a transient layer, deformed coarse grains containing a high fraction of tension twins, and the as-received structure in the metal interior (coarse grains). Each layer has a distinct feature to control yield/ultimate strength and ductility, Hence, various combinations of them lead to a unique performance. This study establishes a relationship between the microstructure, the hardening mechanism, and the tensile behavior of the gradient structure. A method was established to estimate the yield strength of the designed structure based on each layer hardness via a modified rule of mixtures. Tensile and microhardness measurements along with microstructures quantified by electron backscatter diffraction and optical microscopy were utilized to identify strengthening and toughening mechanisms. The optimized structure had a remarkable enhancement in both yield and ultimate tensile strength.