Microstructural evolution and improvement in mechanical properties of extruded AZ31 alloy by combined addition of Ca and Y

This study demonstrates that the combined addition of 0.5 wt% Ca and 0.2 wt% Y to a commercial AZ31 alloy leads to the promotion of dynamic recrystallization (DRX) behavior during hot extrusion, which significantly improves the mechanical properties of the extruded alloy. (Mg,Al)2Ca and Al8Mn4Y particles, which are formed during solidification by the addition of Ca and Y, cause an increase in the area fraction of dynamically recrystallized (DRXed) grains through particle-stimulated nucleation. These undissolved particles also cause the material to deform more severely during extrusion, which, in turn, causes a decrease in the size of the DRXed grains and an increase in the residual strain within unDRXed grains. The tensile yield strength of the extruded alloy improves considerably from 248 MPa to 290 MPa by the combined addition of small amounts of Ca and Y, without any loss of ductility. This drastic improvement in strength is attributed mainly to the combined effects of grain-boundary hardening caused by the increased DRX fraction and decreased DRXed grain size, strain hardening caused by the increased dislocation density in the unDRXed grains, and particle hardening caused by the numerous undissolved particles. Despite the significant improvement in strength and the presence of many brittle particles in the extruded alloy containing Ca and Y, the tensile elongation of the alloy hardly deteriorates; this is because a decrease in the area fraction of coarse unDRXed grains suppresses the formation of twins that act as initiation sites for microcracks during tensile deformation. In addition, decreases in the average grain size and texture intensity cause a larger increase in the compressive yield strength than in the tensile yield strength, and this consequently reduces the tension-compression yield asymmetry of the extruded alloy.