Effects of Zn addition on microstructure and tensile properties of Mg–Y–Co alloy

Effects of Zn addition on microstructure and mechanical properties of the extruded Mg97.8Y1.4Co0.8 (at.%, WC) alloy were investigated at room temperature. It was found that the as-cast WC alloy was mainly composed of α-Mg, MgYCo4, Mg3(Co,Y) and 15R-LPSO phases distributed at the grain boundary. All particles of as-homogenized WC alloy were remarkably refined after extrusion, but there was no phase transition during the subsequent homogenized and extruded states. With the replacement of Co from Zn, the as-cast Mg97.8Y1.4Co0.2Zn0.6 (at.%, WZC) alloy consisted of α-Mg, 18R-LPSO with lamellar and blocky shape, and a small amount of rich-Y phases. Parts of 18R-LPSO phases transformed into fine 14H-LPSO phases and stacking faults (SFs) in the grain interior during homogenization and extrusion. The extruded WZC alloy exhibited a smaller average grain size (∼3.2 μm) than the extruded WC alloy (∼3.9 μm) owing to fully dynamic recrystallization (DRX). Both the extruded WC and WZC alloys had a basal plane texture parallel to the normal direction (ND). Tensile testing results indicated that the Zn replacement improved tensile strengths of the extruded WC alloy and maintained the high elongation of the alloy (∼32.2%). The tensile yield strength, ultimate tensile strength and fracture elongation of the extruded WZC alloy were 212 MPa, 281.5 MPa and 32.2%, respectively. The alloy exhibited a good strength and ductility balance. The high tensile strengths of the extruded WZC alloy at room temperature were mainly attributed to grain refinement and the precipitation strengthening of different particles, and the good ductility was mainly due to grain refinement and texture weakening.