Microstructure, texture evolution and tensile properties of extruded Mg-4.58Zn-2.6Gd-0.16Zr alloy

Microstructure, texture evolution and mechanical properties of extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy were investigated at extrusion temperatures of 260 °C, 280 °C and 300 °C, extrusion ratios of 10, 15 and 30, and ram speeds of 3 mm s⁻¹ and 6 mm s⁻¹, respectively. The results indicated that the as-cast experimental alloy was composed of α-Mg matrix, coarse α-Mg + W(Mg₃Zn₃Gd₂) eutectic and icosahedral quasicrystalline I(Mg₃Zn₆Gd) phase. A small amount of undissolved W phases were detected after the homogenization treatment at 505 °C up to 16 h, while most I phase dissolved into the α-Mg matrix. The initial crystallites before extrusion were randomly oriented, and showed a typical random texture. After extrusions, all samples exhibited a bimodal microstructure consisting of fine Dynamic recrystallization (DRX) grains and coarse elongated un-DRX grains, and the formation of ultra-fine DRX grains only occurred in the sample with an extrusion ratio of 10 at 260 °C, 3 mm s⁻¹, which was caused by the solute segregation in homogenized billets and non-uniformly distributed W phase during the extrusion process. A fiber texture with {0002} planes and <1̄21̄0> directions paralleled to the extrusion direction was observed to be dominant in all the extruded samples. Furthermore, the increase of extrusion ratio was found to be beneficial for the DRX process and refinement of grain size, and the maximum texture intensity was accordingly weakened, which resulted in a decrease of tensile yield strength but an increase of elongation. With increasing the extrusion temperature or ram speed, both the grain size and DRX fraction gradually increased. Consequently, the texture was randomized and the maximum texture intensity decreased, which led to a reduction of the elongation and tensile yield strength. The tensile failure behaviors under different extrusion conditions were found to be related with the contraction twin lamellas formed in the un-DRX grains and the string-like W phases.