Effect of annealing on microstructure and tensile properties of as-extruded Mg-5Gd-4Y-0.5Zn-0.5Zr alloy

In order to investigate the effect of heat treatment on the microstructure and tensile properties of an Mg-5Gd-4Y-0.5Zn-0.5Zr alloy, the phase diagram was calculated by Pandat software, and accordingly the as-extruded alloy was annealed at 450, 510 and 550 °C followed by water quenching. The microstructure and mechanical properties of the specimens were analyzed subsequently by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and tensile testing. The results showed that the average grain size of the as-extruded specimen was about 4 μm, in which Mg24Y5 was detected, and fine-grain strip was formed during the extrusion process. The grain size of the specimen annealed at 450 °C for 24 h was about 11 μm, and the main secondary phases were 14H-LPSO, δ-Zr3Zn2, β1-Mg3Gd and (HCP)Zr particles. The plate-like 14H-LPSO and cubic β1- Mg3Gd phases were coherent with the matrix, and the surface layer of (HCP)Zr was enriched with Zn atoms. When the annealing temperature reached 510 °C and 550 °C, the grain size increased to about 83 and 151 μm, respectively. The 14H-LPSO phases were gradually dissolved while the other secondary phases still existed. It was shown by uniaxial tensile testing that the ultimate tensile strength (UTS), yield tensile strength (YTS) and elongation (EL) of the as-extruded specimen were 291 MPa, 236 MPa and 24.7%, respectively. Increase of UTS, EL and decrease of YTS was resulted after annealed at 450 °C. However, both the strength and ductility dropped obviously after annealed at 510 °C and 550 °C. Ductile fracture features were found on the fracture surface of the as-extruded and 450 °C annealed specimens, whereas brittle topology, such as cleavage, was the main feature on the fracture surfaces of the specimens annealed at higher temperatures. Based on the strength analysis, it was concluded that the fine-grain strengthening was more important than solution strengthening and precipitation strengthening in this work.