Effect of Sm on microstructures and mechanical properties of Mg–Gd(–Sm)–Zr alloys by hot extrusion and aging treatment

Abstract

We investigated the effect of Sm on microstructures and mechanical properties of Mg–Gd (–Sm)–Zr by hot extrusion and aging treatment. The dynamic recrystallization (DRX) and precipitation behavior were analyzed by the electron backscattering diffraction (EBSD) and transmission electron microscope (TEM). For as-extruded alloys, the addition of Sm promotes DRX and dynamic precipitation, and the dynamic precipitation can activate the particle-stimulated nucleation (PSN) mechanism to promote the nucleation of DRX. The DRX grains of the as-extruded Mg–Gd–Zr show a random texture, but the as-extruded Mg–Gd–Sm–Zr shows an abnormal texture (<0001>Mg was parallel to the extrusion direction). For the extruded-aged alloys, the matrix precipitates of the Mg–Gd–Zr and Mg–Gd–Sm–Zr were all β′. But the size of β′ in Mg–Gd–Sm–Zr was smaller. The grain boundaries precipitates of the two alloys were all β. The β phase of Mg–Gd–Zr was granular and distributed discontinuously, while that of Mg–Gd–Sm–Zr was continuous distribution. Furthermore, there was an obvious precipitation-free zone of extruded-aged Mg–Gd–Sm–Zr. The high-temperature mechanical properties of extruded-aged Mg–Gd–Sm–Zr were obviously improved by the addition of Sm. The tensile strength, yield strength and elongation at 200 °C were 336 MPa, 271 MPa and 7.4%, respectively. Finally, the strengthening mechanism of extruded-aged Mg–Gd–Sm–Zr was analyzed in detail.