Effect of Yb Addition on the Microstructure and Mechanical Properties of the Mg–4Sm–3Gd–0.5Zr Alloy

The microstructure and mechanical properties of Mg-4Sm-3Gd-xYb-0.5Zr (x = 0, 1, 2, 3 wt%) alloys under different heat treatment states were systematically investigated, as well as the influence mechanism of the Yb on the mechanical properties. The results show that the addition of Yb can obviously refine the grain size, homogenize the microstructure and improve the strength and plasticity. The addition of Yb changed the lattice constant, reduced the value of c/a, shortened the peak aging time, and accelerated the precipitation of β′ phase. With the increase of the Yb content, the number of β′ phases increases. The spacing between adjacent β′ phases decreases, and the critical shear stress \(\Delta {\tau }_{p}\) $\mathrm{\Delta }{\tau }_p$ demanded for the basal dislocation to bypass the β′ phase increases. The β′ phase improves the strength of the aged alloy through the Orowan mechanism. And the β′ phase forms a nearly closed triangular prism space along the three directions of [1 \(\bar{1}\) $\overline{1}$ 00] _Mg, [01 \(\bar{1}\) $\overline{1}$ 0] _Mg and [ \(\overline{1 }\) $\overline{1}$ 010] _Mg. During the process of tensile deformation, the basal dislocations are trapped in a closed triangular prism space and it was difficult to escape. The aged Mg–4Sm–3Gd–2Yb–0.5Zr alloy showed the optimal mechanical properties, and its yield strength, ultimate tensile strength and elongation were 198 MPa, 275 MPa and 6.6%.