Simultaneously improving strength and ductility of AZ91-type alloys with minor Gd addition

The microstructure and mechanical properties of as-cast AZ91 + xGd (x = 0, 0.1, 0.3 and 0.5 wt%) alloys were investigated in this work. The results showed that the α-Mg grains and Mg₁₇Al₁₂ particles were significantly refined with minor Gd addition. This was because that the addition of Gd resulted the formation of Al₈Mn₄Gd phase, which can act as the heterogeneous nucleation site for the α-Mg grains and Mg₁₇Al₁₂ particles. Besides, the Al₈Mn₄Gd phase were spherical and dispersed in the matrix. With the increase of Gd content, the tensile strength and ductility were improved simultaneously, which was attributed to the grain refinement strengthening and second-phase strengthening. During tensile test, the cracks initiated from the Mg₁₇Al₁₂ and Al₈Mn₅/Al₈Mn₄Gd particles, from which the cracks propagated into the Mg matrix. The fracture mode changed from the cleavage fracture in as-cast AZ91 alloy to the ductile fracture in Gd-containing AZ91 alloys.