Microstructures and room temperature mechanical properties of Mg-5Zn-3Al-0.2Mn casting alloy

The as-cast microstructures of sand-casting magnesium alloys Mg-5Zn-3Al-0.2Mn were studied, which was found to contain δ-Mg and τ(Mg₃₂(Al, Zn)₄₉) phases. By adding trace elements Sr, Ti and B into the melt of the alloys, the primary grain of δ-Mg is refined, meanwhile the morphology of τ phase is changed from semi-netted shape to short rods or spheres, and the distribution of τ phase tends to be more homogeneous. In the case of additions of 0.06%Ti, 0.012%B, and 0.1%Sr, δ-Mg grain is greatly refined and the best room temperature mechanical properties are gotten. It is also found that the ultimate strength and the elongation of Mg-5Zn-3Al-0.2Mn alloy greatly increase when the alloy is quenched after annealed at higher temperature. The microstructure of Mg-5Zn-3Al-0.2Mn alloy is transformed from binary phase microstructure into δ-Mg solid solution due to the dissolution of τ phase after annealed at 343°C for 17 h, and meanwhile the mechanical properties reach the maximum values, i.e., σ_b 245 MPa, δ12%.