Study On The Microstructure And Mechanical Properties Of Sand Mould Cast Mg-10Gd-3Y-Zr Alloy

Mg-Gd-Y alloys are considered to be a kind of novel high strength and heat resistant magnesium alloys with promising in aerospace. The microstructure and properties of Mg-Gd-Y alloys formed in metal mould have been well documented, however the literature about sand mould cast Mg-Gd-Y alloys are not yet available. In this paper, the microstructures and their evolution, processing for heat treatment, mechanical properties, fracture behavior of sand mould cast Mg-10Gd-3Y-Zr alloy were studied by using optical microscopy, X-ray diffraction, scanning electron microscopy, thermal analysis, mechanical test of hardness and tension.The results show that the microstructure evolution of sand mould cast Mg-10Gd-3Y-Zr alloy from as-cast to T4 to peak-aged and high temperature aged conditions involves solid solution + eutectic compound→supersaturated solid solution + cuboidalγphase→solid solution + cuboidalγphase +β′precipitates→solid solution + cuboidalγphase +βprecipitates. In addition, Zirconium cores exist in all these conditions. Microstructural evolution of the alloy during homogenization involves three coinstantaneous processes: dissolving of eutectic phases, formation of cuboid-shaped phases and coarsening ofα-Mg grains. The alloy could be substantially homogenized by the optimized solution heat treatment leaving little eutectic phases and relatively fine grains.The optimum heat treatment condition for the sand mould cast Mg-10Gd-3Y-Zr alloy are as follows: 525℃×12h+250℃×10h, corresponding to tensile properties as follows: UTS～297MPa , TYS～238MPa ,Elongation～1.1%. The strength and elongation goes up as temperature gets to 200℃, and the strength lowers quickly as temperature goes above 200℃.The large capsule was produced successfully with heat treatment mentioned above, and the property of the product was satisfied with the request.The rupture of cast-T4, cast-T6 sand mould cast Mg-10Gd-3Y-Zr alloy mainly belongs to quasi-cleavage fracture, and the inter-granular fracture only occurs in the as-cast alloy. From 100℃to 300℃, the rupture of the cast-T6 alloy is mainly quasi-cleavage fracture; when the temperature is 100℃, the rupture of the alloy consists of cleavage planes and tear ridges; the dimple comes into being when the temperature goes to 200℃; and over 200℃, the proportion of dimple in fracture increases with the temperature elevated.