Effect Of Solidification Conditions And Heattreatment On Microstructure And Properties Of Mg-xZn-3Y-0.7Zr-0.2sb Alloy

In this paper, the selected Mg-xZn-3Y-0.7Zr-0.2Sb alloy has been casted into stepplate under low-pressure using sand mold, die mold and graphite mold. The effects ofdifferent solidification conditions and heat treatment on the microstructure and propertiesof Mg-xZn-3Y-0.7Zr-0.2Sb alloy has been analyzed using optical microscope, scanningelectron microscope and tensile machine.The microstructural analysis indicates that the as-cast microstructure ofMg-xZn-3Y-0.7Zr-0.2Sb alloy obtained under low pressure casting tends to formdendrites. As the capability of cooling increases, and the wall thickness decreases, themicrostructure is fined, and the volume fraction of secondary phase increases. Theas-cast microstructure of Mg-5Zn-3Y-0.7Zr-0.2Sb alloy comprises of-Mg and W phase.W phase is coherently distributed along the grain boundary. To the theMg-8Zn-3Y-0.7Zr-0.2Sb alloy, the dendritic feature is more obvious and the I phaseappears.The mechanical properties of as-cast Mg-5Zn-3Y-0.7Zr-0.2Sb alloy is better thanthe Mg-8Zn-3Y-0.7Zr-0.2Sb alloy. The ultimate tensile strength (UTS), yielding strength(YS) and elongation (EL) is226.0MPa,128.1MPa and8.2%respectively for theMg-5Zn-3Y-0.7Zr-0.2Sb alloy. The UTS, YS and EL is159.1MPa,117.1MPa and1.82%respectively for the Mg-8Zn-3Y-0.7Zr-0.2Sb alloy. The mechanical properties of theas-cast alloy increases with increasing cooling capability of the casting mold anddecreasing wall-thickness.After the500℃×4h+170℃×20h heat treatment, the originally coherentlydistributed W phase in the Mg-5Zn-3Y-0.7Zr-0.2Sb alloy is disrupted, and the residue ispresent at the triangular grain boundary. After the480℃×10h+170℃×20h heat treatment,the plate-like eutectic structure is completely gone, only the W particles remains whichhas underwent spheroidization. The alloy has a better mechanical properties after480℃×10h+170℃×20h heat treatment, especially those casted in a12mm-thick graphitemold. the UTS, YS and EL is262.3MPa,135.2MPa and8.9%respectively.The initial melting temperature of I phase in the Mg-8Zn-3Y-0.7Zr-0.2Sb alloy is436℃, therefore it is not suitable for the liquid-phase heat treatment. After a long timeaging at420℃for, the grains have not significantly coarsened, the melt-down of thefish-bone precipitates can be observed and the coherently network distribution ismodified. After420℃×10h+170℃×20h heat treatment, the mechanical properties of thealloy is significantly improved than the as-cast sample, the ultimate tensile strength exceeds260MPa, and the yield strength exceeds200MPa with a relatively smallelongation. The UTS, YS and EL of the alloy casted in a10mm-thickis sand mold is262.3MPa,135.2MPa and8.9%respectively, which indicates a85.7%、98.4%�'�17.6%improvement than the as-cast sample.The fracture of the as-cast Mg-xZn-3Y-0.7Zr-0.2Sb alloy obtained underlow-pressure casting is mostly quasi-cleavage fracture. After T6heat treatment, thecleavage plane increases and the fracture behavior is dominated by cleavage fracture. Inboth as-cast sample and heat treatment sample, cracks are initially emerge from thesecondary phase during the tensile test.