Solidification Microstructure And Room Temperature Compression Behavior In Mg-Zn-Y Alloy Solidified Under High Pressure

The thermodynamics and kinetics conditions of the solidification process are greatly changed when the pressure reaches the GPa order of magnitude. The final solidification microstructure under super-high pressure is different from those under normal casting process. In this thesis, Mg-8Zn-l Y and Mg-14Zn-4Y alloys has been prepared with the2RRL-M8vacuum resistance smelting furnace. The two kinds of conventional casting alloys were solidified at2-6GPa using CS-1B type hexahedron anvil machine. The solidification structures and phase compositions of Mg-8Zn-1Y and Mg-14Zn-4Y alloys solidified under different pressure were investigated by transmission electron microscopy(TEM), scanning electronic microscopy(SEM) coupled with the energy dispersive spectroscopy(EDS) and X-ray diffraction(XRD). The compression behaviors at room temperature of Mg-Zn-Y alloy solidified under super-high pressure were studied by WDW3100electron universal testing machine controlled by microcomputer. The results showed that the as-cast microstructure solidified under normal pressure was composed by the coarse "ice" dendrite a-Mg matrix and the second phases(Mg10Y2Zn and I-Mg3YZn6)distributed between dendrite. The microstructures of the Mg-8Zn-lY alloys solidified under2-6GPa were regional columnar grains. Solidification phase has not changed, but the solidified microstructure was significantly refined. The distance between the secondary dendrite decreased from41μm(normal pressure) to8μm(6GPa). The length of the dendrite arm reduced from456μm(2GPa) to180um(6GPa). The interplanar crystal spacing(0110) of the a-Mg matrix in the Mg-8Zn-lY alloy was reduced from0.27782nm under normal pressure to0.2705nm under6GPa. The as-cast microstructure solidified under normal pressure of the Mg-14Zn-4Y alloy was composed by the coarse equiaxed dendrite a-Mg matrix and the second phases(MgYZn3and I-Mg3YZn6) distributed between dendrite. The second phase distributed as net-like. The microstructure was refined significantly and the net-like morphology was broken under high pressure. The granular second phase distribution was more uniform under super high pressure. There was one more phase precipitated under high pressure, i.e. MY phase. The faster the cooling rate, the tinier the solidification microstructure will be.The compression strength, the yield strength and the compression ratio of the Mg-8Zn-lY alloy under normal pressure were262.6MPa,244.4MPa and13.3%, respectively. With the increase of the pressure, the compression strength, the yield strength and the compression ratio were raised sharply. The compression strength, the yield strength and the compression ratio under6Gpa were437.3MPa,368.9MPa and24.7%, respectively. A lot of deformation twins existed in the Mg-8Zn-1Y alloy after compression. The compression strength, the yield strength and the compression ratio of the Mg-14Zn-4Y alloy under normal pressure were343.81MPa,330.84MPa and16.81%, respectively. The compression strength, the yield strength and the compression ratio under6Gpa were454.66MPa,426.43MPa and24.74%, respectively. The compression rate of the alloy has a great improvement. The plasticity has been increased under high pressure, and the alloy has certain strain hardening effect. The compression fracture of Mg-8Zn-1Y and Mg-14Zn-4Y under normal pressure was cleavage fracture. The cleavage plane was smooth, the cleavage steps were flat and showed a river pattern characteristics. With the increase of pressure, the cleavage plane reduced gradually, and the tear ridge, toughening nest and "tongue"-like pattern occurred. The fracture mechanism was more close to the quasi-cleavage fracture.