Solidification Microstructure And Phases Evolution In Mg-6Zn-1Y Alloy Solidified Under High Pressure

When the pressure reaches the GPa order of magnitude, the thermodynamics and kinetics of the alloy solidification process are greatly changed. The final morphology and phase distribution of alloy are completely different under super-high pressure compared with those under normal pressure. The sequence of phase transitions under normal conditions can even be changed; thus, some new phases or structures can be generated under super-high pressure. The experimental material used was cast Mg-6Zn-1Y alloy (93wt%Mg,6wt%Zn,1wt%Y) solidified on a CS-1B type hexahedron anvils press. The crystalline morphology and phase evolution of Mg-6Zn-lY alloy solidified under super-high pressure were investigated by metallographic microscope, X-ray diffraction (XRD) and scanning electronic microscopy (SEM) coupled with the energy dispersive spectroscopy (EDS). The results show that the solidified microstructure is significantly refined under super-high pressures. The secondary dendrite arm spacing decreases from41μm(normal pressure) to8～11μm. With increased solidification pressure, crystalline morphology transition from coarse equiaxed dendrite to super-fine equiaxed dendrite, to columnar dendrite, and to cellular dendrite was observed in the microstructure, suggesting that the solid/liquid interface tends to stable. When the Mg-6Zn-lY alloy is solidified under2and4GPa, there exists serious segregation in the dendrite of a-Mg, Segregation ratio is respectively8.33and7.53. And there also exists segregation on the cellular interface under6GPa, but the segregation degree decreases significantly and solute distribution tends to be evenly. The microstructure of Mg-6Zn-1Y alloy solidified under normal pressure consists of the a-Mg, I-Mg3Zn6Y phase and S-Mg12ZnY phases. Under super-high pressure (GPa level), not only a-Mg phase but also S-Mg12ZnY phase is crystallized from liquid phase directly, and the S-Mg12ZnY phase grows up into petal-like or strip-like shape in latter solidification process. In addition, a new ternary phase of Mg-Zn-Y with a high content of Y is found as granular shape under super-high pressures. Therefore, the microstructure of Mg-6Zn-lY alloy solidified under2-6GPa consists of the α-Mg phase, the S-Mg12ZnY phase, the I-Mg3Zn6Y phase, and the new ternary phase of Mg-Zn-Y. XRD analysis shows that diffraction peak of the a-Mg phase migrates slightly towards the right at high pressure. Lattice constant a and c of the α-Mg phase respectively decreases from0.32112nm and0.52488nm (normal pressure) to0.31938nm and0.51747nm (6GPa). The microhardness of Mg-6Zn-1Y alloy is56HV under normal pressure, under6GPa the hardness increases up to85HV, which correspond to44%increases compared with normal pressure. Meanwhile, thermal expansion coefficient of the Mg-6Zn-1Y alloy increases under high pressure, but elastic modulus decreases from68.2GPa to58.6GPa.