High Pressure Solidification And Thermal Deformation Behavior Of Mg_97.6Zn_1.8Y_0.6 Alloy

The thermodynamics and kinetics conditions of the solidification process are greatlychanged when the pressure reaches the GPa order of magnitude. The final solidificationmicrostructure under super-high pressure is different from those under normal castingprocess. The experimental material used was cast Mg_97.6Zn_1.8Y_0.6alloy, solidified on aCS-IB type hexahedron anvil press under high pressure of2,4,6GPa; The microstructureand phase composition of Mg_97.6Zn_1.8Y_0.6alloy solidified under different pressures wereinvestigated by metallographic, scanning electronic microscopy （SEM）, the energydispersive spectrometer （EDS） and X-ray diffraction （XRD）. The compression behaviorsat room temperature of Mg_97.6Zn_1.8Y_0.6alloy solidified under super-high pressure weremeasured by WDW3100electron universal testing machine controlled by microcomputer.The Mg_97.6Zn_1.8Y_0.6alloy were performed on Gleeble-3500thermal/mechanical simulationtester and the effect of Zn/Y ratio on hot deformation behavior of Mg-Zn-Y alloy wereanalysed comparing with Mg_96.2Zn_3.5Y_0.3alloy. Meanswhile the microstructure of the alloyafter hot compression testing were observed and analysed by optical microscopy （OM）and scanning electronic microscopy （SEM）.The results showed that the coarsely as-cast microstructure solidified under normalpressure of the Mg_97.6Zn_1.8Y_0.6alloy, which is coarse, consisted of-Mg matrix and thenet-like second phases (I-Mg₃YZn₆and Mg_6.8Y_0.35Zn_2.81) in the form of eutecticdistributed on the grain boundary. The microstructure of the alloy solidified under highpressure was refined significantly and the granular second phase （I-Mg₃YZn₆andW-MgYZn3） of rod-like and dot-like shape was more uniformly distributed on the-Mgmatrix. There was one more phase precipitated under high pressure of6GPa, the newM-Y phase was like dots. With the increase of the pressure, the hardness,compressive strength, yield strength and other mechanical properties of the alloy wereraised sharply. The compression strength, the yield strength and the compression ratio ofthe Mg_97.6Zn_1.8Y_0.6alloy solidified under normal pressure were259.0MPa,230.4MPaand26.1%, respectively. The compression fracture was cleavage fracture. Thecompression strength and yield strength of the alloy solidified under6GPa were361.4 MPa and256.5MPa, respectively. The cleavage plane was small, with the characteristicsof tear ridge. The degree of cleavage fracture reduced. The fracture mechanism was moreclose to the quasi-cleavage fracture.The hot deformation behavior of the Mg_97.6Zn_1.8Y_0.6alloy was significant influenced by the deformation temperature, strain rate and thereduction. The flow stress decreases with increasing deformation temperature, andincreases with the increase of the strain rate and reduction of the samples. During hotcompression under150°C, the sample fracture occurs before dynamic recrystallization.When the compression temperature is200°C, the dynamic response characteristic of alloyis obviously. When the temperature is higher than300°C, dynamic recrystallization occursin the alloy. By calculating, the relationship between strain rate and peak stress of theMg_97.6Zn_1.8Y_0.6alloy is obtained as m14.347ln182.583. The factor of temperaturecompensation and deformation rate is gained as Z exp（139.86103/RT）. The alloywith appropriate Zn/Y ratio has higher content in it. Quasicrystal phase is a strengtheningphase, therefore when hot deformating, Mg_97.6Zn_1.8Y_0.6alloy with quasicrystal phase in itsmicrostructure has better compressive properties and strain rate sensitivity.