Investigation On The Micro Structures And Properties Of The Fine-grained Mg-Zn-Zr-RE Wrought Magnesium Alloys

Fine-grained magnesium alloy sheets have wide applications in the fields of automotive,aerospace,armament systems,electronic products and etc.because of their advantages such as low density,high strength to weight ratio and stiffness,good thermal conductivity,high plasticity,superior damping capacity properties and the excellent plastic formability for thin-walled structure parts.Although some domestic and overseas researches have done some studies on the fabrication of fine-grained magnesium sheets,they have been unable to achieve a breakthrough on the fabrication of fine-grained magnesium alloy sheets with excellent strength and high plasticity.Hence,how to improve both the strength and the plasticity of these sheets is still a difficult problem in the field of magnesium alloysin the present.In this thesis,the effects of minor Y and Gd addition on microstructures and mechanical properties of ZK60-based alloys throuth the high strain rate rolling(HSRR)are systematically investigated and the mechanisms that fine grained Mg-Zn-Zr-RE magnesium sheets exhibited high strength and high ductility the excellent strength and high plasticity were discussed.The resarch results showed that:(1)Minor Y and Gd addition can effectively refine grains the as-cast ZK60 alloys and change the compositions,morphology and volume fraction of the grain boundary second phases.The ZK60-xY alloy mainly consists of the ?-Mg matrix,Mg7Zn3,I-Mg3Zn6Y(the quasicrystal phase)and W-Mg3Zn3Y2.The ZK60-yGd alloy mainly consists of the ?-Mg matrix,Mg7Zn3 and the quasicrystal ?-phase(Mg-Zn-Gd ternary phase).The volume fraction of the second phase increases with the increasing Gd addition.Large amounts of the elongated bone-like second phase mainly aggregated along the grain boundaries,close to the network distribution.(2)The Y and Gd addition can reduce the stacking fault energy of the ?-Mg matrix and thus promote the dynamic recrystallization(DRX)during the HSRR process.The broken quasicrystal ?-phase particles resulted from the HSRR process play a role in promoting the nucleation of DRX,inhibiting the growth of the DRX grains and refining grains.Meanwhile,the dispersion strengthening and precipitation strengthening effects of the ?-phase particles and the strong bonding interfac between the ?-phase particle and the matrix contribute to the enhanced strength.It can be seen that the strain-induced precipitation for the nano-sized second phase occur in the HSRR alloys.The nano-sized second phases mainly include the rod like?1-MgZn and the ellipsoid-shape quasicrystal I-phase.(3)The fine grained Mg-Zn-Zr-RE magnesium sheets with high strength and high ductility were successfully produced by on-pass HSRR.Rolled at 400 ?,with a strain-per-pass 80%and a strain rate of 9.1s-1,the average grain size of the ZK60-1.0Y sheet was 1.4 ?m.The ultimate tensile strength(UTS)was 335 MPa and the yield tensile strength(YTS)was 254MPa and the elongation to failure was 25.5%;The average grain size of the as-rolled ZK60-0.8Gd sheet was 1.4 ?m,it exhibited an optimal strength-ductility balance,with the ultimate tensile strength of 327MPa,the yield strength of 242MPa and the elongation to rupture of 22%,respectively.(4)The addition of Y and Gd can modify the tensile fracture mechanism and effectively weaken the(0002)basal texture.The maximum intensity of the ZK60-0.8Gd sheets is 2.01,which is about 10.3%lower than that of ZK60 alloy sheets.