| Mg-Gd-Y-Zr alloy exhibits good tensile properties at room temperature and high temperature,fatigue resistance and creep resistance.It has important application value and wide application prospect in aviation,aerospace,weapons and equipment,etc.,and is the key alloy system in the research field of heat-resistant magnesium alloy.However,the existing studies tend to focus on Mg-Gd-Y-Zr alloys with high Gd and Y rare earth elements content,but the defects of such alloys are high density and high cost.Although reducing Gd and Y addition can reduce the density and production cost of this kind of alloy,but its strength is low,which is very adverse to application.The effective way to solve this problem is to refine the grain size of alloy by plastic deformation and improve the precipitation strengthening effect of aging precipitated phase.Therefore,the plastic deformation process of Mg-Gd-Y-Zr alloy with low Gd and Y contents should be studied in depth.In this paper,the effects of deformation temperature(340～500 ℃)and strain rate(0.01～25 s-1)on the critical strain and volume fraction of dynamic recrystallization(DRX)of Mg-6Gd-1.2Y-0.5Zr alloy were investigated by thermal simulation experiment.The range of hot working parameters was optimized by constructing thermal processing diagram.On this basis,the effects of single pass rolling deformation(10%～80%)and rolling temperature(440℃,470℃,500℃)on the microstructure and mechanical properties of MG-6Gd-1.2Y-0.5Zr alloy sheet were investigated,and the fracture mechanism was analyzed.The effects of pass reduction(15%～35%),rolling temperature(460℃,480℃,500℃)and aging temperature(175℃,190℃,205℃)on the microstructure and mechanical properties of sheet metal in multi-pass rolling were studied.The main results are as follows:(1)The true stress-strain curves of the Mg-6Gd-1.2Y-0.5Zr alloy show typical hardening characteristics during thermal compression,and the hardening characteristics are more obvious when ε>10s-1.The peak stress and flow stress of the alloy decrease with the increase of temperature and increase with the increase of strain rate.The critical strain of dynamic recrystallization increases greatly at low strain rate,but decreases slightly at high strain rate.The critical strain of dynamic recrystallization decreases with the increase of strain rate at the same temperature.(2)Based on the Z-value modified Arrhenius equation,the thermal activation energy Q=227.348 kJ/mol of Mg-6Gd-1.2Y-0.5Zr alloy is calculated,and the constitutive relationship of Mg-6Gd-1.2Y-0.5Zr alloy is established:ε=4.28405 × 1016 sinh(0.00849σ)6.71996 exp(-227348/RT)(3)The dynamic thermal processing diagram of Mg-6Gd-1.2Y-0.5Zr alloy is established.When the deformation is between 30%and 80%,Mg-6Gd-1.2Y-0.5Zr alloy has the best hot working temperature range of 400～500 ℃ and strain rate of 0.01～1 s-1 and temperature of 420～500 ℃ and strain rate of 10～25 s-1.The adiabatic shear bands and cracks appear in the unstable deformation zone during hot compression,which corresponds well with the results of the hot working diagram.The critical strain decreases with the increase of strain rate at the same temperature.The volume fraction of dynamic recrystallization increases with increasing strain rate and temperature.The DRX grain size increases with the increase of temperature.The strain rate has little effect on the DRX grain size at 340℃ and 420℃,but the DRX grain size decreases with the increase of strain rate at 500℃.(4)At 440℃,470℃ and 500℃,the maximum deformation of wedge slabs is 25%,30%and 35%,respectively.The dynamic recrystallization of the alloy did not occur under the experimental conditions.At 480℃ and 500℃,the slab from 10 mm thick can be rolled to 2 mm thick by multi-pass rolling process with 15%～35%pass deformation.The recrystallized grain size increases with the increase of temperature and reduction amount.The elongation of sheet decreases with the increase of rolling temperature,and the tensile strength increases first and then decreases with the increase of pass reduction.When the rolling temperature is 480℃ and the pass reduction is 25%,the sheet has the best tensile properties at room temperature,the tensile strength is 303 MPa and the elongation is 5.6%.(5)The alloy after aging at 175～205 ℃ is strengthened significantly.The best aging process is 190℃/174 h,with tensile strength of 405 MPa and elongation of 3.7%.In this paper,the thermal compression deformation behavior of Mg-6Gd-1.2Y0.5Zr alloy and the effect of rolling and aging treatment on its mechanical properties are systematically studied,which provides theoretical and experimental basis for the development of a new type of Mg-Gd-Y-Zr alloy with low Gd and Y contents. |
