Study On Thermal Compression Deformation Behavior And Differential Thermal Deformation Mechanism Of Mg-Sn-based Alloy

Magnesium（Mg）and its alloys have the advantages of low density,high specific strength/stiffness,biodegradability,and excellent electromagnetic shielding,which have potential advantages in the fields of automobile,aerospace,and biomedicine.However,due to Mg belonging to the hexagonal close-packed structure,it exhibits problems of poor plasticity,low absolute strength,and uncontrollable degradation at room temperature,which greatly limits Mg application in industrial fields.Based on the above problems,the researchers’hot points were to control the microstructure by optimizing process parameters and developing advanced plastic deformation methods.Therefore,the paper was select the Mg-5Sn-2Zn-0.5Zr（TZK）alloy with the strength aging response-ability as the research object starts from industrial application consideration,and the preparation of high strength and plastic Mg alloy and development of a new ECAP deformation process were realized by methods of the heat treatment,hot compression,extrusion,and differential temperature equal channel angular pressing.The main research contents and conclusions were summarized as follows.Firstly,the true stress-strain curves,constitutive equations,and hot processing maps of the heat-treated TZK alloy under different deformation parameters were systematically studied.The research results indicated that the true stress-strain curves exhibited typical dynamic recrystallization characteristics,and the true stress increased with the increases in strain rates or decreases in deformation temperatures.The constitutive equations were calculated under different strains and fitted with the experimental data,the results showed that the constitutive equations were suitable for the hot process of the TZK alloy,and according to the calculated values of hot activation energy and stress index,the deformation mechanisms of TZK alloy were dislocation cross-slip and particle stimulated nucleation（PSN）.Additionally,the hot processing maps of TZK alloy were drawn under different strains,and the safe deformation zones were determined as deformation temperatures of 380℃～430℃and strain rates of 0.01 s^-1～0.03 s^-1.Secondly,the microstructure and dynamic recrystallized behaviors of the compressed samples under different deformation regions of the hot processing maps were characterized to optimize the deformation parameters,and the extrusion was executed on the heat-treated TZK alloy based on the above results.The research results indicated that the fine dynamic recrystallization grains were preferentially generated at the grain boundaries with the deformation temperatures increases,and twinning gradually appeared and dynamic recrystallization grains gradually decreased on the TZK alloy with the strain rate increases.The microstructure of safe zones on the hot processing maps was composed of fine and uniform equiaxed grains,its dynamic recrystallization mechanism was discontinuous dynamic recrystallization,and the slips deformation methods were mainly basal and pyramidal slips.In addition,the microstructure of extruded TZK alloy was composed of dynamic recrystallization grains,deformed grains,and twins,and the average grain size was 11.27μm,and its ultimate tensile strength,yield strength,and elongation were 237.05 MPa,149.87MPa,and 18%,respectively.The change in the strength and plasticity were caused by the fine-grains strengthening,PSN,and the high SF values of basal,prismatic,and pyramidal slips.Ultimately,the ECAP deformations of the different mold temperatures and sample temperatures（differential thermal ECAP deformations）were performed on the extruded alloy to investigate the effects of mold and sample temperatures on the microstructure,mechanical properties,and corrosion behaviors.The research results indicated that when the mold was preheated at 300°C and the sample temperatures were at room temperature,200°C and 300°C（DT-1,DT-2,and DT-3）,the microstructure of TZK alloys was composed of equiaxed grains.On the contrary,when the mold was not preheated and the sample was held at 300°C（DT-4）,substantial twins appeared.Through tensile experiments,it was found that the mechanical properties were the best after DT-1 deformation.Its ultimate tensile strength and elongation reach 251.94 MPa and 30%,and the fracture consisted of abundant dimples.The excellent strength and plasticity trade-off can be attributed to fine-grains strengthening,texture weakening,and high SF of pyramidal slips.Additionally,based on the immersion and electrochemical tests,the DT-3 sample had prime corrosion resistance.It had the minimum polarization current density,the maximum diameter of the capacitance loop,and the lowest corrosion rate.The morphology of corrosion products of the DT-3 sample after 3 days of immersion corrosion was a dense lamellar structure.In summary,this work systematically studied the hot deformation behaviors of a new low-cost Mg-Sn-Zn-Zr alloy,to lay a theoretical foundation for the subsequent deformation processing of the TZK alloy.At the same time,this work focuses on the different control effects of mold temperatures and sample temperatures during ECAP deformation,which provides new ideas for the development of a new ECAP deformation process and lays an experimental foundation.