| As light green structural metals with high specific strength,high specific modulus,no pollution and easy recycling,magnesium alloys are widely applied in engineering,such as aerospace and 3C digital,etc.However,magnesium alloys have the disadvantage of poor plasticity and difficulty in forming,especially at room temperature.In order to improve this defect and obtain low cost magnesium alloy with high plasticity at room temperature,the effects of Mn content and extrusion temperature on the microstructure and mechanical properties of binary Mg-Mn alloys were studied through water-cooled casting,homogenization treatment and extrusion deformation.Mg-Mn alloy with tensile elongation higher than 80% at room temperature was developed and the high plastic mechanism of Mg-Mn alloy was researched.As-cast Mg-Mn binary alloy with Mn content greater than 0.7wt % is composed of ?-Mg matrix and Mn particles.With the increase of Mn content,the grain size of as-cast structure decreased first and then increased,and the volume fraction of the second phase increased.After homogenization treatment at 500?,part of Mn is backdissolved into the matrix,with a maximum solid solubility of about 0.8wt%,while the other part still exists as block second phase particles.After extrusion deformation,the grain of Mg-Mn alloy is remarkably refined and the average recrystallization grain size is about 1.0?2.0?m.With the increase of Mn content,the proportion of unrecrystallized area increases and the grain refines,leading to texture strengthening and fine grain strengthening,both of which will lead to the increase of alloy strength.In addition,with the increase of Mn content,Mn in the solid solution increases.After reaching the maximum solid solubility,Mn particles in the second phase also increase.With the increase of ?-Mn particle content,the strengthening effect of the second phase is more significant,and the strength and plasticity of the alloy are also increased.In the extruded Mg-Mn alloy,Mn element is segregation at the grain boundary of recrystallization.Extruded Mg-Mn alloy at room temperature tensile process consists of work hardening stage 3 and stage 4.Elastic-plastic transition into the work hardening stage 3,the work hardening rate is related to the initial dislocation density and the grain boundary barrier dislocation motion,and the second phase also has some influence.When the alloys containing different Mn content were extruded at 125?,there were more recrystallized grains with low Mn content,grain boundary hindered dislocation movement,dislocation proliferation,and work hardening rate increased.When the extrusion temperature was 175 ?,the Mn content had little influence,the recrystallization ratio was similar,and the recrystallization grain number was similar.With the increase of Mn content,the second phase particles in the extruding alloy increased,the pinning effect of the dislocation was stronger,the dislocation proliferation was faster,and the dislocation proliferation effect was greater than the dislocation recovery effect.At lower extrusion temperature,the proportion of recrystallization zone is lower,the initial dislocation density is higher,and the working hardening rate of the extrusion alloy is higher.The elongation-to-failure(tensile strain rate at 1.1×10-3s-1)of the Mg-0.7Mn alloy after being extruded at 125? reached 82%.The alloy is sensitive to strain rate at room temperature.Through the strain rate "jump" test,the strain rate sensitive index m value is about 0.15(strain rate at 1.1×10-3s-1?1.1×10-4s-1),which is significantly higher than that of the commercial deformed magnesium alloy,indicating that grain boundary slip contributes greatly to the ultra-high plasticity of Mg-0.7Mn alloy.The ultra-high plasticity of Mg-Mn binary alloys at room temperature is mainly due to the fine dynamic recrystallization grains and the segregation of Mn at the recrystallization grain boundary,which reduces the grain boundary binding force and promotes the grain boundary slip at room temperature. |
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