Researches On The Microstructures And Thermal Deformation Behaviors Of Micro-alloyed Mg-Mn Alloys

Magnesium alloy can be employed in more areas after deformation,because the grains of wrought magnesium alloy have more excellent performance. However, the deformation ability of Mg alloy is limited by its intrinsic hexagonal closed-packed (hcp) crystal structure which makes Mg alloy lack of slip system and difficult to deform in room temperature. So it's urgent to improve the deformation ability and complete the plastic deformation theory of Mg alloy.In this paper,Mg-Mn alloy,one of the earliest commercially used wrought magnesium alloys,was studied. Mg-Mn alloy with sound welding properties,good corrosion-resistance and moderate strength can be produced in the form of sheets, forgings,extruded profiles,rods,and tubes. As-cast Mg-Mn alloy exhibits pretty coarse grains,which could be refined by the addition of Er and Al in this study. And heat treatment,isothermal hot compression test,hot extrusion,etc.,were conducted to Mg-Mn alloys. Optical Microstructure (OM),Differential Scanning Caborimetry (DSC) , Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectrometer (EDX),Hardness test,etc.,were used to examine the relationship of microstructure,deformation behavior and mechanical properties.The results showed that,the grain size of semi-continuous casting Mg-Mn binary alloy was over 800μm. Adding Er alone to the Mg-Mn alloy didn't evidently influence the grain size and Er was consumed by forming Er-Mn phase with Mn. Whist a significant grain refinement was obtained by simultaneous addition of Er and Al. The grain size decreased remarkablely to 150μm with addition of 0.4% Er/Al. Further increase of the addition amount of Er/Al to 0.7% caused coarsening of the grain size. Small amount of Al-Mn phase and Al₃Er phase were detected in the alloys with Er and Al. It's found that Al₃Er particles could act as the efficient heterogeneous nuclei of primaryа-Mg during the solidification of Mg-Mn alloys.The distribution of the secondary particles( the most are elememtal Mn),for example,the volume,size and inter-space of particles,could be controlled by different heat treatment regimes,and three typical alloys with different particle distributions were produced based on the alloy Mg-Mn-0.4Er/Al. The alloys exhibited the feature of dynamic recrystallization in the process of hot compression test. The homogenization treatment could reduce the resistance to deformation of Mg-Mn alloys during hot working. The constitutive equation was corrected utilizing the data obtained from the compression test,and the deformation activation energy Q for the three solution-treated alloys with solution-treated time-durations being 2h,4h,8h,were computed to be 220,160 and 200 KJ/mol,respectively. The values of the flow stress predicted by the corrected constitutive equation were closed to the experimental values with average error of 2.01%.The deformation behaviors of Mg-Mn alloys were influenced by the distribution of the secondary particles. Excessive amount Excessive amount of micro-sized particles resulted in an increase of the flow stresses and working-hardening degree. A large volume fraction of nano-sized particles also inhibited softening process,but with less harmful effects on the hot workability than bigger-sized particles. The dispersion of Mn particles of 1¹.5μm in size with appropriate volume fraction (1²%) and inter-particle spacing (²0μm) can impart superior hot work deformation ability to the Mg-Mn alloys.The three solution-treated alloys recrystallized with different degree corresponding to the solution time after hot extrusion at 450℃. The longer solution time was,the higher the degree of recrystallization was. There was no evident yield point on the mechanical tension curves at room temperature of three solution-treated alloys. The tensile strength and yield strength of alloy solutioned for 2h were the highest among the three alloys,whilst the elongation of alloy solutioned for 8h was the best indicating this alloy having better plasticity.