| Magnesium alloys are prone to strong basal texture during extrusion and forming,which leads to material anisotropy and hence deterioration in mechanical properties.The texture strength can be reduced by introducing shear stress deformation,and the resulting processes of torsion,compression torsion and surface grinding can be used to obtain gradient structured magnesium alloys.The gradient structure can combine the advantages of high strength of fine grains and good plasticity of coarse grains,weakening the texture while producing heterogeneous deformation induced(HDI)strengthening,thus increasing the yield strength and improving work hardening.On this basis,the centre developed a new method of continuous low-temperature compression twisting,so that the deformed organisation combines the advantages of small grain size of low-temperature deformation and uniform organisation of high-temperature deformation,further improving the organisation and strengthening the mechanical properties of the magnesium alloy.In this experiment,using Gleeble-3500 thermal simulation experimental equipment,using compression and torsion bars made of AZ80 extruded bars as raw materials,and under the same strain control,continuous cooling compression and torsion experiments were conducted at a cooling rate of 10 ℃/s and different cooling ranges(380-260 ℃,380-280 ℃,380-300 ℃,and 380-320 ℃).The data collected by Gleeble were plotted as axial stress-strain and torque torsion curves.Using OM,EBSD and other characterisation methods and OIM software analysis,the evolution of grain size and texture was worked out.By selecting typical grains,dynamic recrystallisation and twinning mechanisms were further analysed and their contribution to texture weakening was highlighted.The research shows:Under combined compression-torsion loading,the softening effect of DRX in the axial stress-strain curve is significant,and there is a significant stage in the rheological mechanics curve where the stress decreases with the increase in strain,while the softening effect of DRX in the torsion-torsion angle curve is small and the slope of the curve is relatively flat;in the later stage of deformation,DRX cannot compensate for the work-hardening effect,and both sets of curves show an upward trend.In compression-torsion deformation,a relatively obvious gradient structure can be obtained,and the grain size is refined as DRX progresses,gradual weakening of the basal texture from the centre to the edges.The analysis of typical grains reveals that the new grains are still basal texture when the basal texture matrix undergoes ordinary continuous dynamic recrystallisation(CDRX),but the chain-like CDRX nucleated at the shear deformation zone has a disordered orientation that weakens the basal texture.Both non-basal CDRX and DDRX can rotate the grain C axial ED-TD plane and weaken the texture along the radial direction.To study the law of microstructure evolution during temperature reduction deformation,three sets of continuous temperature reduction,compression and torsion experiments were conducted at 380-290 ℃,380-320 ℃ and 380-350 ℃ based on four equal parts of the deformation process.The research shows:During the experimental process,there was a grain growth behaviour.Due to the small amount of recrystallisation in the early stage,the refinement effect is not obvious,which cannot offset the coarsening effect of grain growth,resulting in the phenomenon that the grain size in the early stage of deformation is larger than that in the homogenised state of the original rod material.Based on the behaviour of DRX,a coarsening model during deformation is proposed: grain growth is accompanied by penetration of small-angle grain boundaries,and as the deformation intensifies,it changes to large-angle grain boundaries and chain-shaped DRX.At the end of the deformation,rotational dynamic recrystallisation continues at the chain-like structure to thicken the deformation band and achieve the effect of refining the matrix. |
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