Research Of Deformation Twins And Mechanical Properties Of AZ31Magnesium Alloy Subjected To Dynamic Plastic Deformation

In this thesis the hot-rolled AZ31Mg alloy sheet (Mg–3%Al–1%Zn) withhexagonal close packed (HCP) structure and a strong fiber texture was chosen as thestarting material. And dynamic plastic deformation (DPD)(i.e. plastic deformation athigh strain rates) was employed to achieve deformation of samples. AZ31sample wassubjected to DPD parallel to the rolling direction (RD) with the aim of introducing{10-12} twins. Subsequent tensile tests were carried out to investigate effects of {10-12}twin lamellar structure on the mechanical properties of materials. And four samples withdifferent initial textures for DPD were also cut from this hot-rolled sheet with theircompression axis aligned0o、30o、60o and90o to the normal direction (ND) in the rolledsheet, respectively. The microstructure and texture evolution before and afterdeformation had been quantitatively characterized using electron backscatter diffraction(EBSD). The EBSD data had also been applied to analyze in detail the influence ofinitial orientation and deformation conditions on the twinning behavior of Mg alloy, andgive an insight into the effect of different microstructure on the yield behavior,plasticity and work-hardening behavior of materials. These results thus reveal a route toimprove workability of hcp magnesium alloy.The characteristic of {10-12} twin lamellar structure caused by DPD wasinvestigated, and activated twin variants are identified using misorientation methodbased on orientation data from EBSD. The main results are as follows:At the stage of twinning-dominated deformation (ε<~8%), lamellar thicknessdecreases significantly with strain, from5.55μm to2.49μm. This evolution of lamellarthickness during deformation is directly related to {10-12} twin activity. During DPD,the {10-12} variant pairs with the maximum Schmid factor are most frequentlyobserved. This result was also obtained by quasi-static compression (QSC), in which theloading direction was perpendicular to the c-axis of crystal lattice. And different {10-12}variants are generated relative to their Schmid factors, when initial grains have definedorientations with one a-axis of the crystal lattice at roughly0°or30°from thecompression direction. The volume fraction of twins strongly influences the strainaccommodated by twinning. The {10-12} variant pair with the maximum Schmid factoraccommodated about90%of the twinning strain. Its high volume fraction indicated thatboth nucleation and growth mechanisms played important roles in the strain accommodation.The unannealed/annealed AZ31samples after pre-deformation are tested along twodirections, with the tensile axis parallel to the pre-deformed direction and the initialtransverse direction (i.e. perpendicular to the pre-deformed direction), referred to as0°and90°respectively. The investigation of effect of twin lamellar structure and twinningstrengthening on the mechanical properties of materials shows:Work hardening caused by pre-deformation leads to the tensile yield stressincreased slightly with pre-strain for the pre-deformed and unannealed Mg alloy,irrespective of tensile path. During90°tension, higher strength and better ductility areobtained when the highest twin density values are obtained for pre-deformation alongthe RD, indicating that the plasticity improvement caused by twins depends on thespecial strain path. The pre-deformed materials are annealed to eliminate thedislocations. During0°tension, untwinning causes a significant increase in themaximum flow stress. And the tensile yield stress and the maximum flow stressincreases significantly with the volume fraction of twins. The texture hardening whichis caused by the texture change attributable to untwinning plays an important role inimproving mechanical properties of materials. And the strengthening magnitude ofuntwinning increases significantly with the volume fraction of twins. During90°tension, the tensile yield stress increases slightly with pre-strain and the maximum flowstress is not significantly affected, suggesting that the hardening contribution of initialgrain refinement by {10-12} twin lamellae is not very significant during deformation.The investigation of microstructure and twinning behavior for the0°、30°、60°and90°Mg alloy samples during DPD shows:For the0°sample, non-basal slip plays an important role during DPD; For the30°and60°sample, some twins are activated after deformation, though it is limited due tothe initial textures.{10-12} twin behavior follows Schmid factor rule in the60°sample,when initial grains have defined orientations in which the misorientation angledistribution for both one a-axis of the crystal lattice and the ideal orientation of basaltexture is in the range10-40°; When this misorientation angle distribution is in therange30-50°, twin behavior does not follow Schmid factor rule, though {10-12} twinsare still activated in these grains. For the30°sample, there are still {10-12} twinsactivated in grains having large deviations from ideal orientation of basal texture, buttwinning behaviors does not follow Schmid factor rule, similar to the case in the60°sample. And many twin variants with the small Schmid factor values are generated earlier during DPD. For the90°sample, a lot of {10-12} twins are generated afterdeformation.