【作者】 娄超；

【导师】 张喜燕；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2013， 博士

【摘要】 本论文以密排六方结构（HCP）且具有很强基面织构的商用AZ31热轧镁合金板材为研究对象，变形方式选用具有高应变速率特点的动态塑性变形（DPD）。从我们的研究目的出发，设计出DPD方向与板材的轧制方向（RD）平行的变形方式，以引入较高密度的{10-12}孪晶组织。然后对具有孪晶片层组织的AZ31镁合金进行力学性能测试，目的是了解由于孪晶介入引起的组织结构变化对材料力学性能的影响。同时，设计了四类不同初始取向的样品，取其压缩方向分别与板材的法向（ND）成0o、30o、60o、90o，进行DPD。利用电子背散射衍射（EBSD）技术手段定量表征了变形前后样品的微观组织及织构演变规律，详细分析了初始取向及变形条件对AZ31镁合金孪生行为的影响，深入研究了不同的微观组织结构对材料的屈服行为、塑性以及加工硬化行为的影响，从而制定出理想的加工工艺，以改善镁合金的力学性能。对动态塑性变形后AZ31镁合金样品内部形成的{10-12}孪生片层组织结构进行定量分析。然后利用取向差法对晶粒中实际产生的{10-12}孪生变体进行类型判定，并计算其Schmid因子，研究结果表明：{10-12}孪生机制主导着镁合金塑性变形的初级阶段（ε<~8%）。在这一阶段，片层厚度从5.55μm减小到2.49μm，随着应变量的增加，发生了明显地减小。这种片层结构尺寸的演变与{10-12}孪晶在变形中的形核及生长机制密切相关；在高应变速率过程中的{10-12}孪生行为与准静态压缩过程中的相符，都遵循着Schmid因子法则，即具有最大Schmid因子的{10-12}孪生变体优先形核。同时发现晶粒能否形核不同的{10-12}孪生变体很大程度上受其应力加载方式的影响。当平行于晶格基面的压缩载荷与晶格a轴的夹角成0°或30°时，不同的{10-12}孪生变体有可能被激活在同一晶粒中。而具有大的Schmid因子的变体优先形核，导致它的形核数量多，孪生体积分数大，所以这种变体对塑性应变的贡献也很大，协调了几乎90%的孪生应变。对动态塑性变形下退火前后不同应变量的AZ31镁合金样品进行沿DPD方向和垂直于DPD方向（即平行于初始横向(TD)）的拉伸试验(分别简化命名为0°和90°拉伸)，分析孪生片层结构对镁合金力学性能的影响，研究不同的孪生强化机制在改善力学性能方面的作用。研究结果表明：对于未退火的形变镁合金来说，不管是0°拉伸还是90°拉伸，由于预变形导致的加工硬化原因，其屈服强度都随着预变形量的增加有所升高。在90°拉伸中，当孪晶片层密度较高、片层结构比较紧实时，材料的塑性得到极大的改善，这说明孪晶结构的强化能够同时改善材料的强度和塑性；在退火去应力后的预变形样品的0°拉伸过程中，退孪生行为明显地改善了AZ31镁合金的最大流变应力，而且随着{10-12}孪晶体积分数的增加，屈服强度以及最大流变应力均有明显地提高。退孪生引起织构的改变，使得软取向转变成硬取向，从而达到了织构强化的作用，而起这种强化的量级与孪生织构的强度有关。退火去应力后的预变形材料在90°拉伸过程中，{10-12}孪晶片层结构的强化作用导致屈服强度有略微地增加，而最大流变应力几乎没有变化。这说明{10-12}孪晶片层分割导致的晶粒“细化”对AZ31镁合金的力学性能的影响并不明显。对0o、30o、60o、90o四种镁合金样品动态塑性变形后的微观组织进行研究，并对{10-12}孪生行为进行分析，得出以下结论：0°样品主要依靠非基面滑移来协调塑性变形；而对于30°和60°样品来说，虽然宏观织构是不利于{10-12}孪晶形核，但变形后仍有少量的{10-12}孪晶被激活。在60°样品中，一部分晶粒的{10-12}孪生活动是遵循Schmid因子法则，它们的取向主要分布在偏离理想基面织构的10°～40°的范围内；而孪生活动不遵循Schmid因子法则的晶粒的取向主要分布在偏离理想基面织构的30°～50°的范围内，理论上这种取向已经不利于{10-12}孪晶的激活；在30°样品中，仍有少量偏离90°取向较大的晶粒形核{10-12}孪晶，这些激活的孪晶变体与60°样品中类似取向晶粒形核的孪晶一样，并没有遵循最大的Schmid因子的变体优先形核的原则，很多具有极小Schmid因子的孪晶变体优先被激活；90°样品在变形后激活了大量的{10-12}孪晶。更多还原

【Abstract】 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.更多还原