超细晶镁及镁合金的再结晶组织调控和超常室温塑性行为研究

Magnesium (Mg) alloys have become a worldwide hot topic for research and application due to their excellent specific strength. However, Mg and its alloys usually show limited ductility and very poor formability at room temperature due to their anisotropic hexagonal close-packed (HCP) crystal structure, which severely restricted their widespread applications as structural components. The applicant’s recent work suggested that superior room temperature ductility could be realized in bulk pure Mg and Mg alloy with fully-recrystallized ultrafine grained (UFG) microstructures, although the plastic deformation behaviors were different. The result indicated that the above-mentioned phenomenon is closely related to the addition of alloying elements and the UFG microstructures. Thus, commercial pure Mg and several simple solid solution Mg-based binary alloys (Mg-Zn, Mg-Zr, Mg-Ca) will be selected as the representative materials in the present project, for investigating the effect of alloying elements on the grain refinement and recrystallization behavior of Mg. As a result, the formation mechanisms of the fully-recrystallized UFG microstructure will be clarified. Furthermore, the effect of grain size and alloying elements on the activity of dislocation slip, deformation twinning, grain rotation and grain boundary sliding will be investigated, and the underlying mechanisms for the superior room temperature ductility of the UFG Mg and Mg alloys will be revealed. The above proposed work will provide important theoretical guidance as well as technological support for the development of advanced Mg alloys with high strength and good toughness.

镁合金因其优异的比强度而成为当前世界范围内的研究和应用热点。但是，镁合金的密排六方晶体结构特征导致其室温塑性低、变形加工困难，限制了其大规模工业化应用。申请人近期的探索研究发现，具有完全再结晶组织特征的超细晶纯镁及其多元合金均展现出十分优异的室温塑性，尽管其塑性变形行为并不相同。上述结果预示，这可能与合金化元素的添加及超细晶组织密切相关。因此，本项目以纯镁及相对简单的镁基二元固溶体合金(Mg-Zn、Mg-Zr、Mg-Ca)为研究对象，研究合金化元素添加对镁合金组织细化、再结晶行为的影响规律，阐明完全再结晶超细晶镁及镁合金的组织形成机理；进一步开展晶粒尺寸及合金化元素添加对位错滑移、形变孪晶、晶粒转动、晶界滑移等塑性变形机制的影响规律研究，深刻揭示超细晶镁及镁合金实现超常室温塑性的物理机制。通过本项目的开展，可为发展高强韧镁合金提供理论指导和技术支持。

本项目以纯镁（Mg）及简单的Mg-0.3at.%Y二元固溶体合金（Mg-Y）为研究对象，采用剧烈塑性变形技术对纯Mg和Mg-Y合金的晶粒尺寸进行了大范围调控，对其力学行为及强韧化机理进行了深入研究。本项目发现，纯Mg的强度和塑性随晶粒的细化而同步提高，但是当晶粒尺寸d ＜1.5μm时即出现了显著的软化现象，这一临界晶粒尺寸比Cu、Ni等金属大了约两个数量级。基于系统的原位电子显微观察和理论计算，我们比较了位错滑移、孪生变形及晶界滑移等潜在塑性变形机制的临界激活应力与晶粒尺寸之间的依存关系，发现晶界滑移开动所需的临界应力随晶粒细化迅速降低，并逐渐成为超细晶纯Mg（d ＜1μm）在室温准静态加载条件下的主导塑性变形机制，从而导致材料的“反常软化”及室温塑性的大幅增加。在液氮温度（77K）下，导致纯Mg软化的晶界滑移机制被显著抑制，超细晶纯Mg的屈服强度在77K下重新回归Hall-Petch线性关系。进一步，我们探索了微量Y元素添加对纯Mg力学性能的影响规律。我们发现Y元素的添加可以显著降低合金的晶粒尺寸细化极限并弱化（0001）基面再结晶织构。室温力学性能测试表明，Mg-Y合金的屈服强度在很宽的晶粒尺寸范畴都遵循Hall-Petch关系，这是因为Y元素固溶偏聚在晶界处，显著降低了晶界能并抑制晶界滑移。另一方面，Mg-Y合金的塑性随晶粒细化呈先增大后减小的趋势，并在晶粒尺寸~2μm左右获得了强度和塑性的最佳匹配。电子显微观察表明，随着晶粒的细化，{10-12}孪晶被显著抑制，但是大量具有<c>柏氏矢量的非基面滑移系被激活，这对匹配合金在c轴方向的塑性变形和提高合金的加工硬化能力具有重要作用。本项目阐明了超细晶纯Mg的屈服强度偏离Hall-Petch线性关系的临界晶粒尺寸和微观机制，揭示了微量稀土元素添加对纯Mg微观组织和力学行为的影响规律，对高强韧镁合金的设计和制备可提供理论指导和技术支持。