双模态纳米孪晶金属的强韧特性优化及其在吸能结构中的应用

In the early twenty-first century, the nanostructured metals with bimodal grain size distribution have been proposed as the new metallic materials with high yield strength and goood ductility. Recent experiments demonstrated that the nanotwinned polycrystalline metals also possess the good combination of high yield strength and high ductility. Therefore, the nanotwinned metals, in which various distribution of microstructural size such as the grain size and twin spacing is involved, will become another kind of metals with excellent performance in strength and ductility. For the nanotwinned metals with bimodal microstructural size distribution, surface mechanical attrition treatment (SMAT) technology is proposed to prepare such bimodal nanotwinned polycrystalline metals. The deformation mechanisms and mechanical properties are explored in details by the mechanical testing experiments. Then, one can develop the corresponding elastoplastic constitutive model and the failure criterior to probe the relationship between the mechanical properties and microstructural size and distribution in the bimodal nanotwinned metals. With controlling the size and distribution of grains and nanotwins in the metals, the plasticity performance including the yield strength and ductility could be optimized. As a result, such nanostructured metals with optimal mechanical properties will be used in the metal-based thin-wall energy absorbing structures. Through comprehensive study on these key issues of the nanotwinned polycrystalline metals, the corresponding results obtained in this proposal will brighten the microscopic mechanisms of plastic deformation in these bimodal nanotwinned metals, and shed lights on seeking out the effective approaches for improving mechanical properties and optimizing the strength and ductility in the metallic materials. This proposal will provide the applicable experimental and theoretical approaches for the achievement of nanostructured metals with high yield strength and good ductility as well as the effective application in metal-based energy absorbing structures.

双模态晶粒尺寸分布(即双峰分布)纳米结构金属作为一种较高强度和较好韧性的材料，早在21世纪初就被提出。近期实验显示，纳米孪晶金属同样具有较高强度和良好塑性。不同微结构尺寸分布(如晶粒或孪晶片层尺寸的双模态分布)的纳米孪晶金属有望成为新的高强高韧金属材料。本项目将针对双模态纳米孪晶面心立方金属，基于表面机械研磨技术获得此类纳米孪晶金属，从变形机理分析和力学建模入手，建立包含双模态尺寸分布、晶粒和孪晶片层大小的弹塑性理论模型，并根据破坏机理分析建立失效准则。通过理论研究双模态纳米孪晶金属的塑性性能随微结构的大小和分布的变化规律，优化此类金属材料的强韧特性，使高强高韧金属在薄壁吸能结构中得到充分的应用。基于以上关键问题的研究，本项目将给出此类双模态金属材料变形的微观机理，指出提高和优化金属强韧特性的可能途径，为获得高强高韧金属材料及其在吸能结构中的应用提供有效的实验工艺和理论依据。

金属材料在航空、航天工业以及民用工业等领域具有广泛的应用，如何获取同时具备高强度和良好塑性的金属材料一直是金属材料领域长期以来亟待解决的重大难题。本项目针对双模态纳米孪晶、梯度纳米孪晶等新型纳米孪晶金属，基于急速塑性变形方法制备多种纳米孪晶金属，建立包含双模态尺寸分布、晶粒和孪晶片层大小的弹塑性理论模型，提出失效准则，从而优化此类金属材料的强韧特性，研究基于此类材料的薄壁结构的吸能特性。. 首先，给出了预测多级孪晶片层结构金属材料应力应变关系的本构模型，提出了失效判据来预测材料的延展性。理论计算结果表明：随着孪晶片层多级结构的级数增多，材料的屈服强度随着增加而延展性反而削弱。建立了考虑纳米孪晶和纳米晶的强化作用和微裂纹影响的细观力学模型，结果表明：所建立的理论模型可以很好的描述实验测得的应力应变关系和强韧特性。. 其次，通过表面机械研磨技术和预变形等工艺,设计出多种梯度复合纳米结构金属，它们均具有高强高韧的力学性能。采用细观力学方法，分析了由纳米孪晶相和粗晶相组成的梯度双模态纳米孪晶铜的强度和延展性，计算结果表明：基于细观力学的方法得到的应力应变关系，与实验结果吻合很好。建立了描述梯度双模态纳米结构金属和梯度复合纳米孪晶金属的强韧特性的力学模型，计算结果表明：我们的模型很好地描述梯度双模态结构和梯度纳米孪晶复合结构的应力应变关系，计算结果和实验结果吻合很好。. 最后，通过表面机械研磨技术制备得到梯度纳米孪晶不锈钢丝，采用压缩试验机和霍普金森压杆测得不同应变率下的应力应变曲线，建立了梯度纳米孪晶率相关塑性力学模型，结果表明：梯度纳米孪晶不锈钢材料表现出负应变率效应；所建模型成功预测此类纳米孪晶金属材料的负应变率效应。采用基于变形机理的应变梯度塑形模型和Johnson-Cook失效模型，通过有限元模拟研究了双模态纳米孪晶等的冲击性能，计算结果表明：较小孪晶尺寸和均匀微结构分布有利于得到较好的冲击性能。. 在本项目的资助下，相关研究结果已在著名国际期刊发表SCI论文18篇，包括在国际固体力学著名期刊JMPS、国际金属著名期刊Acta Mater、国际塑性力学著名期刊Int J Plasticity发表论文。本项目的研究成果，可为设计金属材料的微结构组成、大小和分布而获得高强高韧力学特性提供有效的理论指导和实验工艺方法。