梯度纳米结构金属的应变硬化行为及微观机理

The GNS/CG (Gradient nanostructured/coarse grained) architecture is generated in austenitic stainless steel plates by means of surface mechanical attrition treatment (SMAT) to reveal the strain hardening behavior and inherent mechanism during tensile deformation. Mechanical properties are tested by quasi-static tensile, repeated stress-relaxation, and micro-indentation methods, upon which the related strength, ductility and strain hardening are analyzed. Meanwhile, Finite element analysis is used to research the micro-mechanical process of compatible deformation between GNS layers and coarse grained matrix.Furthermore, the gradient features of distribution for grain size,deformation twin, and martensite are characterized by X-ray diffraction, transmission electron microscope techniques and so on. Most importantly, close attention is paid to the formation, activation, and interaction of crystalline defects, such as dislocation, stacking fault,twin, and martensite. The results would reveal the micro-mechanical compatible deformation process occuring between GNS layers and coarse grained matrix, and uncover the inherent strain hardening mechanism of GNS/CG architecture resulting in extrordinary high tensile ductility. This proposal is aiming at elucidating the intrinsic tensile plasticity of nano-grained metals, and establishing the connection between macro-mechanical properties of GNS/CG architecture and the key microstructural parameters. Moreover, the solution of the brittle problem of nano-metals would benefit a lot from this proposal.

利用表面机械研磨技术制备奥氏体不锈钢的梯度纳米结构块体样品，为揭示梯度纳米结构金属的应变硬化行为及微结构机理，进行拉伸、循环应力松弛和显微压入等力学性能测试，研究表层的强度和塑性等力学性能；利用有限元模拟，研究表层与粗晶基体塑性协调的微观力学过程；利用X射线衍射、透射电子显微镜等研究梯度纳米结构表层的晶粒尺寸、孪晶和相组成的梯度分布特征，重点关注不同拉伸应变下梯度纳米结构表层中位错、层错、孪晶和相变的形成、发生及交互作用，揭示(1)梯度纳米结构表层应变硬化的微结构机理；(2)表层与粗晶基体塑性协调的微观力学过程。通过本项目的研究，阐明梯度纳米结构金属高拉伸伸长率的微观机制，建立梯度纳米结构金属宏观力学性能与关键梯度参量的关联，为澄清纳米金属本征力学性能、突破纳米金属低塑性瓶颈提供思路。

为揭示梯度纳米结构金属的应变硬化机理，利用SAMT技术在IF钢、304不锈钢、AZ31镁合金中制备了多种梯度纳米结构样品，通过拉伸、循环应力松弛、微结构表征以及有限元模拟，研究了梯度纳米结构表层与粗晶基体协调变形的微观过程、力学性能与应变硬化规律、以及应变硬化过程对应的可动位错形成与增殖、马氏体相变等微结构行为。得出以下主要结论：(1) 梯度纳米结构在拉伸过程中发生单轴向多轴的应力状态转变。粗晶基体发生屈服后，因泊松比不同而在梯度表层产生拉─压、在粗晶基体形成拉─拉多轴应力状态；整体进入屈服以后，梯度表层失稳受基体抑制，表层产生拉─拉、而粗晶基体形成拉─压多轴应力状态；(2) 梯度表层与粗晶基体在侧向的协调变形使梯度纳米结构在拉伸过程中形成应变梯度，导致几何必需位错密度增加从而提高应变硬化；同时应力状态由单轴向多轴的转变过程使可动位错密度不断形成并增殖，进一步提高了位错交互作用与应变硬化；(3) 梯度纳米结构表层细小的晶粒结构和多轴应力状态使马氏体相变持续到较高的应力与应变水平，使得在高应力水平下获得高应变硬化，从而得到超过1500 MPa屈服强度和20%均匀拉伸伸长率；(4) 在镁合金中，梯度纳米结构表层与粗晶基体不同的滑移模式在界面处形成高应力，使锥面滑移在室温下被激活，从而使屈服强度提高2─5倍且保留超过10%的拉伸伸长率。此外，由于负责人工作单位发生变动，开展了部分适应新单位要求的研究工作，得到主要结论如：(5) TiAl合金热处理过程中微孔膨胀以致微裂纹形成是室温脆性的主要因素，通过集成热等静压和热处理工艺，得到无缺陷组织并使室温强度与拉伸延性同时提高；(6) 位错可在石墨烯/金属基体界面上滑移，从而影响并弱化石墨烯金属的变形织构；同时石墨烯可抑制晶界迁移而提高金属的热稳定性。