表面层片状梯度纳米结构金属塑性行为与微观机制

The GNLS/CG(Gradient Nanolaminated Structure/Coarse grained)architecture metallic plate (pure Ni, Cu and Al) samples will prepared by means of surface mechanical rolling and grinding treatment(SMRGT). The plastic deformation behaviors and mechanical prosperities of the GNLS-plate sample will be tested by tensile, micro-indentation and recycle stress relaxation experimental technique. The microstructures，including grain size, grain boundaries type, texture gradient and crystal defect will be characterized by SEM/EBSD, XRD, TEM, HRTEM and others. The evolution process of crystal defects and the migration/rotation of low angle boundaries in the GNLS/CG lamellae with deformation strain increase will be investigated by in-situ tensile test under and TEM/SEM, respectively. The process of dislocation activation, pile up or passing through the low angle boundaries will be revealed by molecular dynamics(MD) simulation at atom scale. Furthermore, The stress/strain distribution and deformation compatibility behavior between GNLS layer and coarse grained (CG) center will be investigated by crystal plasticity finite element method (CP-FEM). This proposal is aiming at establishing the connection model between GNLS microstructure and macro-mechanical properties, revealing the interaction between the crystal defects and low angle boundaries, clarifying the evolution mechanisms of low angle boundaries under stress/strain driven situations, and finally revealing the plastic deformation behavior and micro-mechanism of GNLS/CG. This study would provide a possible approach to develop a new architecture nano-structured metal with higher strength, more stability and excellent plasticity.

利用SMRGT装置，制备表面层片状梯度纳米结构(GNLS/CG)板状样品。采用拉伸、压入和循环应力松弛等方法，研究GNLS/CG塑性行为和力学性能。采用SEM/EBSD, XRD, TEM, HRTEM等方法，定量表征晶粒尺寸、晶界类型、织构梯度、晶体缺陷等微结构特征。结合TEM/SEM原位拉伸测试，研究GNLS/CG纳米层片状晶粒中晶体缺陷的演化以及小角度晶界的迁移和转动。利用分子动力学模拟从原子层次上研究位错的产生、塞积以及通过小角度晶界的情况。基于晶体塑性有限元，分析梯度层与粗晶层之间的应力/应变分布和协调塑性变形行为。通过研究，旨在建立微结构特征参数与力学性能之间的关联模型；揭示晶体缺陷与小角度晶界的交互作用；明晰小角度晶界在应力/应变驱动下的迁移、转动行为与机理；最终阐明GNLS/CG的塑性行为和微观机理；为设计出具有更高强度和稳定性，同时兼具优良塑性的纳米金属材料提供可能途径。

采用自主设计的表面机械滚动碾压处理(SMRGT, Surface Mechanical Rolling and Grinding Treatment)装置，合理调节碾压速度、单次碾压深度、循环次数等工艺参数，成功制备出具有不同微观组织特征与梯度层含量的表面层片状梯度纳米结构(GNLS/CG)板状、棒状金属镍、铝、铜样品。结合显微压入、金相观察、SEM/EBSD等实验方法，定量表征了GNLS/CG结构中晶粒尺寸、晶界类型、织构梯度、晶体缺陷等微结构参数的梯度分布特征。开展了室温单轴拉伸、循环应力松弛等实验，研究了GNLS/CG塑性变形行为和力学性能。结合SEM/EBSD准原位拉伸测试，研究了GNLS/CG纳米层片状晶粒在塑性变形过程中晶体取向、晶粒形貌以及小角度晶界的迁移、转动过程。基于晶体塑性有限元方法，建立了GNLS/CG梯度结构的有限元模型，分析了梯度层与粗晶层之间的应力/应变分布和协调塑性变形行为。研究结果表明，GNLS/CG结构能显著提高材料强度的同时依然保持其优异的均匀塑性变形能力：GNLS-CG金属镍屈服强度由粗晶态的65MPa提高到378MPa，其均匀塑性应变依然高达25%。不同体积含量的GNLS/CG梯度结构将导致不同的塑性变形行为，分别呈现出连续屈服、瞬态效应、屈服降(点)、屈服降(点)+瞬态效应的屈服特征。应变硬化率的分析表明，GNLS-CG金属结构拥有优异的均匀塑性变形能力与其加工硬化能力“再获得”密切相关。塑性变形过程中微观组织演化的准原位观察表明，GNLS-CG金属特殊的梯度结构对应变局域化现象有着显著的抑制作用，有效防止塑性失稳的过早发生。晶体塑性有限元的模拟结果表明，梯度结构在塑性变形过程中存在显著应力/应变的交互作用和协调塑性变形行为，这是其拥有优良均匀塑性的微观机制。GNLS-CG金属的屈服强度-均匀塑性之间呈现出特殊的“反香蕉型”关系，提供了设计、制备出兼具高强度和优良塑性纳米金属材料的可行途径。