超精密切削表面纳/微观塑性机理与实验研究

The operating mode of machining at the critical depth of cut can be considered as a process of interfacial friction between the cutting tool of diamond and the rough surface of the work-piece, since surfaces of work-piece in ultra-precision machining (UPM) are generally far from being perfectly smooth. The stress concentration due to the presence of asperities on the rough surface is held accountable for the mechanisms of dislocation nucleation near and on the rough surface, which will result in layers of tensile or compressive residual stresses depending on their respective distances from the surface, and thus, have an immediate effect on a precise evaluation of the integrity of the machined surface. An appreciable discrepancy has been observed between the results of such a contact induced plastic problem at small scale by employing the classical continuum theory of plasticity and those of experimental investigations. At the same time, only qualitative conclusions on the above-mentioned problem can be drawn by adopting the existing 2D discrete dislocation models and 2D analytical dislocation models on account of their inability of providing a comprehensive description of multiple mechanisms of dislocation motion. A novel approach will be established for a quantitative solution of the same problem by utilizing the multi-scale paradigm of incorporating the 3D discrete dislocation dynamics into the framework of continuum finite element, by means of which the finite element analysis will be implemented relying on the macro plastic strain increment obtained from the simultaneous description and solution of motion mechanisms of a large number of dislocation segments admitting an explicit physics foundation. Such an implementation procedure will make it possible for the present 2D discrete dislocation models, only capable of qualitative analysis of plasticity problems at small scale, to be enhanced to a 3D counterpart for quantitative predictions. Precise experimental measurement of the induced residual stresses will be carried out to confirm and evaluate the computational results. With an appropriate implementation of the proposal, a deep insight will be gained in understanding the mechanics of nano- and micro-scale contacts of rough surfaces involved in UPM based on physics realities. The proposed modeling approach and the corresponding experimental techniques and principles can be directly employed for the evaluation of machined surfaces, and provide guidance on practical ultra-precision machining.

超精密车削工件表面在纳/微米尺度下远非理想光滑,临界切削工况可看作金刚石刀具同粗糙表面间的摩擦作用。粗糙表面凸台导致的应力集中提供了位错在近表面的不同形核机制，产生离表面距离不同的拉／压残余应力层，这将直接影响加工表面完整性的准确评估。经典塑性理论处理这一小尺度接触所致的塑性问题时出现明显偏差。同时，因缺乏对位错多种运动机制的全面描述，现有二维离散位错动力学及解析模型仅能给出该问题的定性分析。三维离散位错动力学同有限元的多尺度耦合为定量求解同一问题提供了新的思路，他通过对大量离散位错多种运动机制更接近物理实际的描述来导出宏观塑性应变增量，进行有限元分析，可使解决方案从目前二维定性分析上升到三维定量预测。同时构建高精度残余应力测量实验对计算结果进行验证评估。项目的实施将在物理层面上加深对超精密加工中微纳米力学问题的进一步理解，模拟方法和实验手段可直接用来评估加工表面，指导实际加工.

超精密切削加工技术是一门集机械、光学、电子、计算机、测量和材料科学等先进技术于一体的综合性技术。超精密加工技术在尖端产品和现代化武器制造中占有重要地位。晶体材料工件表面在纳微观尺度下远非理想光滑，而是存在着一系列分布和高度不同的微小凸台及其他微结构缺陷。当其平均尺寸处在几个纳米到几个微米范围内时，实验观察及理论分析表明，经典塑性理论不再能精确地模拟这一尺度下的弹塑性力学问题，原因在于此时位错这一作为塑性变形物理基础的离散性趋于占主导地位。因而离散位错塑性模型在处理上述问题时便能发挥重要作用。以此为背景，项目开展了一下几方面的研究工作：.1).基于多边形位错环在弹性各向异性半空间的位移及应变解，将Mura等的用于各向同性材料压痕模拟的分布位错环模型推广到单晶体纳米压痕数值模拟，研究了压头相对于试样的不同晶体方位同卸载后回弹及塌陷量的关系；同时对试样近表面的残余应力大小和分布进行了计算分析。建立的此分析模型可用于微纳米力学中的纳米压痕试验中表面位移及残余应力的计算分析。.2).基于多边形位错环在弹性各向异性全空间的位移解，同有限元结合建立了三维有限域弹性各向异性多晶体材料离散位错力学模型，将相应的二维模型推广到三维多晶模型；此模型能有效计算三维位错环同多晶界面及薄膜同基体的相互作用。.3).开发了基于MPI的并行无限域三维弹性各向异性位错动力学程序，可用于分析单晶体中位错的多种运动机制。.4).开发了有限域三维弹性各向同性位错动力学程序，分析了位错环同孔洞的相互作用。.5).考虑几何必需位错密度及其演化对晶体滑移系开动阻力的影响，开发了Abaqus平台上应变梯度晶体塑性用户子程序UMAT，计算了单晶铜丝的扭转尺度效应。.6).开发了基于级数渐进展开的多尺度晶体塑性均匀化计算程序，模拟了多晶板材的织构演化，此方法放弃了经典的Taylor变形近似假设，更符合物理实际。.7).用大型分子动力学程序模拟了纳米压痕实验中的位错行为。