Mechanisms of Cyclic Plastic Deformation in Metals

这项资助在基础水平上研究循环变形或疲劳。人们对金属疲劳的了解很少，部分原因是位错动力学和反向变形过程中的内应力状态尚未表征。位错运动和相互作用的细节以及内部应力状态对这一过程至关重要。在本研究中，将使用先进的实验技术来完成循环变形的详细描述，包括包辛格效应，变形方向逆转的大量可逆应变，以及流动应力的最终饱和。传统的暗场（DF）透射电子显微镜（TEM）将用于铜单晶，以评估位错偶极子间距和分布，这将允许在循环塑性过程中确定局部应力状态。实验将包括TEM中的会聚束电子衍射（CBED），它可以探测小（20纳米直径的光束）区域，以评估未加载箔以及原位或负载下晶格参数的变化。这将允许以相对较高的精度直接评估局部内应力。在高压透射电子显微镜（HVEM）下通过原位变形研究了位错动力学。螺旋位错的作用将被研究，包括堆积和交叉滑移的存在。与卸载时的非弹性相关的位错的细节也将被研究。在HVEM中，特殊取向箔将补充早期的反向变形实验，以特别确定疲劳期间螺杆位错动力学的性质。过去，在循环变形过程中直接观察位错，如原位循环或反向塑性变形试验，如在透射电子显微镜中，只取得了有限的成功。内部应力状态也没有得到充分的确定。人们普遍认为，在循环（以及单调）变形的显微组织中，内应力存在于位错非均质附近。非均质性包括边缘位错偶极束（脉）和持续滑移带（PSBs）边缘偶极壁。从这个项目中得到的理解将有助于设计承受循环应力的薄膜器件

This grant examines cyclic deformation, or fatigue, at a fundamental level. Metal fatigue is poorly understood, partly because the dislocation dynamics and internal stress-states during reversed deformation have not been characterized. The details of dislocation motion and interaction and the internal stress-state are critical to this process. In this research a detailed description of cyclic deformation, including the Bauschinger effect, the substantial reversible strains with reversal of the direction of deformation, and the eventual saturation of the flow stress, will be accomplished, using advanced experimental techniques. Conventional dark- field (DF) transmission electron microscopy (TEM) will be used on Cu single crystals to assess dislocation dipole spacings and distributions, which will allow a determination of the local stress-state during cyclic plasticity. Experiments will include convergent beam electron diffraction (CBED) in the TEM that can probe small (20-nm diameter beam) areas to assess changes in the lattice parameter in unloaded foils as well as in-situ, or under load. This will allow direct assessment of the local internal stress with relatively high accuracy. The dislocation dynamics will be studied by in-situ deformation in the high voltage transmission electron microscope (HVEM). The role of screw dislocations will be investigated, including the existence of pile-ups and cross-slip. The details of dislocations associated with anelasticity on unloading will also be studied. Specially oriented foils will complement earlier reversed deformation experiments in the HVEM to especially determine the nature of screw dislocation dynamics during fatigue. %%%In the past there has been only limited success with direct observation of dislocations during cyclic deformation, such as with in-situ cyclic or reversed plastic deformation tests such as in the transmission electron microscope. Also the internal stress-state has not been adequately determined. Internal stresses are widely suggested to exist in the vicinity of dislocation heterogeneities in cyclically (as well as monotonically) deformed microstructures. The heterogeneities include edge dislocation dipole bundles (veins) and the edge dipole walls of persistent slip bands (PSBs). The understanding developed from this project will aid in the design of thin film devices that undergo cyclic stresses.***