Mechanisms of Cyclic Plastic Deformation in Metals

这个补助金审查循环变形，或疲劳，在一个基本的水平。对金属疲劳的了解很少，部分原因是反向变形过程中的位错动力学和内应力状态尚未得到表征。 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。 在这项研究中，循环变形的详细描述，包括包辛格效应，大量的可逆应变与变形方向的逆转，并最终饱和的流动应力，将完成，使用先进的实验技术。将在Cu单晶上使用常规暗场（DF）透射电子显微镜（TEM）来评估位错偶极子间距和分布，这将允许确定循环塑性期间的局部应力状态。实验将包括会聚束电子衍射（CBED）的TEM，可以探测小（20纳米直径的光束）的地区，以评估在未加载的箔以及原位，或负载下的晶格参数的变化。这将允许以相对高的精度直接评估局部内部应力。位错动力学将通过高压透射电子显微镜（HVEM）中的原位变形来研究。螺旋位错的作用将被调查，包括存在的堆积和交叉滑移。与卸载滞弹性有关的位错的细节也将被研究。特别取向箔将补充早期的反向变形实验中的HVEM，特别是确定在疲劳过程中的螺旋位错动力学的性质。%在过去，在循环变形期间直接观察位错仅取得有限的成功，例如在透射电子显微镜中进行原位循环或反向塑性变形试验。此外，内部应力状态尚未得到充分确定。内应力被广泛认为存在于位错的不均匀性附近的循环（以及单调）变形的微观结构。不均匀性包括刃位错偶极束（脉）和持续滑移带（PSB）的刃偶极壁。 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.***