Mechanisms of Cyclic Plastic Deformation in Metals

这项研究从根本上考察了循环变形或疲劳。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还没有得到表征。位错运动和相互作用的细节以及内部应力状态是这一过程的关键。在这项研究中，将利用先进的实验技术，对循环变形进行详细的描述，包括包辛格效应、随变形方向反转的大量可逆应变，以及最终的流动应力饱和。用常规的暗场透射电子显微镜对铜单晶体进行位错偶极子间距和分布的研究，可以确定循环塑性过程中的局部应力状态。实验将包括在透射电子显微镜中的会聚束电子衍射(CBED)，它可以探测较小的(20 nm直径的束)区域，以评估未加载以及原位或加载时晶格参数的变化。这将允许以相对较高的精度直接评估局部内部应力。在高压电子显微镜(HVEM)中用原位形变的方法研究位错动力学。将研究螺位错的作用，包括堆积和交叉滑移的存在。还将研究与卸载时的滞弹性有关的位错的细节。特别取向的薄片将补充早期的反向变形实验，特别是确定疲劳过程中螺杆位错动力学的性质。在过去，直接观察循环变形中的位错只取得了有限的成功，例如通过原位循环或反向塑性变形测试，例如在透射电子显微镜中。此外，内部应力状态也没有得到充分的确定。在循环(以及单调)变形的微观组织中，位错不均匀附近普遍存在内应力。非均质性包括边缘位错偶极子束(脉)和持续滑移带(PSB)的边缘偶极子壁。从这个项目中发展出来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.***