U.S.-France Dissertation Enhancement: Dislocation Induced Backstresses in Crystalline Materials

几十年来，人们一直认为在塑性变形的晶体材料中存在背应力或长程内应力。然而，直到最近，还没有明确的证据证明这一现象。认为这些应力与变形组织中的位错非均质性有关。非均质性包括单调变形材料的胞壁和亚晶壁，以及循环变形材料的边缘位错偶极束（脉）和持续滑移带（PSBs）的边缘偶极壁。最近（尚未全部发表）使用同步加速器进行的x射线微束衍射实验能够确定位错细胞内部和细胞壁内的弹性应变。这些都是使用定向、单调和循环（压力饱和，即无psb）变形的Cu单晶完成的。结果表明，可测量的远程内应力（LRIS）存在于细胞内部和细胞壁。这些LRIS在0-50%的施加应力范围内因细胞而异。细胞内部的应力大约为0.13-0.29，而细胞壁的应力与内部的应力相反，大约为0.07，尽管后一个值是初步的（未发表）。南加州大学（USC）的研究生Peter Geantil将在USC对位错亚结构进行详细的透射电子显微镜研究，这是他的论文研究的一部分。通过与法国CNRS-ONERA微结构研究实验室的研究人员合作，这些位错信息将被替换为强大的位错动力学代码microMegas，以验证实验LRIS测量结果。做这个实验有双重好处。位错动力学模拟将验证获得的实验数据，而位错建模必须准确地模拟物理现象，因此练习将是对模型本身的测试。我们对塑性变形的理解将得到加强。此外，位错动力学模拟将解析地显示这些应力的来源，并有助于发展这种现象背后的理论。该项目涉及两个主要研究小组之间的合作，努力探索材料强度领域的重要问题。南加州大学的博士生彼得·吉安提尔将被派往法国巴黎的主要研究机构CNRS-ONERA。他将与一些世界领先的材料力学行为建模领域的专家合作，以解决有关材料强度细节的重要问题。该项目还涉及与三个美国政府实验室的合作：NIST、橡树岭国家实验室和阿贡国家实验室的先进光子源。这个项目将提供对塑性变形材料更深入的了解。更具体地说，这项工作将导致对疲劳的更深层次的理解，这是大多数结构材料失效的原因。更好地了解疲劳的细节将反过来导致更强，更可靠的材料。这次合作包括表征材料微观结构中应力状态变化的最佳实验成果，以及同一领域中最好的理论/计算小组。

Backstresses, or long-range internal stresses, have been suggested to exist in plastically deformed crystalline materials for decades. However, until recently, unambiguous evidence of this phenomenon has been absent. It is believed that these stresses are associated with dislocation heterogeneities in the deformed microstructures. The heterogeneities include cell and subgrain walls in monotonically deformed materials, and edge dislocation dipole bundles (veins) and edge dipole walls of persistent slip bands (PSBs) in cyclically deformed materials. Very recent (not yet all published) x-ray microbeam diffraction experiments performed using a synchrotron are able to determine the elastic strains within the dislocation cell interiors and cell walls. These are accomplished using, oriented, monotonically and cyclically (presaturation, i.e., no PSBs) deformed Cu single crystals. The results suggest that measurable long-range internal stresses (LRIS) are present in cell interiors and cell walls. These LRIS vary substantially from cell to cell as 0-50% the applied stress. Cell interiors have stresses of roughly 0.13-0.29 the applied stress and cell walls have stresses of opposite sign to interiors at roughly 0.07 the applied stress, although the latter value is preliminary (unpublished). Peter Geantil, a University of Southern California (USC) graduate student, will be performing detailed transmission electron microscopy on the dislocation substructure at USC as part of his dissertation research. Through collaboration with researchers at the Laboratoire d'Etude des Microstructures at CNRS-ONERA in France, this dislocation information will be substituted into the powerful dislocation dynamics code microMegas, in an effort to verify the experimental LRIS measurements. There is a two-fold benefit in performing this experiment. The dislocation dynamics simulations will verify the experimental data obtained, and dislocation modeling must model physical phenomena accurately, so the exercise will be a test of the model itself. Our understanding of plastic deformation will be enhanced. Additionally, the dislocation dynamics simulations will show analytically the origin of these stresses, and aid in developing the theory behind this phenomenon. This project involves collaboration between two leading research groups in an effort to explore important questions in the field of strength of materials. University of Southern California PhD student, Peter Geantil, will be sent to a major research institution, CNRS-ONERA in Paris, France. He will collaborate with some of the worlds leading experts in the area of modeling the mechanical behavior of materials to resolve important questions regarding details of the strength of materials. The project also involves collaboration with three US government laboratories: NIST, Oak Ridge National Laboratory, and the Advanced Photon Source at Argonne National Laboratory. This project will provide a deeper understanding of plastically deformed materials. More specifically, this work will lead to a deeper understanding of fatigue, the cause for most structural material failures. A better understanding of the details of fatigue will in turn lead to stronger, more reliable, materials. This collaboration consists of the best experimental effort that characterizes the variation in stress-states in materials microstructures, with the best theoretical / computational group in the same area.