Synchrotron Studies of Long Range Internal Stresses in Plastically Deformed Materials

在使用过程中，材料经常有意或无意地承受载荷或应力。有证据表明，由于材料中存在原子缺陷的聚集，这些应力可能不会均匀分布在材料中。这些缺陷负责促进或“携带”由外部应力引起的材料的正常永久或塑性变形。然而，这种应力的重新分配会导致材料中的长范围内应力（LRIS）。表征和理解导致背应力的长距离内应力的细节和起源是至关重要的。这将由首席研究员和他的学生在阿贡国家实验室的先进光子源（同步加速器）上使用新的尖端x射线微束表征技术进行研究。了解背应力的来源和大小的细节可以导致对材料重要性能方面的新理解。这尤其包括疲劳和金属成形。疲劳是结构材料中最常见的突变破坏机制之一。对疲劳的更好理解将提高结构材料的性能。了解金属成形作业中的背应力可以大大节省金属成形制造部门的成本。PI还将与洛杉矶中南部的高中开展外展项目。在过去，人们普遍认为在塑性变形的晶体材料中存在背应力或长程内应力（LRIS）。在变形的显微组织中，位错密度升高或位错不均匀的区域会出现应力升高。长期内应力的存在对于理解循环变形和单调变形都是非常重要的。x射线微束衍射实验将由首席研究员和学生使用同步加速器进行，该同步加速器首次能够确定位错细胞壁和内部的应变。这项工作包括尝试测量细胞内部和细胞壁的全弹性应变张量。这也是有史以来第一次进行这种类型的实验。PI还将进行原位微梁实验，并测量试样在外部载荷下的晶格参数。PI和他的学生将检查严重塑性变形（SPD）的材料，其中已经建议存在相对较高的LRIS。这些实验将由PI和研究生在阿贡国家实验室的先进光子源进行。总的来说，我们对塑性变形的理解有望得到加强。该项目将了解对疲劳很重要的背应力，大多数结构材料失效的原因，以及金属成形中的回弹。了解金属成形操作中的LRIS可以为制造业带来巨大的节省。PI还将与洛杉矶中南部的高中开展外展项目。

Non-Technical SummaryMaterials are often intentionally or unintentionally subjected to loads, or stresses, during service. There is evidence that these stresses may not be uniformly distributed in materials due the presence of clustering of atomic defects in the material. These defects are responsible for facilitating or "carrying" the normal permanent or plastic deformation in materials that results from the external stresses. However, this redistribution of the stress can lead to long range internal stresses (LRIS) in materials. It is critical to characterize and understand the details and origin of long range internal stresses that lead to backstresses. This will be researched by the Principal Investigator and his students using, new cutting-edge, x-ray microbeam characterization techniques at the Advanced Photon Source (synchrotron) at Argonne National Laboratory. Learning the details of the origin and the magnitude of the backstresses can lead to a new understanding of important performance aspects of materials. This particularly includes fatigue and metal forming. Fatigue is one of the most common catastrophic-failure mechanisms in structural materials. Better understanding of fatigue would lead to enhanced structural-materials performance. Understanding backstresses in metal forming operations could lead to enormous savings in the metal-forming manufacturing sector. The PI will also work with outreach programs to high schools in south-central Los Angeles. Technical summaryBackstresses or long range internal stresses (LRIS) have been, in the past, widely suggested to exist in plastically deformed crystalline materials. Elevated stresses can be present in regions of elevated dislocation density or dislocation heterogeneities in the deformed microstructures. The existence of long range internal stress is especially important for the understanding of cyclic deformation but also monotonic deformation. X-ray microbeam diffraction experiments will be performed by the principal investigator and students using a synchrotron that are able to determine the strain within dislocation cell walls and interiors for the first time. This work includes an attempt to measure the full elastic strain tensor in the cell interiors and cell walls. This will also be the first experiment ever performed of this type. The PI will also perform in-situ microbeam experiments and measure the lattice parameter while the specimen is under external load. The PI and his students will examine severely plastically deformed (SPD) materials, where relatively high LRIS has been suggested to be present. These experiments will be performed by the PI and graduate students at the Advanced Photon Source at Argonne National Laboratory. Our understanding of plastic deformation, in general, is expected to be enhanced. This project will understand backstresses that are important to fatigue, the cause for most structural material failures, and also spring back in metal forming. Understanding LRIS in metal forming operations could lead to enormous savings in the manufacturing sector. The PI will also work with outreach programs to high schools in south-central Los Angeles.