Long-Range Internal Stresses in Plastically Deformed Materials

这是一项资助国内博士候选人明确评估塑性变形材料中存在的远程内应力（LRIS）的提案。在过去，许多人（但肯定不是全部）认为在塑性变形的晶体材料中存在背应力或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。非均质性包括循环变形材料中的边缘位错偶极束（脉）和持续滑移带（PSBs）的边缘偶极壁，单调变形材料中的胞壁和亚晶壁。长期内应力的存在对于理解循环变形和单调变形尤为重要。初步的x射线微束衍射实验能够确定细胞内部的弹性应变，由首席研究员使用同步加速器进行。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，小的（17%至29%的施加应力）长期内应力可能存在于细胞内部。这些LRIS在0-50%的施加应力范围内因细胞而异。该建议包括尝试测量细胞壁中的弹性应变，由于高缺陷密度，这一直具有挑战性。这将有助于完成变形材料中LRIS的最终分析。最后，对位错子结构(Burger？s矢量分析和壁面位错的精确空间定位)，通过位错动力学代码进行评估，验证实验LRIS。总的来说，对塑性变形的理解有望得到加强。我们期望成为第一个明确地评估塑性变形材料的长期内应力的人。这是一个利用x射线微光束揭示材料强度秘密的项目。在伊利诺伊州的先进光子源上使用强烈的亚微米x射线束，发现结构材料在亚微米长度尺度上处于与位错子结构相对应的显著的、可变的反方向内应力下。这一结果对理解材料的力学强度和力学行为具有深远的意义。二十多年前，人们就预测到变形材料的微观体积（或细胞）内存在平衡应力，并从许多间接实验中推断出来。然而，它们存在的直接证据一直难以捉摸，因为在同步加速器源的高分辨率x射线微光束发展之前，对测试理论和计算机建模至关重要的应力大小和分布的空间分辨测量是不可能的。这项工作的含义与重要的实际问题直接相关，例如金属板成形（例如，在汽车生产中）和金属疲劳，后者是大多数结构材料失效的原因。

TECHNICAL SUMMARY This is a proposal to fund a domestic PhD candidate to definitively assess the existence of long-range internal stresses (LRIS) in plastically deformed materials. Backstresses or LRIS in the past have been suggested by many (but certainly not all) 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 heterogeneities include edge dislocation dipole bundles (veins) and the edge dipole walls of persistent slip bands (PSBs) in cyclically deformed materials and cell and subgrain walls in monotonically deformed materials. The existence of long-range internal stress is especially important for the understanding of cyclic deformation and also monotonic de-formation. Preliminary x-ray microbeam diffraction experiments that are able to deter-mine the elastic strains within the cell interiors were performed by the principal investi-gator using a synchrotron. These were accomplished using, oriented, monotonically and cyclically (presaturation, i.e., no PSBs) deformed Cu single crystals. The results suggest that small (17 to 29 % of the applied stress) long-range internal stresses may be present in cell interiors. These LRIS vary substantially from cell to cell as 0-50% the applied stress. This proposal includes an attempt to measure elastic strains in the cell walls which has been challenging due to the high defect density. This will help complete the definitive analysis of LRIS in deformed materials. Finally, detailed transmission electron micros-copy will be performed on the dislocation substructure (Burger?s vector analysis and pre-cise spatial positioning of wall dislocations), to assess through a dislocation dynamics code, a verification of the experimental LRIS. The understanding of plastic deformation, in general, is expected to be enhanced. We expect to be the first to definitively assess long-range internal stresses in plastically deformed materials. NON-TECHNICAL SUMMARYThis is a project to uncover the secrets of the strength of materials using x-ray mi-crobeams. The use of intense, submicron, x-ray beams at the Advanced Photon Source in Illinois has led to the discovery that structural materials are under significant, variable internal stresses of opposite direction on submicron length scales corresponding to the dislocation substructure. This result has profound implications for understanding the me-chanical strength and behavior of materials. The presence of counterbalanced stresses within microscopic volumes (or cells) in deformed materials was predicted more than two decades ago and has been inferred from numerous indirect experiments. Yet, direct proof of their existence has been elusive, as spatially-resolved measurements of the stress mag-nitudes and distributions critical for testing theories and computer modeling were not possible before the development of high-resolution x-ray microbeams at synchrotron sources. The implications of this work are of direct relevance to important practical prob-lems such as sheet metal forming (for example, in automobile production) and metal fa-tigue, the latter being responsible for most structural materials failures.