Long-Range Internal Stresses in Plastically Deformed Materials

这是一项资助国内博士生明确评估塑性变形材料中长程内应力（LRIS）存在的提案。背应力或LRIS在过去已经被许多人（但肯定不是所有人）认为存在于塑性变形的晶体材料中。升高的应力可存在于变形的微结构中的升高的位错密度或位错不均匀性的区域中。不均匀性包括刃位错偶极子束（静脉）和持久滑移带（PSB）的循环变形材料和细胞和亚晶壁在单调变形材料的边缘偶极子壁。长程内应力的存在对于理解循环变形和单调变形是特别重要的。初步的X射线微束衍射实验，能够确定的弹性应变内的细胞内部进行的主要调查员使用同步加速器。这些是使用定向、单调和循环（预饱和，即，无PSB）变形的Cu单晶。结果表明，小（17%至29%的所施加的应力）的长程内应力可能存在于细胞内部。这些LRIS在细胞与细胞之间变化很大，为所施加应力的0 - 50%。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，这一直是具有挑战性的。这将有助于完成变形材料中LRIS的确定性分析。最后，详细的透射电子显微镜将进行位错亚结构（汉堡？的矢量分析和精确的空间定位壁位错），通过位错动力学代码进行评估，实验LRIS的验证。一般而言，对塑性变形的理解有望得到加强。我们希望成为第一个明确评估塑性变形材料中的长期内应力的机构。 非技术总结这是一个项目，以揭示秘密的材料强度使用x射线microbeams。在伊利诺斯州的高级光子源中使用强烈的亚微米X射线束导致发现结构材料在亚微米长度尺度上处于与位错子结构对应的相反方向的显著的可变内应力下。这一结果对于理解材料的力学强度和行为具有深远的意义。二十多年前就预测到变形材料中微观体积（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，因为在同步辐射源的高分辨率X射线微束发展之前，对测试理论和计算机建模至关重要的应力磁和分布的空间分辨测量是不可能的。这项工作的意义是直接相关的重要的实际问题，如金属板成形（例如，在汽车生产）和金属fatigue，后者是负责大多数结构材料的故障。

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.