Cyclic Plasticity and Fatigue of Ultrafine Grain FCC Metals at Low Plastic Strain Amplitudes

低塑性应变幅下超细晶FCC金属的循环塑性和疲劳本项目旨在对具有低初始位错密度的超细晶FCC金属的低塑性应变幅循环变形和疲劳行为有一个基本的了解。超细晶粒金属是那些晶粒直径在亚微米范围内，但通常大于约10纳米的金属。这种材料才刚刚开始大量生产，相对于传统晶粒尺寸的材料，人们对它们的机械行为知之甚少，但超细晶粒金属已被证明具有非凡的强度，在许多情况下，具有合理的延展性。它们通常以三种方式之一生产：通过惰性气体冷凝和粉末冶金，通过常规晶粒尺寸材料的严重塑性变形和电沉积。由剧烈塑性变形产生的材料可以表现出高强度和良好的延展性，但它们具有非常高的位错密度，这一特性对决定随后的力学行为非常重要。惰性气体冷凝和粉末冶金生产的材料通常表现出较差的延展性-它们有点脆。只有电沉积的材料既具有低位错密度又具有高强度和良好的延展性。因此，相对于传统晶粒尺寸材料和其他方法生产的超细晶粒材料，它们提供了获得良好疲劳寿命特性的潜力。由于可以通过电沉积生产大量的超细晶粒金属样品，因此在结构用途的商业开发方面具有巨大的潜力，但需要更多地了解这些材料的机械性能。该项目将执行一个重点实验项目，使用电沉积生产的超细颗粒镍。该项目的目标是开发和分析超细晶粒镍在低塑性应变幅值下的详细循环塑性数据库。疲劳试验将在镍电沉积试样上进行。各种技术，如光学显微镜，扫描和透射电子显微镜，以及x射线衍射将用于表征电沉积产生的材料和研究疲劳载荷的影响。其他目标包括对超细晶粒镍的疲劳裂纹萌生行为的理解。疲劳裂纹的萌生是承载能力最终丧失的前兆。该项目将涉及至少一名研究生的论文工作，包括材料科学，电化学，固体力学和铁磁学的元素。它还将涉及本科生，特别是那些来自代表性不足群体的学生。研究结果将通过会议报告和相关期刊上的出版物以及由主要研究人员及其学生维护的网站进行传播。

Cyclic Plasticity and Fatigue of Ultra Fine Grain FCC Metals at Low Plastic Strain AmplitudesThis project is aimed at achieving a fundamental understanding of low plastic strain amplitude cyclic deformation and fatigue behavior in ultra fine grain FCC metals that have a low initial dislocation density. Ultra fine grain metals are those that have grain diameters in the submicron range, but generally greater than about 10 nm. Such materials are only beginning to be produced in large quantities and little is known about their mechanical behavior relative to conventional grain size materials, but ultra fine grain metals have been shown to exhibit exceptional strength with, in many cases, reasonable ductility. They are generally produced in one of three ways: by inert gas condensation and powder metallurgy, by severe plastic deformation of conventional grain size materials, and by electrodeposition. Materials produced by severe plastic deformation can exhibit both high strength and good ductility, but they have a very high dislocation density, a property that is very important in determining subsequent mechanical behavior. Materials produced by inert gas condensation and powder metallurgy generally exhibit poor ductility - they are somewhat brittle. Only materials produced by electrodeposition have both low dislocation density and exhibit high strength and good ductility. For this reason they offer potential for achieving good fatigue life characteristics relative to conventional grain size materials and ultra fine grain materials produced by other means. Because it is possible to produce large samples of ultra fine grain metals by electrodeposition, there is significant potential for commercial development for structural uses, but more needs to be known about the mechanical properties of these materials. The project will execute a focused experimental program using ultra fine grain nickel produced by electrodeposition. The objectives of the project are to develop and analyze a detailed cyclic plasticity database for ultra fine grain nickel at low plastic strain amplitudes. Fatigue experiments will be performed on specimens produced using electrodeposition of nickel. Various techniques such as optical microscopy, scanning and transmission electron microscopy, and x-ray diffraction will be used for characterizing the materials produced by electrodeposition and for studying the effects of fatigue loading. Additional objectives include the development of an understanding of fatigue crack initiation behavior in ultra fine grain nickel. Fatigue crack initiation is a precursor to the eventual loss of load-carrying capacity.The project will involve at least one graduate student in thesis work that encompasses elements of materials science, electrochemistry, solid mechanics and ferromagnetics. It will also involve undergraduate students, particularly those from underrepresented groups. The results will be disseminated through conference presentations and publications in referred journals and via a website maintained by the principal investigators and their students.