SGER: Incipient Kink Bands, Damping, Micro- and Macroyielding in Hexagonal Metals

技术：PI已经取得了几项关键发现，为开发新的六角金属基合金铺平了道路，这种合金不仅具有高屈服点和高阻尼能力，而且还可以用于自我监控其应力状态。使PI能够进行这项高风险/高回报和变革性工作的突破是识别一大类被标记为扭结非线性弹性(KNE)的固体，因为它们的重要(且在许多情况下)变形模式之一是形成完全可逆的、基于位错的初始扭结带(IKB)，在更高的应力和/或温度下演变成可移动的位错壁，即MDW，即。低角度晶界和塑性变形。后者的结合导致扭结边界(KBS)的形成。等电点分析表明，IKB向MDW的转变与某些六方金属，如镁、钴和钛的屈服点相吻合。另一方面，MDW到KB的转换减小了域大小并导致硬化。虽然目前PI了解IKBS、MDW以及最终KBS对机械性能和阻尼的影响，但PI还没有系统地了解这一点，因为到目前为止，PI的重点一直是识别和分类KNE固体。换句话说，在利用新获得的知识来设计具有特殊性质的固体方面，几乎没有做什么。在这项工作中，PI将通过系统地研究微观结构(如晶粒度、织构、孔隙率)、化学(如固溶体、第二相)和热机械加工(如低温、室温和高温预应变、蠕变)对一些重要的六方金属，如镁、钛、钴及其合金中的IKBS、MDW和KBS的形核和生长的影响来做到这一点。为了实现后者，PI将使用以下实验技术：简单的压缩和拉伸测试，循环球形纳米压痕，电子背散射衍射，原位TEM和SEM，声耦合技术，(ACT)和共振超声光谱(RUS)，以及原位中子和X射线衍射。对于KNE固体，应力或应变与声音衰减之间存在一一对应关系。因此，通过监测后者，可以很容易地监测由KNE固体制成的结构部件的健康状况--或者如果KNE固体被粘贴到结构部件上--作为时间的函数。这种自应力监测能力--取决于所选择的材料--在延长的温度范围和/或腐蚀性环境中非常有用。非技术性：最重要的是，PI注意到，IKB是固体变形拼图中最后几个但关键的缺失部分之一；这项工作将使PI能够勾勒出这部分缺失的形状。从对KNE元素和化合物的细读中，很明显，自然界的大部分确实是KNE。因此，从这项工作中学到的东西将对从地质学到微电子学和设备制造商的许多其他研究领域产生重要的影响和影响。这项工作是以研究生和本科生紧密合作的方式进行的。UG从发现的兴奋中受益，并学习有用的分析和其他技能。对于研究生来说，这种交叉培养造就了更好的研究生、导师、教师、领导者和科学家。这个项目的另一个主要目标将是说服大多数，如果不是所有的话，在这个项目中精心挑选的UG学生去攻读一般科学，特别是材料科学的研究生学位。

TECHNICAL: PI's have made several key discoveries that pave the way to develop new hexagonal metal based alloys that not only combine high yield points and high damping capabilities but can also be used to self-monitor their stress state. The breakthrough that allows PI to conduct this high-risk/high-payoff and transformative work is the identification of a large class of solids labeled as kinking nonlinear elastic, (KNE) solids, because one of their important (and in many cases only) deformation mode is the formation of fully reversible, dislocation-based incipient kink bands, IKBs, that at higher stresses and/or temperatures devolve into mobile dislocation walls, MDW, viz. low angle grain boundaries and plastic deformation. The coalescence of the latter leads to the formation of kink boundaries (KBs). PI has shown that the IKB to MDW transformation coincides with the yield points of some hexagonal metals, such as Mg, Co and Ti. The MDW to KB transformation, on the other hand, reduces the domain size and leads to hardening. While currently PI understands the effects of IKBs, MDW and ultimately KBs on the mechanical properties and damping, PI has not done so systematically, because to date PI's focus has been on identifying and classifying KNE solids. In other words, little was done in terms of making use of newly gained knowledge to engineer solids with exceptional properties. In this work PI will do just that by systematically studying the effects of microstructure (e.g. grain size, texture, porosity), chemistry (e.g. solid solutions, second phases) and thermo-mechanical processing (e.g. pre-strains at cryogenic, room and higher temperatures, creep) on the nucleation and growth of IKBs, MDW and KBs in some important hexagonal metals such as Mg, Ti, Co and their alloys. To carry out the latter PI will use the following experimental techniques: simple compressive and tensile testing, cyclic spherical nano-indentations, electron-backscattered diffraction, in situ TEM and SEM, acoustic coupling technique, (ACT) and resonant ultrasound spectroscopy (RUS), and in situ neutron and XRD diffraction. For KNE solids, there is a one-to-one correspondence between stress or strain and sound attenuation. Thus by monitoring the latter, the health of a structural component made with a KNE solid - or if a KNE solid is affixed to a structural component - can be easily monitored as a function of time. This self-stress monitoring capability can - depending on material chosen - be useful over extended temperature regimes and/or corrosive environments. NON-TECHNICAL: Most importantly, PI notes that IKBs are one of the last few, but crucial missing pieces in the deformation-of-solids puzzle; this work would allow PI to outline the shape of this missing piece. From a perusal of the elements and compounds that are KNE it is apparent that most of nature is indeed KNE. Thus what is learned in this work will have important ramifications and implications in many other fields of inquiry, from geology to microelectronics and device manufacturers. The work is structured in such a way that graduate and undergraduate (UG) students work closely together. UG benefit from the excitement of discovery, and learn useful analytic and other skills. For the graduate student this cross-fertilization creates better graduate students, mentors, teachers, leaders and scientists. Another major goal of this project will be to convince a majority, if not all, of the carefully chosen UG students working on this project to go for graduate degrees in the sciences in general, and materials science in particular.