CAREER: Dislocation-level Understanding of Shear Banding in Magnesium and Magnesium Alloys

第一部分：非技术含量镁是一种重量轻的金属，几乎比钢轻四倍，这使它在汽车和航空航天应用中非常有吸引力。然而，纯镁和镁合金在室温下很容易破裂。当镁被拉伸时，它不会均匀地分布它所经历的压力。相反，镁倾向于将应变集中在称为“剪切带”的局部区域。这些剪切带成为镁最常断裂的位置，也是为什么镁比其他金属更容易破裂的原因。这个项目将剪切带与金属在拉伸或变形时出现的原子级缺陷联系起来。这些原子级缺陷被称为“位错”。更大数量的位错更分散，预计会促进材料更均匀的变形或拉伸，从而延迟剪切带。从这项工作中获得的见解将有助于识别新的镁合金，这些合金表现出延迟的剪切带状、更高的抗裂性和更强的镁金属形成不同形状的能力。这项研究的发现还将有助于促进镁在各种汽车和航空航天应用中的更多使用。此外，该项目还提供跨学科的教育机会，培训学生在冶金、电子显微镜和固体力学方面的知识。该项目还为教授和研究生提供了一个平台，通过Louis Stokes少数民族参与联盟计划与未被充分代表的学生密切合作，使他们接触到本研究项目的主题。第2部分：技术总结镁合金作为轻质节能材料具有巨大的潜力，但镁的结构应用因其在室温下的低应变到失效性能而受到阻碍。与其他金属相比，镁的应变到破坏的应变相对较低，这主要与局域剪切带的形成有关。这项工作的目的是促进对镁及其合金变形和破坏机制的理解，特别是发现触发或延迟它们中塑性不稳定性的途径。这里检验的假设是，非基态c+a位错活动的增加导致更均匀的变形，并在整个组织中提供更均匀的应变硬化，从而通过离域剪切带在镁及其合金中延缓塑性不稳定性。这项工作的目的是1)确定c+a位错在剪切带形成中的作用；2)确定温度(高达75℃)以及合金化对准静态变形的镁和镁合金中c+a位错活动和剪切带特征(如尺寸、数密度和储存塑性应变)的影响。这些目标将通过实验测量i)操作位错源所需的应力，ii)在给定应力下产生的位错滑动距离，以及iii)相关的交叉滑移频率来实现。然后，后一种措施将与第二个目标中提到的剪切带特征直接相关。这个项目的更广泛的影响是双重的。首先，找出更具延展性和延展性的镁和镁合金的途径，将显著增加镁合金在各种应用中使用的机会和可能性，从而显著减轻重量和提高能源效率。其次，这个项目将通过i)为本科生提供实践经验，ii)一个数字平台，以提高本科生对材料科学的兴趣，以及iii)扩展现有的YouTube频道，让公众参与到与材料科学相关的问题和现象中。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

PART 1: NON-TECHNICAL SUMMARYMagnesium (Mg) is a light-weight metal that is nearly four times lighter than steel, making it very attractive for automobile and aerospace applications. However, pure Mg and Mg alloys tend to crack fairly easily at room temperature. When Mg is stretched, it does not evenly distribute the strains it experiences. Instead, Mg tends to concentrate strains in localized zones called "shear bands". These shear bands become locations where Mg most often breaks and are responsible for why Mg cracks more easily than other metals. This project relates shear banding to the atomic level defects that occur in metals when they are stretched or deformed. These atomic level defects are called "dislocations". A greater number of dislocations that are more spread out are expected to promote more uniform deformation, or stretching of a material, and thereby delay shear banding. The insights gained from this work will help identify new Mg alloys that exhibit delayed shear banding, increased crack resistance and a greater ability for Magnesium metal to be formed into different shapes. The findings from this research will also help promote increased use of Mg across a wide variety of automotive and aerospace applications. Moreover, this project also produces interdisciplinary educational opportunities to train students in metallurgy, electron microscopy, and mechanics of solids. This project also provides a platform for professors and graduate students to work closely with underrepresented students through the Louis Stokes Alliances for Minority Participation program to expose them to the topics of this research project.PART 2: TECHNICAL SUMMARYMagnesium (Mg) alloys hold great potential for use as lightweight energy-saving materials, but the structural applications of Mg have been hindered by its low strain-to-failure properties at room temperature. The comparatively lower strain-to-failure exhibited by Mg as compared to other metals is chiefly related to the formation of localized shear bands. The goal of this work is to advance the understanding of deformation and failure mechanisms in Mg and its alloys and, in particular, discover pathways that trigger or delay plastic instabilities in them. The hypothesis examined here is that increased non-basal, c+a dislocation activities lead to more uniform deformation and provide more homogeneous strain hardening throughout the microstructure, hence delaying plastic instabilities by delocalizing shear bands in Mg and its alloys. The objectives of this work are to 1) identify the role of c+a dislocations on the formation of shear bands; and 2) identify the effect of temperature (up to 75 degrees Celsius) as well as alloying on c+a dislocation activities and shear band characteristics (e.g. size, number density, and stored plastic strain) in quasi-statically deformed Mg and Mg alloys. These objectives will be accomplished by experimentally measuring i) the stresses needed to operate dislocation sources, ii) the dislocation glide distances produced under given stresses, and iii) the related cross-slip frequency. These latter measures will then be directly correlated to the shear band characteristics mentioned in the second objective. The broader impacts of this project are two-fold. First, identifying pathways to more ductile and malleable Mg and Mg alloys will significantly enhance the opportunity and likelihood for Mg alloy usage across a variety of applications resulting in significant weight-savings and increases in energy efficiencies. Secondly, this project will facilitate increased educational engagement and outreach to students and persons within the collegiate environment and beyond through i) hands-on experiences for undergraduate students, ii) a digital platform to further undergraduate interest in materials science and iii) expansion of an existing YouTube channel engaging the general public on matters and phenomena related to materials science.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.