Effects of Point Defects on Dislocation Nucleation in Metals

技术综述强化材料的传统方法往往侧重于通过减少位错运动来提高塑性变形的抗力。在许多快速发展的表现出增强强度的金属材料中，例如多层金属和超细晶金属，限制因素可能是位错的产生和增殖，而不是位错的运动。尽管世界各地的许多研究小组都在这方面做了大量的工作，但大多数研究都集中在原始的或极度受控的结构上，而大多数工程合金是具有普遍缺陷结构的复杂的、多组分的系统。暴露于辐射和现有的非平衡加工特别可能产生多余的空位和其他点缺陷。该项目将利用实验和计算研究为金属中具有大范围缺陷结构的源位错的形核和操作提供基础研究。该团队将使用压痕技术在各种金属系统中实验量化空位、杂质和晶界在极端应力下对位错成核和塑性的影响，以探索塑性的开始。点缺陷浓度将通过正电子孔隙率测量仪进行评估。纳米压痕研究也将被用来展示现有缺陷在高应力下对纳米级位错传播的影响，这些结果将与使用分子静力学和动力学的计算模拟进行比较。这种耦合将通过开发处理长度和时间尺度的多尺度建模技术，在早期塑性的计算模型和实验研究之间建立更紧密的关系。在工程师和科学家用来增强金属材料的许多方法中，越来越多的人强调开发纳米尺度的结构，这种结构表现出-越小越强-范例，其中在材料中具有较小的长度尺度提供了更多的变形阻力。这个项目将集中于确定金属中存在的缺陷对塑性开始的根本影响。将有一个实验部分，在其中测量各种金属材料的塑性开始，以量化材料的极限强度。这些结果将与为解决永久变形开始建模中的时间和长度尺度问题而开发的计算模拟进行比较。支持的研究生将与一群材料科学和工程本科生合作，这些本科生为初中学生开发了一套材料外展材料，并将获得组织工程学学生团队和向该地区数十个服务不足的教室分发材料包的经验。

TECHNICAL SUMMARYTraditional methods of strengthening materials often focus on increasing the resistance to plastic deformation by decreasing dislocation mobility. In many of the rapidly developing metallic materials that exhibit enhanced strengths, such as multilayered metals and ultrafine grained metals, the limiting factor is likely the creation and multiplication of dislocations, rather than the motion of dislocations. While there is substantial work on this topic by many groups around the world, most studies focus on pristine or extremely controlled structures, whereas most engineering alloys are complex, multicomponent systems that have prevalent defect structures. Exposure to radiation and existing non-equilibrium processing is particularly likely to generate excess vacancies and other point defects. This project will provide a fundamental study of the nucleation of and operation of sources dislocations with a wide range defect structures in metals using both experimental and computational studies. The team will experimentally quantify the impact of vacancies, impurities, and grain boundaries on dislocation nucleation and plasticity at extreme stresses in a variety of metallic systems using indentation techniques to probe the onset of plasticity. Point defect concentration will be assessed by positron porosimetry. Nanoindentation studies will also be used to demonstrate the impact of existing defects on the propagation of dislocations at the nm scale at high stresses, and these results will be compared to computational simulations using both molecular statics and dynamics. This coupling will develop stronger relationships between computational models of incipient plasticity and experimental studies through the development of multi-scale modeling techniques addressing both length and time scales.NON-TECHNICAL SUMMARYOf the many methods used by engineers and scientists to strengthen metallic materials there is increased emphasis on developing nanoscale structures that exhibit the -smaller is stronger- paradigm, where having a smaller length scale in the material provides more resistance to deformation. This project will focus on determining the fundamental effects of existing defects in metals on the onset of plasticity. There will be an experimental component, wherein the onset of plasticity is measured in a variety of metallic materials to quantify the ultimate strength of the material. These results will be compared to computational simulations developed to address both time and length scale issues in modeling the onset of permanent deformation. The graduate students supported will be partnered with a group of materials science and engineering undergraduates that have developed an outreach kit of materials for junior high students, and will gain experience in organizing teams of engineering students and distributing the kits to dozens of underserved classrooms around the region.