Vacancy Engineering for Enhanced Strength and Toughness of Metals

1.强度（承受机械载荷的能力）和韧性（抵抗断裂的能力）在民用和交通基础设施、制造和加工以及生物医学植入物中是期望的，以使它们更坚固、更安全、更轻和更节能。不幸的是，金属的强度和韧性是相互排斥的-这是一个有待解决的根本挑战，因为控制金属强度的机制与韧性是反向耦合的。该研究项目旨在引入一种新型机制，其中原子尺度的空位与现有缺陷相互作用，以诱导强度和韧性的解耦。这种空位-缺陷相互作用还没有得到很好的理解，因为在常规金属中，空位不起主导作用。此外，这些机制受到金属内部结构和现有缺陷分布以及温度的强烈影响。实验将在高分辨率显微镜内进行，因此可以在控制和测量强度和韧性的同时真实的看到不同变形机制的作用。这项研究的科学成果将是原子尺度的空位-缺陷相互作用的新知识，这将通过既坚固又坚韧的金属的技术创新来证明。研究生和本科生将接受冶金，力学，纳米纤维和显微镜等交叉学科的培训，以解决这一高度耦合的研究问题。学术推广活动将进行，以吸引下一代劳动力对材料科学工程。 2.金属的强度来源于其抑制位错运动或塑性变形的能力.另一方面，韧性在断裂之前需要大量的塑性功。由于这种冲突，它们是相互排斥的。目前的模式是优化，或者换句话说，在强度和韧性之间妥协。首先，本研究旨在通过协同多尺度缺陷相互作用（0 D：空位，1D：位错和2D：晶粒/孪晶界）来最大化强度和韧性。在这里，纳米晶体金属中的位错限制被利用来实现高强度。同时，在环境条件下产生点缺陷（空位），这有利于基于扩散的塑性，而不会解锁位错。最终结果是在晶粒尺寸没有任何变化的情况下，抑制位错活动和显著的扩散塑性的独特共存。这项研究是基于这样的假设，即它是可能的，同时在同一晶粒中的位错为基础的强度和扩散为基础的变形能力在同一时间和在环境温度。提出了一种独特的材料处理技术与温度-电流-应力协同作用，以产生所需的空位浓度。这导致了一个变革性的概念;缺陷的三个维度之间的三向相互作用，以最大限度地提高强度和韧性。纳米级的晶界将通过阻碍位错运动而导致非常高的强度。小晶粒引起的延展性的缺乏通过晶界中的扩散塑性被空位消除。最后，将研究所提出的概念在中观和宏观制造过程中的可扩展性在真实的生活应用中的可行性。

1. Nontechnical Abstract Strength (ability to bear mechanical load) and toughness (ability to resist fracture) are desired in civil and transportation infrastructure, manufacturing and processing, and bio-medical implants to make them stronger, safer, and lighter and energy efficient. Unfortunately, strength and toughness in metals are mutually exclusive - a fundamental challenge that remains to be resolved because the mechanism that controls strength in metals is inversely coupled with toughness. This research project aims to introduce a new type of mechanism, where atomic scale vacancies interact with existing defects to induce decoupling of strength and toughness. Such vacancy-defect interaction is not well understood because in conventional metals, vacancies do not play dominant roles. In addition, these mechanisms are strongly influenced by the internal structure and existing defect distribution in the metal as well as temperature. The experiments will be performed inside high-resolution microscopes, so the role of different deformation mechanisms can be seen in real time as strength and toughness are controlled and measured. The scientific outcome of this research will be new knowledge in atomic scale vacancy - defect interactions, which will be demonstrated through technological innovation in metals that are both strong and tough. Graduate and undergraduate students will be trained in crosscutting disciplines of metallurgy, mechanics, nanofabrication and microscopy to solve this highly coupled research problem. Academic outreach activities will be performed to attract the next generation workforce towards materials science & engineering. 2. Technical AbstractStrength of metals originates from their capability of suppressing dislocation motion or plastic deformation. Toughness, on the other hand, requires large amount of plastic work before fracture. Because of such conflict, they are mutually exclusive. The current paradigm is to optimize, or in other words, compromise between strength and toughness. Contrarily, this research aims to maximize strength and toughness through synergistic multi-scale defect interactions (0D: vacancy, 1D: dislocations and 2D: grain/twin boundaries). Here, dislocation confinement in nano-crystalline metals is exploited to achieve high strength. At the same time, point defects (vacancies) are generated at ambient conditions, which facilitate diffusion-based plasticity without unlocking the dislocations. The net result is the unique co-existence of suppressed dislocation activities and pronounced diffusional plasticity without any change in grain size. This research is based on the hypothesis that it is possible to have both dislocation-based strength and diffusion-based deformability in the same grain at the same time and at ambient temperature. A unique material processing technique with temperature-current-stress synergy is proposed to generate the required vacancy concentration. This leads to a transformative concept; a three-way interaction among three dimensions of defects to maximize both strength and toughness. The nanoscale grain boundaries will lead to very high strength by impeding dislocation motion. The lack of ductility arising from small grains is removed by vacancies through diffusional plasticity in the grain boundaries. Finally, scalability of the proposed concept in meso and macroscopic manufacturing processing will be studied for feasibility in real life applications.