Understanding the Fundamental Deformation Processes of BCC Refractory High Entropy Alloys using Experimentally-Validated Kinetic Monte Carlo Simulations

非技术概述：开发新材料是技术进步的核心，如手机、抗震结构或先进的卫星和空间探测器。一个迫切需要新材料的领域是基于标准蒸汽循环的能源生产。在那里，减少温室气体排放的一种方法是提高植物的热力效率。这可以通过开发能够承受比镍基高温合金目前设定的标准更高的工作温度的材料来实现。高熵合金就是这样一种材料，它有望将工作温度提高300~500度。高熵合金是由四种或四种以上化学元素(特别是过渡金属元素)等比例组成的，因此具有非常复杂的化学成分。这个项目的重点是利用原子尺度上最先进的计算和实验工具来了解这些合金在高温下的变形行为和强度。我们的目标是发现使这些系统在高温下坚固的机制，以便我们能够设计出更好的合金，并使用它们来取代发电厂中的现有材料，以提高它们的效率。为此，将有一批不同的学生和科学家参与进来，包括来自洛杉矶地区的女性、拉丁裔学生和退伍军人，这带来了纪律、专注和对高精度机械和计算机的熟悉。该项目将能够推进我们的目标，减少现有发电厂的碳足迹，并为其他应用做出贡献，如改进喷气式和火箭发动机以及更安全的核电站。技术概述：难熔高熵合金(RHEA)是一类由四种或四种以上等原子比例的难熔金属元素组成的材料。这些合金在很宽的温度范围内具有很高的强度和延展性，甚至有可能超过镍基高温合金，因此在高温应用中显示出巨大的前景。这些体系通常结晶为体心立方(BCC)合金，这表明它们的塑性响应是由螺位错的热激活运动控制的。然而，这些体系在高温下的高强度并不符合晶格电阻和固溶体硬化的标准理论。在本项目中，将建立螺位错滑移的动力学蒙特卡罗(KMC)模型。合金将被表示为一种有效的介质，其特征是所有合金元素的原子平均化，其中每个原子都被视为这种有效环境中的溶质。该项目将重点放在NbMoTaW系统作为代表性的RHEA。计算方法将通过单晶样品的原位透射电子显微镜纳米力学测试来验证，该测试将用于研究合金的温度、取向和应变率相关性。最终，根据这一建议开发的工具将有助于评估耐火高熵合金如何随着温度和应变率的变化而变形，目的是改善这些系统的高温行为，并评估它们提高发电厂效率的潜力。该提案包含一项让本科生和代表性不足的少数族裔学生参与研究活动的计划。此外，提案调查人员将接触攻读理工科本科学位的少数民族学生和退伍军人。这项建议的结果将用于加强加州大学洛杉矶分校的几门课程的内容，其中位错、强化机制和金属塑性是教学大纲的核心部分。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。

NON-TECHNICAL SUMMARY:Developing new materials is at the heart of technology advancements such as cellphones, earthquake-resistant structures, or advanced satellites and space probes. One area where new materials are sorely needed is that of energy generation based on standard steam cycles. There, one way to reduce greenhouse gas emissions is to increase the thermodynamic efficiency of the plants. This can be done by developing materials that can sustain higher operating temperatures than the current standard set by nickel-based superalloys. High-entropy alloys are one such class of materials that hold the promise of increasing the operating temperature by up to 300~500 degrees. High-entropy alloys are made up of four or more chemical elements (specifically transition metal elements) in equal proportions, and thus have a very complex chemistry. This project focuses on understanding the deformation behavior and strength of these alloys at high temperatures using the most advanced computational and experimental tools at the atomic scale. The goal is to discover the mechanisms that make these systems strong at high temperature so that we can design yet better alloys and use them to replace current materials in power generation plants to increase their efficiency. For this, a diverse group of students and scientists will be engaged, including women, latino students from the Los Angeles area, and military veterans, which bring discipline, focus, and familiarity with high-precision machinery and computers. This project will be able to advance our goals towards reducing the carbon footprint of existing power plants, and contribute to other applications such as improved jet and rocket engines and safer nuclear power plants. TECHNICAL SUMMARY:Refractory high entropy alloys (RHEA) are a class of materials consisting of four or more refractory metal elements in equiatomic proportions. These alloys show great promise for high temperature applications due to their high strength and ductility in a wide temperature range, potentially superior to even Ni-based superalloys. These systems usually crystallize in a body-centered cubic (bcc) alloy, which suggests that their plastic response is controlled by thermally activated motion of screw dislocations. However, the high strength of these systems at high temperature does not fit standard theories of lattice resistance and solid solution hardening. In this project, a kinetic Monte Carlo (kMC) model of screw dislocation glide will be developed. The alloy will be represented as an effective medium characterized by an atomic averaging of all the alloy elements, and where each atom then is treated as a solute in this effective environment. The project focuses on the NbMoTaW system as a representative RHEA. The computational approaches will be validated using in-situ transmission electron microscopy nanomechanical tests of single-crystal specimens, which will be used to study the temperature, orientation and strain rate dependence of the alloy. Ultimately, the tools developed under this proposal will be useful to assess how refractory high entropy alloys deform as a function of temperature and strain rate, with the goal of improving the high temperature behavior of these systems and evaluating their potential to increase the efficiency of power plants. The proposal contains a plan to involve both undergraduate students and students of underrepresented minorities in the research activities. As well, the proposal investigators will reach out to minority students and Armed Forces veterans pursuing undergraduate degrees in science and engineering. The results of this proposal will be used to enhance the content of several courses at UCLA where dislocations, strengthening mechanisms, and metals plasticity are a central part of the syllabus.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.