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

非技术总结：开发新材料是手机、抗震结构或先进卫星和太空探测器等技术进步的核心。急需新材料的一个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电厂的热力学效率。这可以通过开发能够承受比目前镍基高温合金设定的标准更高工作温度的材料来实现。高熵合金就是这样一类材料，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的化学元素（特别是过渡金属元素）按等比例组成的，因此具有非常复杂的化学性质。本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，这样我们就可以设计出更好的合金，并用它们来取代发电厂现有的材料，以提高效率。为此，将有一群不同的学生和科学家参与其中，包括女性、来自洛杉矶地区的拉丁裔学生和退伍军人，他们带来了纪律、专注和对高精度机械和计算机的熟悉。这个项目将能够推进我们的目标，减少现有发电厂的碳足迹，并有助于其他应用，如改进喷气发动机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难熔金属元素以等原子比例组成的材料。由于这些合金在宽温度范围内具有高强度和延展性，因此它们在高温应用中表现出很大的前景，甚至可能优于镍基高温合金。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。本课题将建立螺旋位错滑动的动力学蒙特卡罗模型。合金将被表示为一种有效介质，其特征是所有合金元素的原子平均，其中每个原子在这种有效环境中被视为溶质。该项目重点关注作为代表性RHEA的NbMoTaW系统。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进行验证，该测试将用于研究合金的温度，取向和应变速率依赖性。最终，根据本提案开发的工具将有助于评估难熔高熵合金如何变形为温度和应变速率的函数，以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。该提案包含一项计划，让本科生和代表性不足的少数民族学生参与研究活动。此外，提案调查人员还将接触少数族裔学生和攻读科学和工程本科学位的武装部队退伍军人。这项提案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。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.