Entropy and Phase Transformations in Stable Nanocrystalline Alloys

非技术概述纳米晶合金是由许多非常细小的晶粒组成的下一代材料，它们在无序的“晶界”粘在一起。这些材料具有各种特殊的性能，如极高的强度和耐磨性。它们也开始从实验室的好奇心转变为广泛采用的工程材料，这要归功于一个至关重要的概念：通过添加合金元素来稳定其晶界。传统的合金科学忽视了晶界的影响，但在纳米晶金属中，晶界的影响是不容忽视的，因为它们是如此突出。因此，对于这些材料，需要新的合金科学，重点是整合晶界和合金添加之间的相互作用。该项目研究纳米晶体材料的合金科学，特别关注温度的影响和在高温下保持稳定性。该项目使用原子尺度的计算机模拟来探索纳米结构形式的各种合金的稳定性，并确定形成纳米晶体结构的成分和条件的限制。对这些新的理论进展进行了实验验证，并制作了新的纳米晶合金的第一个原型样品。预计这些发展将产生一种预测性科学工具包，用于未来设计新的纳米晶合金家族，用于从电子产品到机械部件再到3D打印的广泛应用。这个项目的研究是由麻省理工学院的本科生、研究生和博士后进行的，作为他们材料科学培训的一部分。该项目团队还与行业科学家合作，将工作重点放在相关材料和应用上，并通过未来的技术转让调整研究的影响。技术总结这一研究计划的智力优势集中在解决纳米结构合金中的熵作用上。具体地说，该项目正在通过原子计算开发多晶体中晶界偏析的构型和振动熵的全景图，该计算可以在一个具有完整的晶界位置的多晶谱的系统中将这两个贡献分开。这些信息反过来又使我们能够全面分析体相和纳米结构状态之间的自由能竞争，包括体相中的边界结构转变(或络合转变)和同素各向同性相变等细节。该项目的首要目标是实现足够定量的热力学计算，以便能够预测具有平衡和亚稳定纳米晶结构的合金相图，然后提供这些预测的实验测试。该方案的更广泛影响包括各种培训、外展和传播活动。本科生、研究生和博士后研究人员在经典材料科学核心概念(合金热力学、相平衡)和纳米科学(纳米结构稳定性)的新主题的交叉点上接受培训。研究结果发表在公开文献上，并通过国际和平协会的外联活动向工业界广泛传播。特别是，工业互动被用于指导相关材料和感兴趣的温度的研究工作，以达到实际目的，促进未来的技术转让。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 SummaryNanocrystalline alloys are next-generation materials that are composed of many very small crystal grains, adhered together at disordered "grain boundaries". These materials have a variety of exceptional properties like extreme strength and wear resistance. They are also beginning to transition from laboratory curiosities to widely-adopted engineering materials thanks to a critically enabling concept: the stabilization of their grain boundaries by adding alloying elements. Traditional alloy science ignores the effect of grain boundaries, but these cannot be ignored in nanocrystalline metals where they are so prominent. Therefore, new alloy science is needed for these materials, with a focus on integrating the interactions between grain boundaries and alloying additions. This project studies the alloy science of nanocrystalline materials, with a special focus on the effects of temperature and maintaining stability at high temperatures. The project uses computer simulations at the atomic scale to explore the stability of various alloys in nanostructured form, and identifies limits to the compositions and conditions under which nanocrystalline structures can be formed. Experiments are conducted to make and test these new theoretical advances, and to produce the first prototype samples of new nanocrystalline alloys. These developments are expected to lead to a predictive scientific toolkit for the future design of new families of nanocrystalline alloys, for use in a wide array of applications ranging from electronics, to machine components, to 3D printing. The research in this project is carried out by undergraduate, graduate, and postdoctoral students at MIT as part of their training in materials science. The project team also engages with industry scientists to focus the work on relevant materials and applications, and to align the research for impact through future technology transfer.Technical SummaryThe intellectual merits of this research program center on resolving the role of entropy in nanostructured alloys. Specifically, the project is developing a full view of configurational and vibrational entropy on grain boundary segregation in polycrystals, through atomistic computations that can separate these two contributions in a system with a full polycrystalline spectrum of grain boundary sites. This information in turn enables a full analysis of the free energy competition between bulk phases and nanostructured states, including details like boundary structural transitions (or complexion transitions) and allotropic phase transformations in the bulk. The overarching goal of the project is to achieve sufficiently quantitative thermodynamic calculations to be able to predict alloy phase diagrams complete with equilibrium and metastable nanocrystalline structures, and then to provide experimental tests of those predictions. The broader impacts of this program comprise a variety of training, outreach and dissemination activities. Undergraduate, graduate, and postdoctoral researchers are trained on topics at the intersection of classical materials science core concepts (alloy thermodynamics, phase equilibria) and new topics in nanoscience (nanostructure stabilization). The research results are published in the open literature and also disseminated widely to industry through the outreach activities of the PI. In particular, industrial interactions are used to guide the research efforts towards relevant materials and temperatures of interest for practical purposes, facilitating future technology transfer.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.