Entropy and Phase Transformations in Stable Nanocrystalline Alloys

纳米晶合金是由许多非常小的晶粒在无序的“晶界”上粘附在一起的新一代材料。这些材料具有各种特殊性能，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，这要归功于一个关键的可行概念：通过添加合金元素来稳定其晶界。传统的合金科学忽略了晶界的影响，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，这些材料需要新的合金科学，重点是整合晶界和合金添加物之间的相互作用。本项目研究纳米晶材料的合金科学，特别关注温度的影响和在高温下保持稳定性。该项目在原子尺度上使用计算机模拟来探索各种合金在纳米结构形式下的稳定性，并确定纳米晶体结构形成的成分和条件的限制。实验对这些新的理论进展进行了验证，并制作了第一批新型纳米晶合金的原型样品。这些发展有望为未来设计新的纳米晶合金家族提供一个预测性的科学工具包，用于从电子、机器部件到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。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。本研究项目的智力优势集中在解决纳米结构合金中熵的作用。具体来说，该项目正在开发一个完整的多晶晶晶界偏析的构型和振动熵视图，通过原子计算，可以在具有完整晶界位的多晶晶光谱的系统中分离这两种贡献。这些信息反过来使我们能够全面分析体相和纳米结构态之间的自由能竞争，包括边界结构转变（或肤色转变）和体中的同素异形体相变等细节。该项目的首要目标是实现足够定量的热力学计算，从而能够预测具有平衡和亚稳纳米晶体结构的合金相图，然后为这些预测提供实验测试。该方案的广泛影响包括各种培训、外联和传播活动。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）和纳米科学新主题（纳米结构稳定）交叉的主题上进行培训。研究结果发表在公开文献中，并通过PI的外联活动向工业界广泛传播。特别是，工业互动用于指导研究工作，以实现实际目的的相关材料和感兴趣的温度，促进未来技术转让。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 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.