Solid Solution Enhanced Synthesis of Multi-Principal Component Alloys via Oxide Reduction

非技术概述多主成分合金(MPCA)是一种全新的合金类型，由四种或四种以上的金属元素组成。这些材料可以拥有新颖和令人兴奋的物理性能组合。特别是，这些合金中的一个子族，即所谓的耐火MPCA，具有超过目前使用的合金的高温机械性能。因此，将这些新合金应用于涡轮发动机可以提高效率并减少温室气体排放。目前用于制造耐火MPCA的方法使用极高的温度，并且可能导致合金中的成分变化。该计划旨在探索一种新的工艺，即通过还原金属氧化物的混合物来生产金属合金。这是通过在氢环境中进行热处理来实现的。这不仅是一项新技术，而且先前的工作表明，有可能转化某些通常非常稳定的氧化物。这项研究试图了解氧化物还原过程中这一意想不到的方面。提出了一种基本原理，即成分环境增强了还原作用。通过研究由不同元素组合组成的一系列MPCA的还原行为，对该模型进行了验证。计算机建模被用来帮助识别具有热力学特征的合金组合，这些组合在模型的基础上显示出希望。这些研究结果可能导致对定制的MPCA组合物进行更好的加工，可能在许多技术领域产生重大社会影响，包括能源储存和发电、运输和医疗设备。与合金设计和成形有关的外联活动侧重于鼓励代表性不足的少数群体在STEM领域寻求机会，并提高初中生和高中生对材料工程的认识。技术概述该计划的目的是开发一个科学框架，用于了解氧化物材料混合物的复杂还原行为，从而为加工具有重要技术意义的多主成分合金(MPCA)建立基于成分的策略。这项研究建立在先前工作的基础上，证明MPCA可以通过还原致密的金属-氧化物混合物来制造。用这种方法成功地合成了由Co、Cr、Fe、Ni和Mn组成的合金，其热处理条件相对温和，不会发生MnO的还原。提出了由于合金固溶体的形成而增强还原热力学驱动力的模型。混合焓的电子结构计算指导了元素组合的选择以进行测试。在控制温度和氧分压(PO2)的条件下，研究了这些氧化物的还原行为。通过使用扫描电子显微镜(SEM)和扫描透射电子显微镜(STEM)技术对部分还原样品进行表征，以及使用同步辐射光源进行原位X射线衍射还原研究，揭示了反应和物相演变的顺序。确定金属相的形核机制、过渡相的作用和速率控制传输机制具有广泛的基础意义，并有利于催化和氧环的相关研究。由于固溶体模型的开发将有助于确定对工艺至关重要的热处理和成分参数，因此MPCA组合物(包括块状和粉末)的强化加工得到了加速。这项研究还通过为选定的MPCA元素组合生成还原-组成图而使研究界受益。与合金设计和成形有关的推广活动包括致力于鼓励中学女生考虑科学和工程职业的选择(绘制工程和科学职业的视野和机会)，以及ASM教师和高中材料夏令营，旨在提高高中生对材料工程的认识。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 SummaryMulti-principal component alloys (MPCAs) are a radical new class of alloys that consists of approximately equal mixtures of four or more metallic elements. These materials can possess novel and exciting combinations of physical properties. In particular, a sub-group of these alloys, so-called refractory MPCAs exhibit high temperature mechanical properties that surpass those of the alloys currently in use. The application of these new alloys in turbine engines could therefore result in higher efficiency and reduced greenhouse emissions. Current methods used to fabricate refractory MPCAs utilize extremely high temperatures, and can result in compositional variations within the alloy. This program seeks to explore a new process whereby the metallic alloy is produced by reducing a mixture of the metallic oxides. This is achieved by heat-treatment in a hydrogen environment. Not only is this a novel technique, but prior work has shown that it is possible to convert certain oxides that normally would be extremely stable. The research seeks to understand this unexpected aspect of the oxide reduction process. A rationale is suggested whereby the reduction is enhanced by the compositional environment. This model is tested by studying the reduction behavior of a range of MPCAs composed of different elemental combinations. Computer modelling is used to help identify alloy combinations with thermodynamic characteristics that show promise based on the model. These research findings could lead to enhanced processing of tailored MPCA compositions, with potential for significant societal impact in many technological areas, including energy storage and generation, transportation, and medical devices. Outreach activities relating to alloy design and forming focus on encouraging under-represented minorities to pursue opportunities in STEM fields, and to increase awareness of materials engineering amongst middle- and high-school students. Technical SummaryThe aim of this program is to develop a scientific framework for understanding the complex reduction behavior of a mixture of oxide materials, and hence establish composition-based strategies for the processing of technologically important multi-principal component alloys (MPCAs). This research builds on prior work that demonstrated that MPCAs can be fabricated via the reduction of a compacted metal-oxide mixture. Alloys composed of Co, Cr, Fe, Ni and Mn have been successfully synthesized by this route, using relatively mild heat-treatment conditions for which the reduction of MnO would not be expected. A model is proposed whereby the thermodynamic driving force for reduction is enhanced due to the formation of the alloy solid solution. Electronic structure calculations of the enthalpy of mixing guide the selection of elemental combinations for testing. The reduction behavior of these oxide compositions are studied under controlled conditions of temperature and oxygen partial pressure (pO2). The sequence of reactions and phase evolution is revealed by the characterization of partially reduced samples using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) techniques, as well as in-situ x-ray diffraction reduction studies using a synchrotron light source. Identification of the nucleation mechanisms of the metallic phases, role of transient phases, and rate-controlling transport mechanisms is of broad fundamental interest, and benefit related research on catalysis and oxygen looping. The enhanced processing of tailored MPCA compositions, both bulk and powder, is expedited by the development of the solid solution model, since it will help identify the heat treatment and compositional parameters that are critical to the process. The research also benefits the research community by generating Reduction-Composition diagrams for select MPCA elemental combinations. Outreach activities relating to alloy design and forming include CHOICES (Charting Horizons and Opportunities In Careers in Engineering and Science), which is dedicated to encouraging middle-school girls to consider careers in science and engineering, and the ASM Teacher and High School Materials Camps, designed to increase awareness of materials engineering amongst high school students.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.