High-entropy Alloys Strengthened by Coherent Precipitates

未来的能源必须是可持续的，污染物最少，碳足迹减少，生产效率更高。为了实现高效的能量转换，热机的工作温度应尽可能高，并且用于发动机部件的材料必须能够承受高工作温度。传统的合金设计是根据特定的性能要求选择主要成分，然后在不牺牲主要性能的情况下使用合金添加物来赋予次要性能。与传统合金不同的是，高熵合金是一种多组分材料，它含有至少五种成分，其原子比例近似相等。传统合金主要由一种主要成分组成，除了少量的合金添加。最近的实验表明，这些合金有望成为高温材料。这一发现为高温合金的发展开辟了一条新的途径。目前的项目有望推动田纳西大学在先进材料领域的研究和工程教育活动。它还纳入了几个项目，以招募和加强妇女和少数民族对科学和工程项目的参与。项目包括全国少数民族研究生教育联盟和德州大学工程学院的管道工程多样性项目，以指导本科生并培养他们对研究的兴趣。此外，首席研究员将参与到当地高中的推广活动，以提高学生的科学和技术意识。技术概述高熵合金是一类新型材料，它通常含有5种以上的成分，其原子比例近似相等，而传统合金主要是由一种主要成分与一些额外的次要元素组成。由于具有较高的组态熵，这些新合金通常在单面心立方相或体心立方相中表现出显著且令人惊讶的相互溶解度。此外，由于几种不同原子的分配，这些合金通常表现出缓慢的扩散，因此是有前途的高温蠕变材料。据报道，这些合金中的一些在时效后可硬化，并表现出显著的强化，类似于传统的铝、钢和镍基合金。该小组最近的实验表明，在一些高熵合金中形成了大体积分数的晶格错配应变为0.58%的相干纳米沉淀，其方式类似于在Ni高温合金中观察到的（错配应变~0.5%）。基于这些有希望的观察结果，该项目旨在发展对高熵合金中沉淀相干纳米颗粒的理解，并由此提高这种新型材料的抗变形能力，特别是在高温下。研究任务包括沉淀动力学的基础研究，决定纳米沉淀物热稳定性的因素，以及位错与纳米沉淀物之间物理/化学相互作用的基础机制。具体来说，该项目将通过有意控制析出相-基体界面能来研究和改善高熵合金中析出相的热稳定性。利用高分辨率电子显微镜、原子探针和高能x射线表征纳米沉淀物的组成、形态和分布，特别关注纳米沉淀物-基质界面的结构和化学性质。确定决定界面热稳定性的因素。在室温和高温下，研究了变形过程中位错和纳米沉淀物之间的力学相互作用，并评估了强化效果。对L12析出物增强面心立方高熵合金（如cocrfemnni基）进行初步研究，并最终扩展到体心立方合金体系。在理论方面，将对简单二元甚至三级体系的界面溶质偏析及其对界面能的影响进行初步模拟。计算将扩大到更复杂的合金系统，只有在实际的工具是可用的。该项目的完成将为结构纳米材料的发展提供有用的技术信息。对沉淀硬化高熵合金的评估也可以帮助设计和改进高温结构部件或功能器件的应用，例如微电子互连的扩散屏障。此外，对高熵合金的研究将激发对多组分系统的研究和理解的科学兴趣，特别是在相图中心附近，这实际上是未被探索的。

Non-Technical SummaryFuture energy must be sustainable, with minimum pollutants and reduced carbon footprint, and more efficiently generated. For efficient energy conversion, the operating temperature of a heat engine should be as high as possible and materials used for the engine components must be able to withstand the high operating temperature. Traditional design of alloys is to select the major component based on a specific property requirement, and further to use alloying additions to confer secondary properties without sacrificing the primary property. High-entropy alloys are multi-components materials containing at least five components in approximately equiatomic proportions, in contrast to traditional alloys, which are primarily based on one major component except with some minor alloying additions. Recent experiments show that these alloys can be promising as high-temperature materials. This discovery opens a new avenue for the development of high-temperature alloys. The current project is expected to advance the research and engineering education activities in the area of advanced materials at the University of Tennessee. It also incorporates several programs to recruit and enhance women and minority participation in science and engineering projects. Programs include National Consortium for Graduate Education for the Minorities and UT College of Engineering's Pipeline Engineering Diversity Program, to mentor undergraduate students and foster their interests in the research. In addition, the the principal investigator will participate in outreach to local high schools to promote student awareness of science and technologies.Technical SummaryHigh-entropy alloys are a new class of materials, which contain typically over 5 components in approximately equiatomic proportions, in contrast to traditional alloys, which are primarily based on one major component alloyed with some additional minor elements. As a result of high configuration entropy, these new alloys generally exhibit significant and surprising degree of mutual solubility in a single face-centered cubic or body-centered cubic phase. Also due to partitioning of several different atoms, these alloys generally show sluggish diffusion, thus are promising materials for high-temperature creep resistance. Some of these alloys were reported to be hardenable upon aging and exhibited significant strengthening, similar to conventional Al, steels, and Ni based alloys. Recent experiments from this group demonstrated that large volume fraction of coherent nanoprecipitates with a lattice mismatch strain as small as 0.58% were formed in some high-entropy alloys, in a fashion similar to that observed in Ni superalloys (mismatch strain ~0.5%). Based upon these promising observations, this project aims at developing an understanding of precipitating coherent nanoparticles in high-entropy alloys and also the resulting improvement on deformation resistance, in particular, at elevated temperatures, in this new class of materials. The research includes tasks of a basic study of the precipitation kinetics, factors determining the thermal stability of nanoprecipitates, and the underpinned mechanisms of the physical/chemical interactions between dislocations and nanoprecipitates. Specifically, the project will study and improve the thermal stability of precipitates in high-entropy alloys through deliberate control of the precipitate-matrix interface energy. High-resolution electron microscopy, atom probe and high-energy x-ray will be employed to characterize the composition, morphology and distribution of nanoprecipitates with special attention on the structure and chemistry of the nanoprecipitate-matrix interface. Factors determining the interface thermal stability will be identified. Mechanical interactions between dislocations and nanoprecipitates during deformation at both room and high temperatures will be examined and the strengthening efficacy will be evaluated. The initial study of L12 precipitate-strengthened face-centered cubic high-entropy alloys, such as the CoCrFeMnNi-based, will be conducted and, eventually, extended to body-centered cubic alloy systems. On the theory side, preliminary simulations will be conducted on interface solute segregation and its effect on the interface energy in simple binary or even tertiary systems. Calculations will be expanded to more complex alloy systems only after practical tools are available. The completion of this project will offer useful technical information to the development of structural nanomaterials. The evaluation of precipitation-hardened high-entropy alloys can also aid in the design and improved application of high-temperature structural components or functional devices, for example, diffusion barriers for microelectronic interconnects. Moreover, research on high-entropy alloys will stimulate the scientific interest on the study and understanding of multicomponent systems, in particular, near the center of the phase diagram, which is virtually unexplored.