Fundamental Study of Low-Cycle-Fatigue Behavior of High-Entropy Alloys

非技术摘要本计画将研究一种称为高熵合金的新型多元合金的低周疲劳行为。近五年来，这类合金以其独特的力学性能引起了人们的极大关注。本工作的变革潜力是作为一个系统的和创新的结构表征实验研究，从而揭示在循环载荷下的变形机制和集成的设计，制造，验证，改进和预测组件，这可以应用于其他先进材料的研究在未来。 参与该项目的学生和研究人员将有机会在国家实验室体验最先进的研究设备。外展活动将包括K-12科学教育，少数民族的参与，并努力使公众参与。该项目的成果将通过科学界和公众可以利用的渠道传播，重点是初中和高中学生以及妇女和少数民族学生。研究结果的适当方面将被纳入主要研究者（PI）的研究生和本科课程材料，向学生介绍现代跨学科材料研究。 本项目的目标是（1）通过改变高熵合金（HEAs）的结构（例如，控制Al含量），（2）使用现有技术的表征方法（例如，先进的显微镜和原位中子衍射），和（3）从本工作中获得的变形行为的基本理解的基础上，设计和开发创新的HEAs具有优异的低循环性能。高性能混凝土由于其独特的力学性能，在近五年来引起了人们的极大关注.尽管人们对材料的力学行为进行了大量的研究，但这些研究大多是针对单调试验的，几乎没有针对疲劳性能，特别是低周疲劳性能的研究。疲劳行为的机制，必须仔细检查之前，HEAs确实可以在实际应用中引入。因此，关键问题变得明显：（1）多主元素如何影响结构，进而影响疲劳性能;（2）高熵配置如何影响变形机制;以及（3）如果上述两个方面对疲劳抗力有积极影响，那么根本的影响因素是什么。因此，提出了一个假设，并在本工作中进行了测试，即具有一些特定组成的HEAs（即，结构）将显示出优于传统合金的上级抗疲劳性，这是由于（1）形成孪晶的大趋势，（2）溶质原子和来自元素尺寸失配的大变形到钉扎位错，和（3）位错和孪晶之间的相互作用，在室温和高温下疲劳裂纹萌生之前和裂纹扩展期间，并且它们的长期性能可以通过冷或热处理来调节和进一步改善（例如，冷轧以控制晶粒尺寸）。预计LCF行为的机制可以通过所提出的实验，理论和建模工作的组合来揭示，从而为单相和多相HEAs的变形行为提供基本的理解。

Non-Technical AbstractThis project will study low-cycle-fatigue behavior of a new class of multi-component alloys called high-entropy alloys. These alloys have attracted huge attention in recent five years for their unique mechanical properties. The transformative potential of the present work is represented as a systematic and innovative investigation for structural characterization experiments, thus revealing the deformation mechanisms under cyclic loading and integrating the design, fabrication, verification, improvement, and prediction components, which can be applied for the studies of other advanced materials in the future. Students and researchers involved in this project will have opportunities to experience the state-of-the-art research equipment at the national laboratories. The outreach activities will include the K-12 science education, minority involvement, and efforts to engage the public. The results of the project will be disseminated through avenues accessible to the scientific community and to the general public with an emphasis on middle- and high-school students, as well as women and minority students. Appropriate aspects of the research results will be incorporated into the principal investigator's (PI's) graduate and undergraduate course materials to introduce students to modern interdisciplinary materials research. Technical AbstractThe goal of this project is to (1) study the low-cycle fatigue (LCF) behavior of high-entropy alloys (HEAs) by varying the structures (e.g., controlling the Al content), (2) clarify the deformation mechanisms of HEAs during LCF using state-of-art characterization methods (e.g., advanced microscopy and in-situ neutron diffraction), and (3) based on the fundamental understanding of the deformation behavior obtained from the present work, design and develop innovative HEAs with excellent LCF properties. HEAs attract huge attention in recent five years for their unique and excellent mechanical properties. Even though extensive studies have been devoted to the mechanical behavior,most of these research activities are for monotonic tests and almost none has focused on the fatigue properties, especially LCF. The mechanism of fatigue behavior must be examined carefully before HEAs can indeed be introduced in practical applications. The critical issues, thus, become obvious: (1) how do the multi-principal elements affect structures, and further, fatigue properties; (2) how does the high-entropy configuration influence the deformation mechanism; and (3) if the above two aspects have positive effects on the fatigue resistance, what are the fundamental contributing factors. Accordingly, a hypothesis is proposed and to be tested in the present work that HEAs with some specific compositions (i.e., structures) will show superior fatigue resistance over traditional alloys due to (1) great tendency to form twins, (2) solute atoms and large distortion from the element-size mismatch to pin dislocations, and (3) the interaction between dislocation and twinning, before fatigue-crack initiation and during crack propagation at room and elevated temperatures, and their long-term performance can be modulated and further improved by cold or hot treatment (e.g., cold rolling to control grain size). It is expected that the mechanism of LCF behavior could be revealed by a combination of the proposed experimental, theoretical, and modeling efforts, thus providing the fundamental understanding of the deformation behavior for single- and multiple-phase HEAs.