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

非技术摘要本项目将研究一类新的多组分合金的低周疲劳行为，这些合金被称为高熵合金。近五年来，这些合金以其独特的力学性能引起了人们的极大关注。本工作的转化潜力表现为对结构表征实验的系统和创新性的研究，从而揭示了循环载荷下的变形机理，集成了设计、制造、验证、改进和预测组件，可用于未来其他先进材料的研究。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、少数群体参与和努力让公众参与。该项目的成果将通过科学界和公众可利用的途径传播，重点是初中生和高中生以及妇女和少数群体学生。研究结果的适当方面将被纳入首席调查员(PI)的研究生和本科课程材料中，以向学生介绍现代跨学科材料研究。本项目的目标是(1)通过改变结构(如控制Al含量)来研究高熵合金(HEAs)的低周疲劳行为，(2)利用最先进的表征方法(如先进的显微镜和原位中子衍射)阐明HEAs在低周疲劳过程中的变形机制，以及(3)基于对本工作所获得的变形行为的基本了解，设计和开发具有优异低周疲劳性能的新型HEAs。近五年来，HEAS以其独特而优异的力学性能引起了人们的极大关注。尽管已有广泛的研究致力于力学行为，但这些研究大多是针对单调试验，几乎没有人关注疲劳性能，特别是低周疲劳性能。在HEA真正应用于实际应用之前，必须仔细研究疲劳行为的机理。因此，关键问题变得显而易见：(1)多主元如何影响结构，进而影响疲劳性能；(2)高熵构型如何影响变形机制；以及(3)如果以上两个方面对疲劳抗力有积极影响，那么根本的贡献因素是什么。因此，本文提出并验证了一种假设，即具有某些特定成分(即组织)的HEAs将表现出比传统合金更好的疲劳抗力，这是因为：(1)形成孪晶的倾向大，(2)溶质原子和从元素尺寸失配到针位错的大变形，(3)位错与孪晶之间的相互作用，在疲劳裂纹萌生之前以及在室温和高温下裂纹扩展过程中，它们的长期性能可以通过冷或热处理(例如，控制晶粒度的冷轧)来调节和进一步提高。通过实验、理论和模型研究的结合，有望揭示低周疲劳行为的机理，从而对单相和多相HEA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.