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

本项目将研究一类新的多组分合金——高熵合金的低周疲劳行为。这些合金以其独特的力学性能在近五年来引起了人们的广泛关注。本研究的变革潜力在于对结构表征实验进行了系统和创新的研究，从而揭示了循环载荷下的变形机制，并将设计、制造、验证、改进和预测等组成部分整合在一起，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。拓展活动将包括K-12科学教育、少数民族参与以及公众参与的努力。该项目的成果将通过对科学界和一般公众开放的途径进行传播，重点是初高中学生以及妇女和少数民族学生。研究成果的适当方面将被纳入首席研究员（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)通过改变结构（如控制Al含量）研究高熵合金（HEAs）的低周疲劳（LCF）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）阐明高熵合金（HEAs）在低周疲劳（LCF）过程中的变形机制，(3)基于从本工作中获得的变形行为的基本认识。设计和开发具有优异LCF性能的创新型HEAs。近五年来，HEAs以其独特而优异的力学性能备受关注。尽管对其力学性能进行了大量的研究，但这些研究活动大多是单调试验，几乎没有对疲劳性能，特别是LCF进行关注。在实际应用中引入HEAs之前，必须仔细研究其疲劳行为机制。因此，关键问题变得很明显：(1)多主元素如何影响结构，进而影响疲劳性能；(2)高熵形态对变形机制的影响；(3)如果以上两个方面对抗疲劳性能有积极的影响，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验证，即具有某些特定成分（即结构）的HEAs将比传统合金表现出更好的抗疲劳性，这是由于(1)形成孪晶的极大倾向，(2)溶质原子和由元素尺寸不匹配引起的大畸变，以及(3)位错和孪晶之间的相互作用，在疲劳裂纹开始之前和在室温和高温下裂纹扩展期间。它们的长期性能可以通过冷轧或热处理（例如，冷轧以控制晶粒尺寸）来调节和进一步改善。期望通过实验、理论和建模的结合，揭示LCF行为的机制，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.