Collaborative Research: deformation mechanisms of fcc and hcp Cobalt with high-density stacking faults

非技术总结：薄膜形式的钴（Co）是一种重要的磁性材料，广泛应用于磁性数据存储设备，微机电和纳米机电系统（MEMS/NEMS），以及环保耐磨和耐腐蚀涂层。然而，无论是面心立方（fcc）还是六角形密排（hcp）形式的Co薄膜的力学性能都知之甚少。作者最近的研究表明，高密度层错（SFs）——破坏原子有序排列的原子平面——可以引入fcc和hcp Co中。这些SFs可能会大大提高Co的力学性能，从而提高Co的强度和延展性。该项目的目的是阐明SFs密度的影响，并系统地研究SFs对Co力学性能的影响。调查人员已有合作，他们的专业知识可以很好地相互补充。是次合作让学生有机会透过互访、讲座和研讨会，在实验和模拟方面获得互补的知识。研究人员还安排研究生参观能源部综合纳米技术中心，以使用先进的显微镜设备。从这个项目中获得的知识可以纳入这两个机构的课程。合作研究者可以利用休斯顿大学成功的外展项目来扩大工程领域的参与。项目负责人可在少数民族院校通过“博士衔接项目”招收一名少数民族研究生。两位研究者都不断地监督本科生，并鼓励他们的学生参加重大会议。技术概述：本项目的目的是研究高密度SFs在fcc和hcp Co中的变形机制。最终目的是了解SFs在控制金属力学性能，提高Co的强度和变形能力方面的意义。研究人员将实验和分子动力学模拟相结合，完成以下主要任务：(1)了解fcc Co中SFs的成核和拦截SFs的形成，并调整fcc和hcp Co中SFs的密度；2)通过原位纳米压痕和原子模型的结合，研究fcc Co的变形机制，包括位错- sf相互作用、尺寸效应和加工硬化；3)研究高密度SFs在hcp Co中的变形机制，了解hcp Co中变形孪核的成核机制，揭示SFs在金属力学行为中的重要作用。此外，新型纳米力学测试工具与分子动力学模拟相结合，通过在原子水平上对fcc和hcp Co与SFs的变形机制进行综合研究，填补了知识空白。

Nontechnical summary:Cobalt (Co), in the form of thin films, is a critical magnetic material with widespread applications in magnetic data storage devices, microelectromechanical and nanoelectromechanical systems (MEMS/NEMS), as well as environmentally benign wear and corrosion resistant coatings. Yet, the mechanical properties of Co films, either in face-centered-cubic (fcc) or hexagonal-close-packed (hcp) form are poorly understood. The principal investigator's recent studies show that high-density stacking faults (SFs) - atomic planes that disrupt the ordered arrangement of atoms - can be introduced into fcc and hcp Co. These SFs may drastically enhance mechanical properties leading to higher strength and ductility of Co. The aim of the project is to elucidate the effect of the density of SFs and systematically investigate the mechanical properties of Co with SFs. The investigators have existing collaborations and their expertise nicely complements each other. The collaboration provides students with the opportunity to gain complementary knowledge in experiments and simulations through mutual visits, lectures and seminars at the participating institutions. The investigators also have arrangement for graduate students to visit the Department of Energy - Center for Integrated Nanotechnologies to access advanced microscopy facilities. The knowledge derived from this project can be incorporated into curricula at both institutions. The co-investigator can leverage successful outreach programs at University of Houston to broaden participation in engineering. The principal investigator can recruit a minority graduate student through the "Pathway to Doctoral Program" from minority institutions. Both investigators continuously supervise undergraduate students and encourage their students to attend major conferences. Technical summary:The objective of this project is to investigate the deformation mechanisms in fcc and hcp Co with high-density SFs. The ultimate goal is to understand the significance of SFs in governing the mechanical properties of metals, and improving the strength and deformability of Co. The investigators combine experiments and molecular dynamics simulations to perform the following major tasks: (1) understand the nucleation of SFs and the formation of intercepted SFs in fcc Co, and tailor the density of SFs in fcc and hcp Co; 2) examine the deformation mechanisms in fcc Co, including dislocation-SF interactions, size effect and work hardening, via a combination of in situ nanoindentation and atomistic modeling; and 3) investigate the deformation mechanisms in hcp Co with high density SFs and understand nucleation mechanisms of deformation twins in hcp Co. This project could reveal the significant role of SFs in mechanical behavior of metals. Furthermore, the combination of novel nanomechanical testing tools with molecular dynamics simulations fills in the knowledge gap through comprehensive interrogation of the deformation mechanisms in fcc and hcp Co with SFs at the atomistic level.