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

非技术综述：钴(Co)以薄膜的形式存在，是一种重要的磁性材料，在磁数据存储设备、微电子机械和纳米机电系统(MEMS/NEMS)以及环境友好的耐磨和耐腐蚀涂层中有着广泛的应用。然而，对于面心立方(FCC)或六方密堆积(HCP)形式的Co薄膜的力学性能还知之甚少。主要研究人员最近的研究表明，高密度层错(SFS)--破坏原子有序排列的原子平面--可以被引入到FCC和HCP Co中。这些SFS可以显著地提高Co的力学性能，从而获得更高的强度和塑性。本项目的目的是阐明SFS密度的影响，并用SFS系统地研究Co的力学性能。调查人员有现有的合作，他们的专业知识很好地相互补充。合作为学生提供了机会，通过在参与机构的相互访问、讲座和研讨会获得实验和模拟方面的互补知识。研究人员还安排研究生参观能源部-集成纳米技术中心，以访问先进的显微镜设备。从该项目获得的知识可纳入两个机构的课程。这位合作研究员可以利用休斯顿大学成功的外展项目来扩大对工程学的参与。首席研究员可以通过“博士之路”从少数民族院校招收一名少数民族研究生。这两名调查人员都不断地监督本科生，并鼓励他们的学生参加重大会议。技术综述：本项目的目标是研究FCC和HCP Co在高密度SFS中的变形机制。研究人员将实验和分子动力学模拟相结合，完成了以下主要工作：(1)了解面心立方Co中SFS的成核和截留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拟相结合的方法，研究FCC Co中的形变机制，包括位错-SF相互作用、尺寸效应和加工硬化；3)研究高密度自蔓延高温合金的变形机制，了解变形孪晶的形核机制，揭示高密度自蔓延高温合金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的结合填补了这一知识空白，在原子水平上全面询问了面心立方和六方钴的变形机制。

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.