Materials World Network/Research in Undergraduate Institutions: The True Three-Dimensional Nature of Aligned Dislocation Boundaries in Deformed Metals

该项目是由Harvey Mudd学院（HMC）与澳大利亚新南威尔士大学（UNSW）材料科学与工程学院和电子显微镜部门合作完成的。这也是一项本科院校研究活动，因为该奖项支持HMC本科生进行国际材料科学研究。该项目致力于解决一个长期存在的争议，即在轧制具有中高层错能的变形面心立方和体心立方金属时产生的排列位错边界阵列的性质。由于它们在决定屈服应力和应变硬化的各向异性方面的作用，这些位错边界的结构和起源引起了人们的极大兴趣。深入了解这些微观结构的真实特征对于提高基于物理模型的宏观力学性能的预测能力至关重要。关于形变多晶体中这些排列边界的结构和起源，有两种相反的理论：(i)它们沿着某些晶体平面取向，或者（ii）它们的排列主要由塑性变形期间的宏观应力状态决定。目前支持这些理论的证据是基于二维数据，不一定能揭示变形微观结构的真实性质，并可能导致错误的解释。为了明确地解决这个问题和进一步的建模能力，需要边界的三维（3D）方向。三维数据的收集使用聚焦离子束-电子背散射衍射（FIB-EBSD）断层扫描。在后处理中结合fib生成的序列切片的EBSD图，生成完整的晶体体积，能够在亚微米分辨率下揭示许多类型的结构特征。HMC学生领导开发新的计算工具，以有效和准确地分析由该方法产生的大型3D数据集。他们还改进了FIB-EBSD方法，并在同行评议的期刊和国家或国际会议上发表了他们的发现。在新南威尔士大学，受资助的本科生和首席研究员可以广泛使用HMC所没有的电子显微镜设备和培训。因此，该项目为新南威尔士大学物理冶金和电子显微镜专家与美国本科生之间的合作提供了难得的机会。该奖项由材料研究部和国际科学与工程办公室共同资助。

This project is based on collaboration between Harvey Mudd College (HMC) and the School of Materials Science and Engineering and the Electron Microscope Unit at the University of New South Wales (UNSW) in Australia. It is also a Research in Undergraduate Institutions activity, as the award supports international materials science research experiences for HMC undergraduate students. The project focuses on resolving a longstanding controversy about the nature of arrays of aligned dislocation boundaries that are generated during rolling of deformed face-centered cubic and body-centered cubic metals with intermediate to high stacking fault energies. The structure and origin of these dislocation boundaries are of substantial interest due to their role in determining anisotropy in yield stress and strain hardening. Insight into the true character of these microscale structures is essential for advancing the predictive capabilities of physically-based models for macroscale mechanical properties. The two opposing theories of the structure and origin of these aligned boundaries in deformed polycrystals are: (i) they are oriented along certain crystallographic planes, or (ii) their alignment is dictated primarily by the macroscopic stress state during plastic deformation. Current evidence supporting these theories is based on two-dimensional data that does not necessarily reveal the true nature of deformation microstructures and can lead to erroneous interpretation. To definitively resolve the issue and to further modeling capabilities, three-dimensional (3D) orientations of the boundaries are required. Data in 3D is collected using focused ion beam-electron backscatter diffraction (FIB-EBSD) tomography. EBSD maps of FIB-generated serial sections are combined in post-processing to generate full crystallographic volumes capable of revealing many types of structural features at submicron resolution. HMC students lead the development of new computational tools required to efficiently and accurately analyze the large 3D data sets that result from this method. They also refine FIB-EBSD methods and present their findings in peer-reviewed journals and national or international conferences. At UNSW, the supported undergraduates and the principal investigator have extensive access to electron microscopy facilities and training unavailable at HMC. The project therefore provides an extraordinary opportunity for collaboration between UNSW's experts in physical metallurgy and electron microscopy and U.S.-based undergraduate students. This award is co-funded by the Division of Materials Research and the Office of International Science and Engineering.