Determining Grain Boundary Solute Segregation Specificity in Nanocrystalline Stability

摘要：当材料的晶粒尺寸减小时，材料的强度就会增加。不幸的是，与大尺寸的颗粒相比，这些小尺寸的颗粒在能量上是不利的。加热后，较小的晶粒会生长，以减少这种能量差，从而导致强度的损失。近年来，特定类型合金的发展表明，较低浓度的原子型或溶质可以稳定这些晶粒，防止这种生长。这些观察的结果是各种模型来解释晶粒尺寸的稳定。然而，这些模型有几个基本的科学空白仍有待解决。例如，模型晶界被认为是等效的，而现实晶界在结构、能量和迁移率方面是不同的。缺乏对这种溶质特定的晶界偏析的实验验证阻碍了模型的进一步发展。这项研究将通过结合电子显微镜和原子探针断层扫描来检测这些特定边界的溶质偏析，从而克服这些先前的限制。然后，这些结果将被转发给解释这种行为多样性的原子模型。这项研究将整合到各种推广活动中，包括为中学教育教师举办的暑期材料夏令营，将材料科学引入初高中课堂。技术摘要：纳米晶体材料由于其独特的尺寸依赖性而成为一个活跃的研究领域。由于界面（边界）占整个结构的很大一部分，来自这些边界的相应界面能会使这些晶粒本身不稳定。这些边界上的溶质分配被设计为稳定这些晶粒生长的一种手段，动力学溶质阻力效应和热力学界面能降低的概念被认为是稳定机制。迄今为止，这些模型假设所有的晶界都是各向同性的，彼此等效；在现实中，晶界在能量、结构和迁移率上是多种多样的，溶质划分是这种可变性的函数。利用交叉相关进动电子衍射和原子探针层析成像，本研究将阐明导致稳定的晶界的溶质特异性。实验结果将与氢化物蒙特卡罗分子动力学原子模型相联系，该模型将阐明特定边界类型的稳定机制。此外，将测量退火过程中这些边界的实时实验量化，以确定迁移率和动力学对稳定性的贡献。集体结果将在揭示特定晶界的热力学和动力学考虑如何导致晶粒尺寸稳定性方面弥合突出的和基本的知识差距。这项研究将无缝地整合到各种外展活动中，以加强STEM学科，包括为中学教育教师举办的暑期材料夏令营，将材料科学引入初中/高中课堂。

Non-technical Abstract: When the grain size of a material is reduced, the strength of a material increases. Unfortunately, these small grain sizes are energetically unfavorable as compared to larger sized grains. Upon heating, the smaller grains will grow to reduce this energy difference which results in the loss of strength. In recent years, the development of specific types of alloys has shown that the lower concentration atom type, or solute, can stabilize these grains against this growth. An outcome of these observations has been various models to explain the grain size stabilization. However, these models have several fundamental scientific gaps that still remain to be solved. For example, the modeled grain boundaries are considered equivalent wherein reality grain boundaries are diverse in their structure, energy, and mobility. The lack of experimental verification of this solute specific segregation to grain boundaries has hindered further model development. This research will overcome these prior limitations by combining electron microscopy and atom probe tomography to detect solute segregation to those specific boundaries. These results will then be forward fed to atomics models that explain this diversity of behavior. This research will be integrated into various outreach activities, including a summer materials camp for secondary education teachers to introduce materials science into the middle/high school classrooms. Technical Abstract:Nano-crystalline materials is an area of active research owing to their unique size dependent properties. Since interfaces (boundaries) constitute a large fraction of the entire structure, the corresponding interfacial energy from those boundaries can make these grains inherently unstable. Solute partitioning to these boundaries has been devised as a means to stabilize these grains against growth with concepts of kinetic solute drag effects and thermodynamic reduction of interfacial energy being suggested mechanisms for stabilization. To date, these models assume all the grain boundaries are isotropic and equivalent to each other; in reality grain boundaries are diverse in energy, structure, and mobility with solute partitioning being a function of that variability. Using cross-correlative precession electron diffraction and atom probe tomography, this research will elucidate the solute specificity to grain boundaries that leads to stabilization. The experimental findings will be linked to hydride Monte Carlo-Molecular Dynamics atomistic models that will elucidate the mechanisms of stabilization to specific boundary types. In addition, real-time experimental quantification of those boundaries during annealing will be measured to ascertain mobility and kinetic contributors to stability. The collective results will bridge outstanding and fundamental knowledge gaps in revealing how thermodynamic and kinetic considerations for specific grain boundaries leads to grain size stability. This research will be seamlessly integrated to various outreach activities to strengthen the STEM disciplines including a summer materials camp for secondary education teachers to introduce materials science into middle/high school classrooms.