Determining Grain Boundary Solute Segregation Specificity in Nanocrystalline Stability

University Of Alabama Tuscaloosa, Tuscaloosa AL

Investigators

Abstract

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.