The Roles of Grain Boundary Energy and Curvature on Grain Growth in SrTiO3 polycrystals

 Predicting the microstructural evolution of polycrystals is important for efficient  microstructure design, which can be used to tailor material properties. However, it is currently  not possible to accurately predict grain growth in polycrystals due to the limited understanding  of 1) the mechanisms that govern the grain boundary (GB) migration in polycrystalline  materials having an interconnected network of GBs, 2) the influence of GB energy in the  microstructure history on those migration mechanisms. Therefore, this thesis focuses on  understanding how the GB curvature and energy in the microstructure history influence the  normal and abnormal grain growth behavior in SrTiO3 polycrystal.  

The GB migration of polycrystalline SrTiO3 exhibiting normal grain growth (NGG)  was analyzed using synchrotron-based high energy x-ray diffraction microscopy (HEDM). This experimental grain growth was compared to an isotropic grain growth simulation to  identify unknown migration mechanisms occurring in experiments that simulations cannot  predict. Based on the investigation, it was found that the GB velocity is linearly correlated to  its local curvature in simulations, as expected. On the contrary, experimental observations of  GB migration showed no linear correlation between GB velocity and local curvature.  Additionally, 37% of GBs in the experiment moved against their center of curvature (anti-curvature GB motion) as opposed to the classical theory of curvature-driven GB motion. Therefore, to investigate why anti-curvature GB motion may be an energetically favorable  process, the correlation of GB area changes to the direction of motion of neighboring GBs in  the triple junction (TJ) was analyzed. The results from the analysis revealed that when GBs are  bounded by GBs migrating towards their center of curvature (curvature-driven GBs) in the TJ,  high-energy & high-area GBs, on average, increase in area, and low-energy & low-area GBs,  on average, decrease in area. This correlation suggests coarsening may be the dominant free  energy minimization mechanism for GBs bounded by curvature-driven GBs. For GBs bounded  by anti-curvature GBs, the mechanism of low-energy GBs replacing high-energy GBs was  slightly dominant and is more pronounced than for GBs bounded by curvature-driven GBs.  However, there was no clear evidence that the energy replacement mechanism occurs at the  lowest or highest energy GBs. Overall, these results suggest that multiple mechanisms act  during GB motion, and it is important to accurately identify markers for when each mechanism  will act for better prediction of GB motion direction. 

To understand the factors that induce abnormal grain growth (AGG), the heating  profiles were modified to potentially engineer different initial microstructural states with  unique GB energy distributions to explore the role of microstructure history’s GB energy on  the AGG of SrTiO3 at 1425°C. The heat treatment that produced fewer high-energy GBs and a  narrow GB energy distribution exhibited a slower growth rate for matrix grains and a smaller  fraction of abnormal grains formed after growth. Hence, this suggests GB energy engineering  can potentially be used to control grain growth rate and likelihood of AGG. 

Overall, the results here have advanced our understanding to better predict and control  microstructure evolution in polycrystalline materials. Hence, these findings will provide an  efficient path in the future for engineering microstructures for different application  requirements.