Probing grain boundary structure-property relationships using directionally solidified, grain boundary engineered oligocrystals
Ware, Logan Gregory
Doctor of Philosophy
Grain boundaries are two-dimensional defects in polycrystalline materials and are one of the main determinants of their macroscopic properties. Due to their complexity, from the atomic level to the macroscopic, experimental study of grain boundary physics, chemistry, and mechanics is limited to small subsets of easily accessible grain boundary structures. Advances in computational materials science and microstructural characterization techniques have expanded the scope of grain boundary study, but there remain several open challenges to full understanding of grain boundary structure-property relationships. Many studies in the past two decades have found that the orientation of the grain boundary plane is an important factor in predicting grain boundary properties, however systematic investigation of the grain boundary plane is hindered by the lack of control of the grain boundary plane during fabrication and processing. This thesis aims to solve the outstanding challenge of grain boundary plane orientation control and greatly open up experimental investigation of grain boundary structure-property relationships. In order to accomplish this, I demonstrate a novel experimental fabrication technique that leverages additive manufacturing to grow bicrystals of complex shape. These complex bicrystals contain equally complex grain boundary geometries, whose structures are a continuous function of the bicrystal shape, meaning one single grain boundary can represent many different grain boundaries simultaneously. In this work, relevant topics are surveyed briefly, including grain boundary structure, grain boundary energy, and grain boundary fabrication through crystal growth, followed by a description of the bicrystal growth technique. A study on the thermodynamic stability of complex grain boundary morphologies found in these bicrystals follows, in which we find that these grain boundaries are metastable within the constraints of the bicrystal. We also consider the effect of local temperature gradients on the kinematic accessibility of a given grain boundary geometry to determine the constraining thermal and geometric factors during crystal growth. The crystal growth technique is then applied to studying grain boundary energy as a function of the grain boundary plane orientation. This study has found the first experimental observation of symmetries in grain boundary properties in a general high angle grain boundary with no special misorientation relationship. This last result has important implications for experimental study of the symmetry of general grain boundaries, as well as for improved property predictions based on the topology of a given grain boundary network in polycrystalline materials.
physical metallurgy; additive manufacturing; crystal growth; grain boundaries