In-Situ SEM Investigation of Individual and Connected Grain Boundaries in Aluminum (PDF)

The recent development in experimental research on grain boundary and triple junction migration and structure in metals has been reviewed in this thesis. The dependence of grain boundary mobility on boundary character (misorientation and inclination) as well as the dependency of the triple junction mobility on the junction crystallography was addressed.
The motion of individual grain boundaries and triple junctions was investigated in specially grown bi- and tricrystal samples of high purity aluminum. Orientation contrast imaging in an SEM combined with an in-house developed heating stage was used for in-situ tracking of the microstructure evolution at elevated temperatures. Furthermore, a novel laser powered heating stage was developed in order to enable the microstructure and -texture characterization at elevated temperature up to 1000°C.

The migration of connected <100> tilt grain boundaries, was studied experimentally as a function of temperature between 400°C and 580°C. The study revealed that the majority of grain boundary systems were controlled by the lower mobility of the curved grain boundaries rather than the triple junction mobility within the investigated temperature range.
Solely one grain boundary system (TJ-41) revealed two different kinetic regimes of coupled triple junction - grain boundary motion dependent on temperature proving the existence of a specific triple junction mobility and its dependence on triple junction crystallography. At low temperatures the motion of the grain boundaries was found to be retarded by the low triple junction mobility. In contrast, at elevated temperatures the system kinetics was found to be grain boundary controlled.
The migration of grain boundary systems with triple junction, composed of mixed <110> twist and 40° <111> tilt grain boundaries, was studied experimentally in dependence of temperature for a range of boundary misorientations. In analogy to the study on connected <100> tilt grain boundaries, the majority of grain boundary systems were found to be controlled by the lower mobility of the curved grain boundaries. The only exception was found during investigation of the grain boundary system TJ-twist-19 revealing a system migration influenced by the triple junction mobility at all investigated temperatures, confirming the existence of a finite triple junction mobility and its dependence on triple junction crystallography. However, the boundary system TJ-twist-19 was not found to change its state of motion as observed for sample TJ-41.
Aluminum bicrystal samples, containing individual <100> and <111>tilt grain boundaries, were investigated experimentally in-situ in a temperature range between 300°C and 640°C for a range of boundary misorientations. The study revealed, for both systems, a different behavior of SAGBs in contrast to LAGBs. Grain boundaries with a rotation angle below 15° were found to be immobile during annealing at a given temperature in conjunction with a faceted grain boundary shape. Owing to grain boundary anisotropy the boundary facets correspond to boundary inclinations which possess a particularly low energy as confirmed by additional MS-simulation studies on bicrystals with equivalent crystallography.
In contrast, LAGBs with a misorientation angle larger than 15° did not tend to form grain boundary facets. Instead, the grain boundaries assumed a curved shape and moved in steady-state in the entire investigated temperature range, as expected for curvature driven grain boundary motion assuming grain boundary isotropy.
The microstructure evolution of individual circular grains in thecourse of grain shrinkage was simulated for bicrystals implementing a 2D-vertex model. The study revealed that the modeled grain shape is consistent with the experimentally observed boundary shape in bicrystal samples with equivalent crystallography.
Furthermore, the study revealed that grain boundary anisotropy can affect the rate of grain area change. In the case of grains with a pronounced grain boundary anisotropy, a preferred formation of straight facets composed of low energy boundary planes was observed. As a result, the mean free energy of the grain boundary was reduced, causing a decrease of the capillary driving force. Owing to the decrease of P, the normal velocity of the respective segments decreased likewise, causing a retardation of grain shrinkage.

Finally, the author believes that the results of the studies presented here lead to a better understanding of solitary and connected grain boundaries and their role in modern materials science and engineering.