In situ investigation of grain boundary migration in aluminium and copper bicrystals

Grain boundary migration plays an important role during the microstructural evolution of polycrystalline materials. Understanding of its mechanisms is therefore crucial to gain further insight on many processes, which take place during material treatment. Systematic investigations of grain boundary migration are preferably done on single, non-connected grain boundaries to avoid a distortion of the migration kinetics due to the influence of triple points or similar interactions with other grain boundaries. This dissertation presents results of such investigations for the migration of selected grain boundaries in high purity Al and Cu. Bicrystalline specimens containing individual grain boundaries with precisely defined crystallographic geometry have been produced by directed solidification. The behavior of the grain boundaries under capillary and stress induced driving force was then investigated in situ, using special stages inside a scanning electron microscope. In general, the reaction of the grain boundaries to stress induced and capillary driving force was found to be different in many aspects. This supports the assumption that the underlying mechanism of grain boundary migration is crucially dependent on the type of applied driving force. The curvature driven migration of low angle grain boundaries and of high angle tilt grain boundaries close to Σ3, in both cases in Al, was found to be significantly influenced by the inclination dependent boundary energy as revealed by atomistic simulations. Grain boundaries with a sufficiently anisotropic energy did not move under capillary driving force because relative minima of the boundary energy either kept them in the initial flat shape or caused the formation of stationary facets. Addition of a twist character to the low angle tilt grain boundaries or deviation from the Σ3 misorientation made the boundary energy largely isotropic. Respective grain boundaries assumed a curved shape and migrated further due to capillary driving force. Low angle and tilt grain boundaries in Al as well as selected tilt grain boundaries in Cu reacted with shear coupled migration to a stress induced driving force. Application of the mechanical load parallel to or at 45° to the boundary plane induced migration with the same kinetics, which proved that only the shear component of the stress tensor contributes to the driving force. Stress driven migration was generally accompanied by a lateral deformation of the grains consistent with the coupling effect. The ratio between lateral displacement and migration rate of the grain boundary in normal direction was in excellent agreement with predictions from geometrical models. High angle tilt grain boundaries close to Σ3 did not migrate steadily under stress induced driving force in general. With the only exception being a grain boundary close to Σ3 and {110} crystallographic plane, which was also notably the only grain boundary that did not cause any grain shear during stress driven migration