On kinetics and thermodynamics of high angle grain boundaries in aluminium : experimental study on grain boundary properties in bi- and tricrystals

In this work, experimental study on grain boundary migration and grain boundary structure has been performed. The dependence of grain boundary (GB) and triple junction (TJ) migration on grain boundary character (disorientation between the grains and inclination of the boundary plane), temperature, and purity was studied. A new method for experimental measuring the grain boundary excess free volume was designed. Experiments were carried out on specially grown bi- and tricrystals of Al of high purity. 1) The motion of curved grain boundaries was studied in Al-bicrystals. The rate of grain boundary displacement and the shape of the moving GB were measured. Two types of boundaries were investigated: curved tilt GBs containing different sets of boundary planes (elements) and curved mixed GBs which consist of tilt and twist elements. Experiments on the tilt GBs revealed that their steady-state motion is not affected by changing the inclination of the boundary elements in the moving boundary. Experiments on the mixed GBs showed non-monotonic dependence of the grain boundary mobility on the angle of misorientation. This dependence is very similar to that discovered in the past for pure tilt GBs. These results allows concluding that the character of boundary, tilt or mixed (tilt-twist), does not affect the motion of the curved GBs. 2) The interaction between the moving GB and the external sample surface was studied in Al-bicrystals. The motion of curved GBs. was studied. The effect of the surface triple junction was studied in terms of the dynamic equilibrium of boundary and surface triple junction velocity. The obtained linear relationship between the boundary velocity and the driving force allows concluding that the migration of GBs is determined by the boundary mobility, and is not affected by the surface triple junction. The impact of anisotropy of the external surface on the GB motion was also studied. Results demonstrated that the motion of curved GBs in samples with a strong anisotropy of the external surface cannot be described by the existing steady-state model of GB migration.3) The steady-state motion of connected GBs (grain boundaries with a triple junction) was studied in Al-tricrystals with different concentrations of Mg (0.2 - 1000 wt. ppm). The effect of Mg-atoms on grain boundary and triple junction mobilities was measured. It was shown that at the low concentration of Mg (0.2-100 wt. ppm) the impurity drag theory of Lücke and Detert is valid, at the higher concentration of Mg – the dependence of grain boundary mobility on impurity concentration cannot by described by this theory. It was also shown that the dragging influence of triple junctions intensifies with increasing the concentration of Mg. 4) A new method to measure the excess free volume of grain boundaries is designed. It is based on the Gibbs adsorption equation, which relates the energy of GB and the autoadsorption. The dependence of the grain boundary energy on the pressure was measured in Al-tricrystals at high gas pressure. The measured contact angle between the boundaries reflects the balance of their energies and the pressure dependence of the energy. The excess free volume was determined for 40° tilt GBs with a 4° twist component and was found to be 6.4E-11 m3/m2