Self-organization of cooperative grain boundary sliding in aluminium tricrystals

As the given theoretical analysis is simplified, one cannot detect a concrete direction of local migration. But due to experimental observations we can conclude that the direction of migration is determined by a non-symmetry of an elastic stress field at the triple junction. In particular, one of the causes of such non-symmetry may be partial stress relaxation at initiation of GBS along the adjacent boundary. In another way, a chain of consecutive processes occurs. The stress from the head of the pile-up initiates GBS along the adjacent boundary where σxy from external stresses were not sufficient. The originated GBS predetermines the direction of local migration which is induced by a slight potential of the boundary with the pile-up. According to the size of long range area of sliding suppression and the grain boundary migration this of pile-up of dislocations has a size ∼ 500 μm. Due to migration, a dihedral angle between boundaries increases. That contributes to break through the triple junction and cooperation of GBS at both boundaries. A non-balanced part of shear deformation is compensated by intragranular slip. The experiments on the aluminium tricrystals show that due to migration two main processes, namely, cooperation and self-organization of GBS, can occur in triple junctions. A unified cooperative process consisting of identical regularities of GBS development takes place there (one and the same GBS rate, index GBS rate sensitivity to shear stress and similar values of the activation energy of GBS for adjacent boundaries with different Schmid-factors) [15, 16]. On the basis of the experimental data comparison one can conclude that one and the same CGBS process occurs during superplastic deformation of polycrystals [1-4] and tricrystals. The cooperation of GBS at boundaries and the straightening of the triple junction actually increase the GBD pile-up (L in eq.11) and raise the stress concentration at the next triple in the direction of sliding band penetration. If the triple junction was less suitable for CGBS than the previous one and CGBS could not occur here the enlarged size of the pile-up assists in expansion of self-organization process. So, the larger the number of grain boundaries consisting the CGBS band is, the easier is the process of accommodation at triple junctions. Thus, the cooperative grain boundary sliding in combination with the local migration forms the self-organization process. The moving force of such local migration is an increase in potential due to pile-ups of grain boundary dislocations near the triple junction. It takes place on randomly orientated grain boundaries mainly with the maximum Schmid factor.