In-situ transmission electron microscopy study of the dislocation accommodation in [101] tilt grain boundaries in nickel bicrystals

The relaxation of stresses associated to extrinsic grain-boundary dislocati ons (EGBDs) in singular Sigma = 3, {111}, in vicinal Sigma = 11, {311} and in general Sigma = 11, {332} grain boundaries (GBs) has been investigated b y in-situ transmission electron microscopy (TEM) using weak-beam conditions . In a singular Sigma = 3, {111} GB, reactions of combination and annihilat ion occur between extrinsic and intrinsic dislocations when they initially intersect each other. These reactions yield the parallelism of the two type s of dislocation which is the preliminary configuration for any of the exis ting incorporation models to operate, but further evolution by decompositio n of the EGBDs in glissile and sessile components, which can move away, as depicted by theory, was observed once only at a very high temperature. Furt hermore, the rearrangement of all the sessile dislocations, extrinsic and i ntrinsic, in a periodic network was not observed, even after maintaining th e thin foil at 930 degrees C = 0.6T(m) (T-m is the melting temperature) for a long time. In vicinal Sigma = 11, {311} GBs, the continuous spreading of EGBD contrasts, previously observed by TEM in bright-field conditions, has been proved to be a decomposition process. Indeed, owing to the weak-beam technique, the wide image of a relaxed EGBD has been resolved in a set of s mall segments resulting from the interaction between the EGBD and the intri nsic dislocations. After annealing the thin foil at temperatures up to 0.4T (m), all the GBDs had rearranged themselves in a quasiperiodic network, ind icating that equilibration is almost achieved. In general Sigma = 11, {332} GBs, the widening of the EGBD contrasts has never been associated with a d ecomposition phenomenon. No discrete product of EGBD decomposition has ever been observed by TEM, even in weak-beam conditions. The differences betwee n the accommodation processes according to the GB type are discussed by ref erring to the GB atomic structure as described by the structural unit model .