MOBILITY OF GRAIN-BOUNDARY DISLOCATIONS DURING THE CONSERVATIVE UNTWISTING OF [001] TWIST BOUNDARIES

We modeled the mobility of grain boundary dislocations (GBD's) during the untwisting of the [001] twist boundaries. Instead of assuming two semi-infinite crystals in calculating the grain boundary energy (i.e., the Read-Shockley approach) and therefore the driving force for untwi sting, we assume equally spaced CBD's moving in the (001) boundary pla ne with the dislocations closest to the surface being pulled out by th e image force. Experimental results from crystallite rotation in fee g old were used to investigate the mobility of the GBD's. Two types of G BD motion were tested: viscous and thermally activated. The observed m otions of the GBD's during untwisting can be described only as thermal ly activated. The Hirth-Lothe approach, which involves a thermally act ivated process overcoming the Peierls barrier, was applied to describe the mobility of GBD's during untwisting into the Sigma 5 cusp/minimum (Sigma is the reciprocal of the density of the lattice sites in coinc idence between two lattices at a misorientation) and the mobility of l attice dislocations {100} [110] during untwisting into the Sigma 1 cus p/minimum. The Peierls barrier for GBD motion confined to the glide pl ane of the boundary (001) is significantly higher than that for lattic e dislocations glide on {111} planes. From the untwisting rates, we es timate the energy barriers for GBD motions as 1.69 eV for Sigma 1 and 1.84 eV for Sigma 5 [001] twist boundaries. These results can explain the high yield stress and its sharp temperature dependence during plas tic deformation of nanoparticle compacts of fee metals. These results can also be used to estimate the largest size of crystallites that wil l rotate.