The visibility of extrinsic grain boundary dislocations in symmetrical [011] tilt boundaries in aluminium

Abstract

Two types of defect have been observed in symmetrical, high angle, [011] tilt grain boundaries in aluminium. One type appeared as an array of fine lines with a spacing which decreased with increasing twist mismatch between the (011) planes normal to the grain boundary. These lines were considered to be structural dislocations with a Burgers vector close to a/4 [011]. The other type of boundary defect had been introduced by deformation at elevated temperatures (extrinsic grain boundary dislocations) and also had a Burgers vector close to a/4 [011]. Their contrast decreased sharply with increasing twist mismatch of the (011) planes, that is, with decreasing spacing of the structural dislocations of similar Burgers vector. This behaviour is accounted for by a strain sharing concept which involves interaction between the array of structural dislocations and the extra grain boundary dislocation introduced.