Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Defects in solids 111 aggregate as a platelet as shown in Figure the resultant collapse of the disc-shaped cavity Figure would bring two similar layers into contact. This is a situation incompatible with the closepacking and suggests that simple Frank dislocations are energetically unfavourable in cph lattices. This unfavourable situation can be removed by either one of two mechanisms as shown in Figures and . In Figure the B-layer is converted to a C-position by passing a pair of equal and opposite partial dislocations dipole over adjacent slip planes. The Burgers vector of the dislocation loop will be of the ơS type and the energy of the fault which is extrinsic will be high because of the three next nearest neighbour violations. In Figure the loop is swept by a Aff-type partial dislocation which changes the stacking of all the layers above the loop according to the rule A B C A. The Burgers vector of the loop is of the type AS and from the dislocation reaction Aơ C ơS AS or 1 1 0 1 0 C 1 0 0 0 1 6 2 0 2 3 and the associated stacking fault which is intrinsic will have a lower energy because there is only one next-nearest neighbour violation in the stacking sequence. Faulted loops with b AS or 1 c C p have been observed in Zn Mg and Cd see Figure . Double-dislocation loops have also been observed when the inner dislocation loop encloses a central region of perfect crystal and the outer loop an annulus of stacking fault. The structure of such a double loop is shown in Figure . The vacancy loops on adjacent atomic planes are bounded by dislocations with nonparallel Burgers vectors . b 2c C p and b 2c p respectively the shear component of the second loop acts in such a direction as to eliminate the fault introduced by the first loop. There are six partial vectors in the basal plane p1 p2 p3 and the negatives and if one side of the loop is sheared by either p1 p2 or p3 the stacking sequence is changed according to A B C A
