Nucleation of deformation twins at slip plane intersections in B.C.C. metals

Abstract

The formation and dissociation of dislocation barriers at slip plane intersections in b.c.c. metals are considered, where one or both of the slip planes are of the {112} form. It is found that barriers may be weak or strong, depending on the characteristics of the glide dislocations which contributed to the formation of the barrier, and on the stress state at the barrier. Weak barriers dissociate by a low energy path to produce twin lamellae in the intersecting {112} planes, at stress concentrations of less than 50 times the applied stress. Evidence obtained by transmission electron microscopy on shock-loaded iron-base alloys shows that twin nucleation can occur at slip plane intersections in b.c.c. metals. If dislocation dissociation producing twinning dislocations is prohibited, sufficiently strong barriers could exist to allow local stresses to reach critical values for nucleation of cleavage cracks. It is proposed that twinning and cleavage stresses are of similar magnitudes because both phenomena are heterogeneously nucleated at similar sites.