Edge dislocations dissociated in {112} planes and twinning mechanism of b.c.c. metals

Abstract

Based upon the recent observations that a slip dislocation in b.c.c. metals is associated with three twinning dislocations (Ogawa and Maddin 1964), the atomic configuration of the dissociated slip dislocation as well as those of mono-layer, two-layer and three-layer twins were studied in detail. The interfacial energies of a two-layer and three-layer twin are concluded to be considerably smaller than that of a mono-layer twin or a stacking fault, and a coherent twin boundary is found to be composed only of compressed bonds. Owing to this estimated lower interfacial energy of a two-layer twin, edge dislocations are considered to dissociate into three twinning dislocations over three neighbouring {112} planes. The dissociated edge dislocation is found to be associated with both a locally expanded and compressed region. Since the motion of the locally expanded region is controlled by the thermal motion of atoms, three twinning dislocations can be nucleated from the dissociated edge dislocation, at a high strain rate or at low temperatures and if the applied shear stress is greater than a certain critical shear stress. It is also found that the dissociated edge dislocation can glide in the {112} plane if the locally expanded region created as a result of the dissociation is able to follow the motion of the dissociated dislocation. Hence the critical strain rate and temperature for twinning were found to be inter-related. It is found that the present model for twinning, i.e. the nucleation of three twinning dislocations from an edge dislocation dissociated over three neighbouring {112} planes, is able to explain a number of facts experimentally observed: (i) the ease of twinning at low temperatures and at high strain rates, (ii) the effect of pre-strain and of ageing after pre-strain, (iii) the interrelation between slip in {112} and twinning, (iv) the calculation of k ^T in the Petch equation, and the good agreement between the calculated value and the value experimentally determined, (v) the effect of interstitial impurities and of alloying elements upon twinning. This model also gives rise to internal friction of the relaxation type and the peak is roughly estimated to be in kc ranges at room temperature in iron.