Thermally activated motion of jogged screw dislocations in NiAl composites

Long screw dislocations with jogs or super jogs are observed in NiAl matrix composites after high temperature plastic deformation. Thermally activated motion of these jogged screw dislocations is postulated as the rate-controlling mechanism of deformation. The mechanism involves the non-conservative motion of jogged screw dislocations, and it was concluded that only the motion of screw dislocations which contain vacancy-producing ( VP) jogs are capable of predicting the correct relation between stress and strain rate in a creep test. A simple model, which is based on the cross-slip of screw dislocations in NiAl, is proposed to account for the occurrence of VP jogs. In this model, kinks change into a single type of jog after a screw dislocation cross slips. The change from kinks into jogs is a simple process because of the crystal geometry of B2 ordered structure. This model will provide the necessary conditions of the mechanism of jogged screw dislocation. An explanation of why jogs prefer to move by producing vacancies is offered. By computer simulation, it was found that this mechanism successfully predicted the temperature dependence of the yield stresses of NiAl composites.