Dynamic dislocation behaviour in the intermetallic compounds NiAl, TiAl and MoSi2

The dynamic behaviour of dislocations in NiAl, TiAl and MoSi₂ on ‘easy’ slip systems is studied by in situ straining experiments in a high-voltage electron microscope. At elevated temperatures, the dislocations are smoothly bent as in NiAl and TiAl or sometimes show superkinks as in MoSi₂, and they move in a viscous way. It is suggested that this dynamic behaviour as well as the flow stress anomaly are connected with the formation of atmospheres around the dislocations. A model is proposed assuming that the lowest energy configuration of a dislocation may require a certain number of antisite defects or other point defects in the dislocation core. This cloud of disordered structure may follow partly the moving dislocations to induce an additional friction, analogous to other diffusion controlled mechanisms. The view of atmospheres controlling the dislocation mobility in intermetallics at elevated temperatures is supported by measurements of the dependence of the strain rate sensitivity on the strain rate itself.