Dislocation Dynamics in Intermetallic and Oxide Dispersion Strengthened (ODS) Alloys

Abstract

ABSTRACTIn situ straining experiments in a high-voltage electron microscope allow the observation of the dynamic behaviour of individual dislocations. Such experiments have been performed on the intermetallic alloys NiAl, NiAl containing 0.2 at% Ta, α-TiAl, and MoSi2, and the oxide dispersion strengthened (ODS) alloys INCOLOY MA956 and INCONEL MA754 in a wide range of temperatures. There are many similarities in the dynamic behaviour of dislocations in the different materials. In the intermetallic alloys, a transition occurs between low temperature mechanisms and a viscous motion in the temperature range of the flow stress anomaly. The viscous motion at high temperatures can be explained by diffusion processes in the dislocation cores, whichcan be described by the theory of the Cottrell effect. The diffusing species can be quite different, alloying components or intrinsic point defects like vacancies and antisite defects existing in the lattice or only in the dislocation cores. If the dislocations are straight and crystallographically oriented during their motion, they may be dissociated and move by a succession of glide and conservative climb between the partial dislocations. In the ODS alloys, the dislocations move between the oxide particles again in a viscous way. The relation between the dislocation dynamics and the strain rate sensitivity of the flow stress is discussed for thedifferent materials.