Revealing the effect of Ta on coarsening behavior of γ' phases during thermal exposure at 1000 °C in a Ni-based SX superalloy

To reveal the effect of Ta on coarsening behavior of gamma' phases during thermal exposure at 1000 degrees C in a Ni-based single crystal superalloy, morphology and particle size distributions of gamma' phases as well as misfit dislocations in alloys with varying Ta content are investigated by scanning electron microscope, transmission electron microscope, high resolution x-ray diffraction and atom probe tomography techniques. The results demonstrate that Ta addition accelerates the non-directional coarsening of gamma' phases during thermal exposure at 1000 degrees C. This can be attributed to two aspects. For one thing, Ta addition increases absolute lattice misfit at both room temperature and 1000 degrees C, thereby enhancing interfacial elastic energy as driving force for the non-directional coarsening of gamma' phases. Moreover, to release interfacial elastic strain energy, high-density misfit dislocations are formed. However, the misfit dislocations can be served as channels that facilitate the rapid diffusion of elements, which promoting the non-directional coarsening of gamma' phases. For another, Ta addition improves gamma/gamma' two-phase interfacial energy based on A.J. Adelle's approach, so further accelerating the non-directional coarsening of gamma' phases. Besides, the relationship between Ta addition and coarsening mechanism of gamma' phases is further discussed in detail. It is found that the Ta addition advances transition of particle size distributions from being initially controlled by matrix diffusion to being governed by interface diffusion.