Grain boundary sliding and its relation to ductility and fracture in fine-grained polycrystalline materials with a particular focus on γ-TiAl

The focus of this paper is the relationship between grain boundary sliding and fracture in polycrystalline γ-TiAl and its significance with regard to ductility as grain size approaches the nanometer scale. Deformation by a number of relevant mechanisms is modeled using a unified approach based on the classical reaction rate theory. We delineate the high ductility grain boundary sliding flow regime, which is characterized by a relatively high deformation rate and a high rate sensitivity. In general, we find that decreasing grain size enhances this deformation mode and thereby, enhances ductility. With regard to fracture, we concern ourselves here with crack nucleation at grain boundary triple junctions caused by stress concentrations resulting from sliding. This focus leaves as the key dimension of interest that of the grain size itself and allows us to unambiguously address the effects of grain size diminishment on ductility. We compare the characteristic time associated with the build up of stress concentrations by grain boundary sliding and their relaxation by a number of flow mechanisms. We find that for γ-TiAl, intergranular crack nucleation is exacerbated as grain size is diminished from the micrometer scale to about 40 nm. However, as grain size is further reduced below 40 nm, we find that crack nucleation becomes increasingly mitigated due to the relaxation of stress concentrations by diffusional flow. The findings discussed in this paper allow us to define the flow regime for nanocrystalline γ-TiAl, wherein both the high ductility sliding mechanism prevails and crack nucleation is mitigated. From this result one can predict the conditions for enhanced ductility in this conventionally brittle material.