Tensile crack initiation in gamma-titanium aluminide

Using electron channeling methods, fracture initiation in γ-TiAl based alloys has been studied with the overall objective of determining the principal fracture initiation mode. Electron channeling methods reveal sufficient information to determine true grain orientation as well as deformation modes for the tetragonal γ-TiAl crystal structure.; Fracture initiation is found to occur principally at γ-γ grain boundaries, with the second most common initiation site occurring at γ-α ₂ interphase boundaries. For both initiation modes, boundary crack initiation appears to be related to impingement of deformation twins from the γ-TiAl phase.; For the γ-γ grain boundary fracture initiation, a factor that incorporates components relating to deformation twinning, grain boundary conformation to incident deformation twins, and the relationship between twin shear and macroscopic stress state is reported. Interpretation of this factor implies that the driving mechanism of γ-γ boundary fracture is the accumulation of grain boundary strain energy associated with deformation twin accommodation by the grain boundary. Further refinement of this grain boundary fracture factor, F, is discussed. The current study suggests an enhancement of the fracture correlation with a modified form of F that incorporates grain boundary orientation; in addition, further avenues of grain boundary fracture factor refinement are suggested, toward the ultimate goal of describing a threshold for which grain boundary fracture will occur. Ultimately, this study suggests that any γ-TiAl based alloy that deforms principally through deformation twinning is unlikely to display extensive tensile ductility, due to the nature of accommodation of the grain boundary to deformation twins.