The contribution to crack growth resistance of colony boundaries in a fully lamellar microstructure in binary Ti-46Al was studied using single-colony thick (but polycolony in-plane) compact-tension specimens. Such specimens were fatigue precracked and subsequently loaded incrementally in a scanning electron microscope. Crack advance was monitored and related to surface microstructure. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation: an in-plane kink angle and a through-thickness twist angle. Thus, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. From these experiments, it was determined that under certain conditions, the colony boundaries could offer significant resistance to crack growth. Within individual colonies however, crack propagated with minimal resistance. Crack propagation across the colony boundary ranged from being continuous with minimal accompanying damage at the boundary to crack arrest, multiple crack renucleation, ligament formation and subsequent failure of the ligament. An examination of a few specimens that contain a mixture of regions that are single-colony thick and regions that are two-colonies thick illustrates that the fracture processes observed within colonies on the surface of the specimen are a direct consequence of the lamellar misorientation between the surface colony and the sub-surface colony.
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