Effect of lamellar structure on fracture behavior of TiAl polysynthetically twinned crystals

The fracture behaviors of Ti-49.1at.%Al and Ti-50.8at.%Al polysynthetically twinned crystals with fine and coarse lamellae, respectively, have been investigated, focusing on the effect of the lamellar structure and loading axis. Type A and B specimens, in which the notch directions were parallel and perpendicular to lamellar planes, respectively, were prepared and compact tension tests were performed at room temperature. The fracture behavior exhibited strong anisotropy, depending on the crack paths in the lamellae. For type A specimens, the crack propagated very easily parallel to the lamellae, exhibiting low KQ values, and a cleavage-like fracture on the (0001) plane in the α2 phase was observed. In type B specimens, the crack propagated across the lamellar boundaries, exhibiting high KQ values, and the boundaries acted as an effective barrier to further crack propagation. Fine lamellae were beneficial in improving the fracture toughness.