Transmission electron microscopy investigation of fatigue crack tip plastic zones in a polycrystalline gamma-TiAl-based alloy

Deformation at the tips of fatigue cracks has been studied in a polycrystal line, fully lamellar TiAl based alloy using transmission electron microscop y. A tiny, intensely deformed plastic zone containing twins, slip bonds and microcracks is generated during fatigue crack growth. Twins are activated extensively, for a wide range of Schmid factors. When the Schmid factor for slip is large, the resulting slip band is compact and well defined. Such s lip bands tend to be associated with microcracks. When the Schmid factor fo r slip is small, the resulting slip band is diffuse and ill defined. Such s lip bands are not associated with microcracks. Microcracks form, not only a long well defined slip bands, but also along alpha (2)-gamma interfaces and , less willingly, along gamma-gamma twin interfaces. The former, however, p resent more resistance to translamellar deformation than do the latter. Def lection of microcracks from interlamellar and twin mode to slip band mode a nd vice versa, and the transfer of translamellar deformation from lamella t o lamella, can be understood by reference to the competing effects of the e xternally applied stress (via the Schmid factor) and neighbouring pile-ups and microcracks.