The smooth and notched three-point bending fatigue behavior of directionally solidified high-Nb TiAl alloy

To understand the fatigue behavior of directionally solidified (DS) TiAl alloys, the Ti46Al7Nb alloy fabricated by cold crucible DS method was investigated by microstructure characterization and three-point bending fatigue tests. Results show that the microstructure of the alloy is composed of alpha 2/gamma lamellae with 13 segregation, and the angle between lamellar arrangement and DS growth direction is 45 degrees. Smooth and notched specimens were prepared to evaluate the fatigue performance of the TiAl alloy under pre-damage conditions. The finite element results indicate that the notched have more obvious stress concentration. The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. The crack propagation path indicates that there are cleavage microcracks with different shapes around the main crack in the notched specimen, while the main crack in the smooth one is surrounded by many corrugated cracks perpendicular to the lamellar direction. The whole process of fatigue crack initiation and propagation under three-point loading is established. Fracture morphology shows that notched specimens are mostly brittle cleavage fracture along the lamellar facets, while the smooth are trans-lamellar fracture with plastic characteristics. The degree of fatigue deformation of the smooth specimen is greater than that of the notched one, which mainly reflects on high density dislocation and dislocation intersection, as well as more sub-grain boundaries and twins.