The microstructure and high cycle fatigue performance of as-cast and directionally solidified Ti46Al7Nb alloy under the three-point bending loading

This study systematically investigated the microstructure and high cycle fatigue deformation behavior of a high-Nb TiAl alloy with a nominal composition of Ti46Al7Nb alloy prepared by cold crucible directional solidification (CCDS) method through TEM, SEM, etc. Macro/microstructure and phase component of as-cast and directionally solidified alloys were characterized. The three-point bending fatigue performance at room temperature was tested, and the corresponding fracture and deformation mechanisms were also revealed. Results show that the columnar grains of the directionally solidified TiAl alloy are well-arranged in the direction of the heat flow. Compared with the as-cast alloy, the segregation of the directionally solidified microstructure is significantly improved. After 10⁷ cycles, the three-point bending fatigue limit of the directionally solidified Ti46Al7Nb alloy is about 60 MPa higher than that of the as-cast state. Fracture morphology shows that cleavage fracture is the main fracture mode of the as-cast alloys. Quasi-cleavage fracture composed mainly of trans-lamellar fracture, supplemented by small amount of plastic fracture, is the major fracture mode of directionally solidified Ti46Al7Nb alloy. The fatigue deformation of this alloy mainly occurs in the γ lamellae. Dislocation multiplication and stress-induced generation of subgrain are the main mechanisms of high cycle fatigue deformation of Ti46Al7Nb alloy. Screw dislocations were also found in the γ subgrains generated after cyclic deformation. This research provides a better understanding of the high cycle fatigue deformation of TiAl alloys.