Fractographic study on naturally initiated short fatigue cracks in a near-lamellar TiAl alloy at room temperature

Short crack phenomena are considered important for lamellar structures in γ-TiAl alloys and have been thoroughly investigated in the past. However, the short cracks in the previous studies were nearly all introduced artificially. No particular investigations have looked into the initiation of fatigue short cracks. Therefore, naturally initiated short fatigue cracks at room temperature under two different stress ratios (0.1 and 0.5) were investigated in a near-lamellar γ-TiAl alloy (Ti-45Al-2Mn-2Nb) in this study. The observations show that the fatigue crack initiation behaved differently at low and high stress ratios. At low stress ratio, the specimens failed at lower ultimate stress levels (σ_max = 450 and 475 MPa), and the crack initiated from the cluster of interlamellar fracture near mode-I orientation or stress concentration areas. At the higher stress ratio, the specimens failed at higher but consistent stress levels (σ_max = 560 and 570 MPa), and in the specimen crack initiation areas, the interlamellar fractures were still the primary fracture mode, whereas some were found at tilted angles due to shear deformation. The results suggest that short fatigue cracks can naturally initiate in lamellar γ-TiAl alloys, thus attention should be paid to their microstructure design, surface finishing and cleanliness.