High temperature low cycle fatigue properties of Ti-48Al-2W-0.5Si gamma titanium aluminide

The low cycle fatigue properties of the γ-TiAl alloy Ti-48Al-2W-0.5Si (at.%) have been examined. Seven batches of the material with slightly varying compositions were used, elucidating the large influence of the microstructure on the low cycle fatigue properties and fracture behaviour. A duplex fine grained microstructure shows superior properties in comparison to a material low in aluminium with a coarse grained nearly lamellar structure due to larger isotropic hardening and smaller Bauschinger effect, which lead to smaller inelastic strains and damage in each cycle. The fractographic study showed that failures are mostly initiated at the surface of the test specimens. Interlamellar crack initiation is predominant in the nearly lamellar structure, whereas the duplex material shows initiation mainly in large γ-grains or internally in interdendritic γ-areas. The larger scatter in fatigue life of the nearly lamellar material is explained by the anisotropic properties of the lamellar colonies in combination with the large colony size. The probability to have a large lamellar colony with the lamellas oriented perpendicular to the loading axis producing premature failure is increased as the content of this microconstituent is increased. Even if the duplex structure has the lowest fracture toughness of the two types of microstructures studied here, its grain size is smaller than the critical crack length for instant failure, leading to some stable fatigue crack growth and thus explaining the smaller scatter in fatigue lives for this microstructure.