Effects of microstructural parameters on the fatigue crack growth of fully lamellar γ-TiAl alloys

A study has been made to investigate the effects of colony size and lamellar spacing on the fatigue crack propagation behavior of fully lamellar γ-TiAl alloys. It has been found that the overall crack growth rates of fully lamellar microstructure are not largely affected by the variation of the colony size up to ≈400 μm, though its respective roles on the intrinsic and extrinsic nature of crack growth resistance are quite different. However, in the coarse colony microstructure (≈1400 μm), the fatigue crack growth threshold (ΔKth) is markedly decreased, while the crack growth resistance remains constant. The lamellar spacing is proved to be the more important factor to control both the fatigue crack initiation and growth resistance at room temperature. The fine lamellar spacing (0.2–0.7 μm) microstructures represent superior ΔKth and fatigue crack growth resistance compared to the coarse lamellar spacing (≈5.5 μm) microstructure. This superior fatigue resistance is mainly attributed to the higher number of lamellar interfaces resistant to crack advance, as well as to the higher closure effects. The colony boundaries and the lamellar interfaces play an important role in retarding the advancing crack at room temperature, serving as barriers for the dislocation movement and as sinks for dislocation pile-ups.