Fatigue-crack growth behavior in a dual-phase ({gamma}+{alpha}{sub 2}) TiAl intermetallic alloy at elevated temperatures

Fatigue-crack propagation behavior in a Ti-47.3Al-2.3Nb-1.5Cr-0.4V (at.%) alloy is examined at 650 and 800 C in two markedly different microstructural conditions, namely a duplex (fine, equiaxed) and fully-lamellar (coarse, microlaminated) microstructures; results are compared with those obtained at room temperature. It is found that the lamellar microstructure exhibits increased fatigue resistance compared to the duplex structure at all temperatures, principally due to shielding from crack deflection, microcracking and shear-ligament bridging, although the effect of the bridging is progressively diminished under cyclic loads. The extent of improvement, however, is significantly lower during cyclic fatigue compared to monotonic fracture. For both ({gamma}+{alpha}{sub 2}) microstructures, maximum resistance to fatigue-crack growth is seen at 800 C; growth rates, however, are faster at 650 C than at ambient temperatures. Such variations in fatigue resistance with test temperature are attributed to intrinsic differences in crack-tip deformation at 800 C and 650 C, that respectively correspond to temperatures above and below the ductile-to-brittle transition temperature for {gamma}-TiAl ({approximately}700 C).