On the growth of small fatigue cracks in γ-based titanium aluminides

Based on an experimental study of the fatigue-crack growth behavior of long ( > 5 mm) through-thickness and small (c < 300 mu m) surface cracks in duplex and lamellar microstructures in a Ti-47Al-2Nb-2Cr-0.2B (at.%) alloy at 25 deg C, the following conclusions can be made: small-crack growth rates in both microstructures are faster than those of corresponding long cracks at the same applied Delta K levels; moreover, small cracks are found to propagate at applied Delta K levels below the long-crack threshold Delta K sub TH . This effect is primarily attributed to the role of crack-tip shielding, principally from uncracked ligament bridging and crack closure, in impeding long-crack growth rates (similitude limitation), as closer correspondence between long- and small-crack data is achieved after "correcting" for such shielding. Small-crack growth, however, is still evident below this lower-bound threshold in lamellar structures even after such normalization, due to statistical sampling of the coarser microstructure (continuum limitation). A comparison of long-crack data suggest that the fatigue crack-growth resistance of the lamellar structure is significantly greater than that of the duplex structure, with growth rates being up to five orders of magnitude lower at equivalent Delta K levels. A comparison of small-crack data, conversely, indicates marginally superior fatigue crack-growth resistance in the duplex structure (although the scatter bands for the two structures overlap). Duplex microstructures appear to offer better properties from the perspective of their potential use in fatigue-critical applications. In addition to their higher strength and ductility, the definition of a shielding-corrected (lower-bound) Delta K sub TH,eff threshold, below which both small- and long-cracks are dormant, appears feasible. Such an approach is less certain for the coarser lamellar microstructures where small cracks of grain-size dimensions continue to propagate below this lower-bound threshold (a continuum limitation).