The contrasting role of microstructure in influencing fracture and fatigue-crack growth in gamma-based titanium aluminides at large and small crack sizes

摘要

Ambient and elevated temperature fracture and fatigue-crack propagation behavior, for both large (>5 mm) and small (<500 mum) cracks, has been studied in gamma-based TiAl alloys, specifically involving single-phase gamma, duplex and lamellar microstructures, the latter being examined over a range of colony and lath dimensions. Specifically, in the presence of large cracks, coarse lamellar microstructures are found to exhibit by far the best ambient temperature cyclic and monotonic crack-growth properties, compared to duplex and single-phase gamma microstructures. At elevated temperatures, lammellar microstructures also show increasing crack-initiation toughness, although rising resistance-curve behavior is not seen above the ductile-brittle transition temperature (DBTT approx 700 deg C). In the presence of small cracks, however, the large differences in fatigue-crack growth properties of the duplex and lamellar structures are no longer evident; moreover, the small cracks grow at applied driving forces lower than the large-crack thresholds, below which cracking is presumed to be dormant. Such behavior is rationalized in terms of (i) the minimal effect of crack-tip shielding, by crack closure and uncracked ligament bridging, with cracks of limited wake, and (ii) the biased microstructural sampling, associated with cracks small compared to the characteristic microstructural dimensions.