Fracture Fundamentals in Titanium Aluminides.

Abstract : Fracture toughness and fatigue-crack propagation behavior at temperatures between ambient and 800 deg C has been investigated in a wide range of (gamma + alpha2) TiAl microstructures, including single-phase gamma, duplex, coarse lamellar (1-2 mm colony size), fine lamellar (^150 micron colony size), and a P/M lamellar microstructure (^65 micron colony size, ^0.1 micron lamellar spacing). The influences of colony size, lamellar spacing and volume fraction of equiaxed gamma grains are analyzed in terms of their effects on resistance to the growth of large (>5 mm) cracks. Specifically, coarse lamellar microstructures are found to exhibit the best cyclic and monotonic crack-growth properties, while duplex and single-phase gamma microstructures exhibit the worst, trends which are rationalized in terms of the salient micromechanisms affecting growth. These mechanisms primarily involve crack-tip shielding (R-curve toughening) processes, and include crack closure, crack deflection, and uncracked ligament bridging. However, since the potency of these mechanisms is severely restricted for cracks with limited wake, in the presence of small (<500 microns) cracks the distinction in the fatigue-crack growth resistance of the lamellar and duplex microstructures becomes insignificant. With respect to temperature, resistance to fatigue-crack growth, as characterized by the fatigue threshold, is found to be superior at 800 deg C, yet inferior at 600 deg C, compared to room temperature behavior. This anomalous temperature effect is ascribed to a dominant role of oxide-induced crack closure at 800 deg C, which acts to retard near-threshold growth rates and to lead to premature arrest of crack growth at a higher threshold value.