Fatigue crack propagation resistance of ductile TiNb-reinforced γ-TiAl intermetallic matrix composites

An experimental study has been made to investigate the cyclic crack propagation resistance of ductile-particle toughened brittle materials, specifically, ductile TiNb-reinforced γ-TiAl intermetallic composites, as a function of microstructure. Under cyclic loading, cracks are found to grow subcritically at stress intensities of 4–5 MPa m 1 2 , far below their maximum toughness levels. Such behavior is associated with the susceptibility of the ductile TiNb phase to fatigue failure, and consequently to the diminished role of crack tip shielding from crack bridging by unbroken TiNb ligaments, in contrast to observations under monotonic loading. No evidence for bridging is seen under cyclic loading and bridging zone lengths in the wake of the crack tip are limited to less than 150 μm, compared with 4–5 mm under monotonic loading. Moreover, crack growth rates are very sensitive to applied ΔK level, with measured exponents for the da/dN-ΔK relationship ranging from 10 to 20. The exponent decreases with an increase in volume fraction of the ductile phase, but is independent of particle thickness; fatigue thresholds are, however, less affected.