FATIGUE THRESHOLDS OF CRACKS RESULTING FROM IMPACT DAMAGE TO �-TiAl

Significant research has been conducted in recent years on the �-based titanium aluminide alloys. The low density of �-TiAl relative to current nickel-based superalloys, has made it an attractive candidate for use in the low-pressure turbine blades of gas turbine engines [1]. In addition to weight savings, the lower density leads to higher specific stiffness and good high temperature strength retention [1,2,3]. However, the ordered crystal structure, typical of intermetallics, leads to inherently low room temper­ ature ductility and fracture toughness. This raises serious concerns about the ability of this material to resist the formation of cracks at impact damage sites, cracks that could subsequently grow in fatigue and lead to failure of the turbine blade. Of particular concern are cracks, which are perhaps too small to be detected, but may still grow under high cycle fatigue conditions. For large cracks, a long-crack fatigue crack growth threshold (�KTH) is an appropriate design criterion to ensure infinite fatigue life. However, in the case of very small damage levels on the order of a few equiaxed gamma grains or less, �KTH fails to accurately portray the threshold for crack propagation [4,5,6]. For a fine grained duplex microstructure, the sub-threshold growth of small cracks can be collapsed with long-crack data by using the closure corrected �Keff stress intensity to define the crack tip driving force. Small cracks are generally dormant at stress intensities below �Keff,TH. However, experiments on a coarse lamellar microstructure suggested that scatter in the small crack threshold was still observed despite correction for closure due to statistical sampling of the larger lamellar colonies [7]. Consequently, the relative size of the dominant microstructural feature can be crucially important in determining the relevant fatigue threshold stress intensity range. The present study investigates the change in fatigue crack threshold with the size of cracks resulting from simulated impact damage in a duplex �-TiAl alloy tested at 600°C. This work is part of a larger effort to support the transition of �-TiAl to industrially relevant gas turbine engine applications.