ROOM TEMPERATURE FATIGUE RESPONSE OF �-TiAl TO IMPACT DAMAGE

Gamma based titanium aluminides have received considerable attention recently as candidate materials in gas turbine applications, particularly low pressure turbine blades (1–3). Their low density and high specific stiffness, result in potentially significant weight savings in structures such as gas turbine engines if substituted for current materials. Two �-TiAl microstructures are predominately identified in the literature: a lamellar microstructure consisting of alternating plates of � and �2, and a duplex microstructure consisting of equiaxed � grains and lamellar colonies (3,4). Lamellar alloys generally exhibit better fracture toughness and fatigue crack growth resistance (5–7). Duplex alloys, on the other hand, exhibit superior ductility and tensile strength, but lower fatigue crack growth resistance (8). In both microstructures fatigue crack growth behavior is characterized by a strong dependence on cyclic stress intensity range, �K (5). This sensitivity can lead to very short lifetimes if initiation lifetime is negated by defects produced by foreign object or manufacturing-related impact damage. The objective of the present study is to characterize and quantify the damage resulting from low-speed impacts on �-TiAl and to relate this impact damage to reductions in fatigue failure stress by the use of a threshold-based model.