PERGAMON BEHAVIOR OF GAMMA TiAl SUBJECTED TO IMPACT DAMAGE AND ELEVATED TEMPERATURE FATIGUE

Gamma titanium aluminide has received significant attention in recent years as a candidate material for use in aerospace and industrial gas turbine engine applications. In particular, these materials offer significant weight reductions (densities are less than half that of nickel-based superalloys), high specific strength retention at elevated temperature and high specific stiffness which is particularly important in vibrating components such as blades. This combination of weight savings and good mechanical properties has led to the possibility that �-TiAl may be a suitable replacement for nickel-based alloys, such as Inconel 718, in low pressure turbine blades without significant redesign of the blade [1]. However, the criticality of relatively low ductility, fracture toughness and fatigue crack growth resistance, must first be assessed. It is well known that fatigue crack growth rates in �-TiAl alloys are very sensitive to stress intensity range and that there is a small difference between threshold stress intensity range and apparent fracture toughness in these materials [2–5]. The result is limited damage tolerance and dramatic reductions in fatigue lifetime in the presence of extrinsic damage, such as that produced from an impact event. To apply a damage tolerance approach to this situation would require improved crack detection techniques and would increase the life cycle cost of the engine by decreasing the inspection interval. Using a threshold-based approach, on the other hand, would ensure that pre-existing or service induced cracks would not grow and that failure by fatigue would not occur [6]. The present study investigates the feasibility of using a threshold calculation to estimate the fatigue strength reduction caused by impact damage at elevated temperatures (600°C). The results are part of a larger investigation into the feasibility of using �-TiAl for low-pressure turbine blades.