EFFECTS OF MICROSTRUCTURE AND TEMPERATURE ON FATIGUE-CRACK GROWTH IN THE TIAL ALLOY TI-46.5AL-3NB-2CR-0.2W

The fatigue crack growth behavior of forged Ti-46.5Al-3Nb-2Cr-0.2W (at.%) was investigated. Heat treatments were used to generate both a nearly fully lamellar microstructure (grains of alpha(2)-gamma lamellae) and a duplex microstructure (equiaxed gamma and lamellar grains) to span the wide range of microstructural conditions available through thermomechanical processing of these alloys. Fatigue crack growth tests using load-shedding threshold and constant-load-amplitude techniques were conducted at room temperature, 600 degrees C (below the ductile-to-brittle transition temperature (DBTT)) and 800 degrees C (above the DBTT). Results show that the fatigue crack growth resistance of the lamellar microstructure is superior to that of the duplex microstructure. The nature of fatigue crack advance depends strongly on microstructure, which explains, at least in part, the differences observed in crack growth rates for the lamellar and duplex microstructures. Fractography was conducted to identify the dominant crack growth mechanisms in both the lamellar and duplex microstructures.