RATE AND ENVIRONMENTAL-EFFECTS ON FRACTURE OF A 2-PHASE TIAL-ALLOY

The influence of strain rate and environment on the fracture behavior of a two-phase TiAl-alloy, Ti-47Al-2.6Nb-2(Cr + V), heat-treated to a nearly fully lamellar microstructure has been studied by performing conventional tensile, compression, and fracture toughness tests in air, argon, and vacuum at 25-degrees-C and 800-degrees-C. Both tensile and compression tests were conducted at strain rates of 1 x 10(-3) and 1 x 10(-5) s-1, and fracture toughness tests were performed under displacement rates of 0.25 to 2.5 mm/min. In addition, in situ fracture toughness tests were conducted at slow rates both in vacuum and in air. The results indicated that both strain rate and environment affected the tensile stress-strain behavior and ductility and the fracture resistance of the TiAl-alloy at 800-degrees-C. In contrast, neither the tensile ductility nor the fracture toughness was significantly affected by the environment at ambient temperature. For compression in air, the stress-strain behavior was insensitive to both strain rate and test temperature within the conditions tested. Studies of fracture surfaces revealed that low tensile ductility in this alloy at ambient temperature is associated with the tendency to delaminate along gamma/gamma and gamma/alpha2 interfaces.