ELEVATED-TEMPERATURE CRACK-GROWTH RESISTANCE OF TIAL UNDER MONOTONIC AND CYCLIC LOADING

Effects of microstructure and environment on elevated temperature fatigue crack growth rates have been studied in Ti-48Al-2Mn-2Nb (at.%). Microstructures containing predominantly alpha(2)/gamma two phase lamellar regions and small amounts of allotriomorphic gamma grains possess much higher fracture toughness and increased crack growth resistance than those containing predominantly twin-related and allotriomorphic gamma grains. Such gamma grains may fail by transgranular cleavage at ambient temperature and by intergranular decohesion at temperatures of 700 and 800-degrees-C, under both fatigue loading and during unstable fast fracture. Such "static" failure modes increase the dependence of crack growth rate on the alternating stress intensity factor and microstructures containing significant amounts of gamma grains should not be subjected to fatigue loading at elevated temperatures.