HIGH-TEMPERATURE FRACTURE AND FATIGUE RESISTANCE OF A DUCTILE b-TiNb REINFORCED g-TiAl INTERMETALLIC COMPOSITE

ÐThe high-temperature fatigue-crack propagation and fracture resistance of a model g-TiAl intermetallic composite reinforced with 20 vol.% ductile b-TiNb particles is examined at elevated temperatures of 650 and 8008C and compared with behavior at room temperature. TiNb reinforcements are found to enhance the fracture toughness of g-TiAl, even at high temperatures, from about 12 to 040 MPa m, although their e€ectiveness is lower compared to room temperature due to the reduction in strength of TiNb particles. Under monotonic loading, crack-growth response in the composite is characterized by resistance-curve behavior arising from crack trapping, renucleation and resultant crack bridging e€ects attributable to the presence of TiNb particles. In addition, crack-tip blunting associated with plasticity increases the crack-initiation (matrix) toughness of the composite, particularly at 8008C, above the ductileto-brittle transition temperature (DBTT) for g-TiAl. High-temperature fatigue-crack growth resistance, however, is marginally degraded by the addition of TiNb particles in the C±R (edge) orientation, similar to observations made at room temperature; premature fatigue failure of TiNb ligaments in the crack wake diminishes the role of bridging under cyclic loading. Both fatigue and fracture resistance of the composite are slightly lower at 6508C (just below the DBTT for TiAl) compared to the behavior at ambient and 8008C. Overall, the bene®cial e€ect of adding ductile TiNb reinforcements to enhance the room-temperature fracture and fatigue resistance of g-TiAl alloys is retained up to 8008C, in air environments. There is concern, however, regarding the long-term environmental stability of these composite microstructures in unprotected atmospheres. # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.