DUCTILE-REINFORCEMENT TOUGHENING IN GAMMA-TIAL INTERMETALLIC MATRIX COMPOSITES - EFFECTS ON FRACTURE-TOUGHNESS AND FATIGUE-CRACK PROPAGATION RESISTANCE

The influence of the type, volume fraction, thickness and orientation of ductile phase reinforcements on the room temperature fatigue and fracture resistance of gamma-TiAl intermetallic alloys is investigated. Large improvements in toughness compared to monolithic gamma-TiAl are observed in both the TiNb- and Nb-reinforced composites under-monotonic loading. Toughness increases with increasing ductile phase content, reinforcement thickness and strength; orientation effects are minimal. Crack-growth behavior is characterized by steep resistance curves primarily due to crack trapping/renucleation and extensive crack bridging by the ductile-phase particles. In contrast, under cyclic loading the influence of ductile phases on fatigue resistance is strongly dependent upon reinforcement orientation. Compared to monolithic gamma-TiAl, improvements in fatigue-crack growth resistance are observed in TiNb-reinforced composites only in the face (C-L) orientation; crack-growth rates for the edge (C-R) orientation are actually faster in the composite. In comparison, Nb-particle reinforcements offer less toughening under monotonic loading but enhance the fatigue properties compared to TiNb reinforcements under cyclic loading.