On the contrasting role of ductile-phase reinforcements in the fracture toughness and fatigue-crack propagation behavior of TiNb/γ-TiAl intermetallic matrix composites

A comparative study has been made on the role of ductile-phase toughening under monotonic and cyclic loading at room temperature, specifically involving the fracture toughness and fatigue-crack propagation behavior of a γ-TiAl intermetallic alloy reinforced with 10 vol.% of ∼ 50 μ-sized ductile TiNb particles. Under monotonic loading, substantial toughening is achieved in the composite primarily from crack bridging by uncracked TiNb particles in the wake of the crack tip; additional contributions are provided by shielding from crack-particle interactions. Compared to a K Ic value of ∼ 8 MPA√m for monolithic γ-TiAl, the composite shows significant resistance-curve behavior with a steady-state fracture toughness exceeding 25–30 MPa√m; in this regime, measured bridging zones approach ∼ 4 mm in length. In contrast, under cyclic loading, such ductile-particle toughening is found to be far less effective, due to the susceptibility of the ductile metallic phase to fatigue failure. No evidence of crack bridging by TiNb particles is observed to within ∼ 150 μm of the crack tip, and fatigue-crack propagation is observed at a stress intensity as low as 6 MPa√m, far below that required to initiate cracking under monotonic loading.