Fracture and R-curve behavior of an intermetallic β-stabilized TiAl alloy with different nearly lamellar microstructures

• Fracture and R-curve behavior of 4 nearly lamellar TiAl microstructures was studied.
• Large twist angles between colonies lead to high R-curves.
• Grain boundaries with cellular reaction and γ-grains enhance fracture toughness.
• A high amount of βO phase along colony boundaries decreases fracture toughness.
• Intrinsic and extrinsic toughening mechanisms were identified.

Four different microstructures of an engineering multi-phase Ti–43.5Al–4Nb–1Mo–0.1B alloy (in at.%) were fracture mechanically tested under monotonic loading conditions from room temperature up to 700 °C. Monotonic loading crack resistance curves were measured using the potential drop technique and the calculated crack lengths were compared to optically measured crack lengths during in-situ experiments under the light-optical microscope. By examining the fracture surfaces and the polished side faces of the samples in the scanning electron microscope the fracture mechanisms were evaluated. It was found that the fracture toughness mainly depends on structure and phase arrangement at the boundaries and can be enhanced up to 30% by having thick boundary areas with a few micrometers in width, consisting of unconnected βO and γ grains. In contrast, a connected βO-phase at the colony boundaries as well as a texture which provide lamellae parallel to the subsequent fracture surface reduce the fracture toughness.