Microstructural influences on fatigue threshold behavior and fracture toughness of an additively manufactured γ-titanium aluminide

The fracture and fatigue behavior of an additively manufactured γ-TiAl alloy (BMBF3) produced by selective electron beam melting and further subjected to two different heat treatment conditions was investigated. As one of the heat-treatments led to a pronounced layer structure, the experiments were also performed with different specimen orientations to account for possible anisotropy effects. In order to characterize the frequently found peculiarities of the local fracture process in this material class, such as shear ligament bridging, static and cyclic R-curves were recorded. In addition, also fatigue crack growth curves were measured. The results were critically compared with the properties of a selected heat treatment condition of an established TNM alloy manufactured by a conventional processing route. The investigations show that large α ₂/γ-colonies lead to high fracture toughness, but have almost no positive effect on the long-crack fatigue threshold. On the other hand, duplex microstructures with small α ₂/γ-colonies and thick lamellae favor high long-crack thresholds, but low fracture toughness. Microstructures with high fractions of globular γ-phase exhibited both, low fracture toughness and low long-crack fatigue threshold values. In comparison to the established TNM reference alloy, the additively manufactured counterparts can compete especially when the evolving anisotropy of the properties is taken into account for component design.