Fatigue crack growth behaviour in titanium aluminides

Fatigue crack growth behaviour of several titanium aluminides has been examined. With increase in Al content from 13 to 50 at.%, microstructures change from (α + β) to (α2 + β) to (α2 + γ) and finally to γ where α2 is Ti3Al and γ is TiAl. The exact microstructure for a given alloy depends on the composition, presence of ternary elements that stabilize β, such as Nb, and heat treatment. The degrees of slip planarity and brittleness increase with increase in Al content as one proceeds from α to α2 to equiatomic α. In addition, there is an increase in the propensity for faceted mode of crack growth with an increase in the degree of slip planarity. Fatigue threshold for a faceted crack growth depends on grain size or obstacle spacing. Hence crack growth rates can vary significantly for a given alloy depending on the microstructural scaling parameters, namely, equiaxed grain size or inter-obstacle spacing for Widmanstatten precipitates with basket-weave or lamellar structure. The load ratio effects have been analysed using the two parametric approach developed by the authors, and it is shown that the results can be explained without invoking an extraneous factor, namely crack closure.