FATIGUE-CRACK GROWTH-BEHAVIOR IN TITANIUM ALUMINIDES

Fatigue crack growth behaviour of several titanium aluminides has been examined. With increase in Al content from 13 to 50 at.%, microstruct ures change from (alpha + beta) to (alpha(2) + beta) to (alpha(2) + ga mma) and finally to gamma where alpha(2) is Ti3Al and gamma is TiAl. T he exact microstructure for a given alloy depends on the composition, presence of ternary elements that stabilize beta, such as Nb, and heat treatment. The degrees of slip planarity and brittleness increase wit h increase in Al content as one proceeds from alpha to alpha(2) to equ iatomic alpha. In addition, there is an increase in the propensity for faceted mode of crack growth with an increase in the degree of slip p lanarity. Fatigue threshold for a faceted crack growth depends on grai n size or obstacle spacing. Hence crack growth rates can vary signific antly for a given alloy depending on the microstructural scaling param eters, namely, equiaxed grain size or inter-obstacle spacing for Widma nstatten precipitates with basket-weave or lamellar structure. The loa d ratio effects have been analysed using the two parametric approach d eveloped by the authors, and it is shown that the results can be expla ined without invoking an extraneous factor, namely crack closure.