Microstructural influences on fatigue crack growth in the near alpha titanium alloy Timetal 834

Fatigue crack growth rates were measured employing a corner crack testpiece under laboratory conditions in the near alpha titanium alloy Timetal 834. The alloy was heat treated to produce two different microstructures: a bi-modal form typical of aerospace applications containing approximately 15% primary alpha in a transformed matrix and a coarse grained, fully transformed Widmanstatten microstructure. Both microstructures demonstrate a significant sensitivity to R value. Distinct growth rate regimes relating to short and long crack behaviour were defined in both structures, however, short crack phenomena were most prevalent in the transformed microstructure. Facet formation and transgranular crack growth was found to be prevalent in the earliest stages with a cyclic striation type mechanism dominating the long crack regime. Implications of these findings for defect tolerant component lifing are discussed.