Effects of morphology and volume fraction of alpha(2) phase on the fatigue crack propagation of a Ti-24Al-11Nb alloy

Abstract

The main objective of the present investigation is to study the individual effect of alpha(2) morphology and volume fraction on the fatigue crack propagation behavior of a two-phase (alpha(2) + beta) Ti-24Al-11Nb alloy. With systematic heat treatments, three distinct morphologies have been obtained: fine colony, fine equiaxed, and fine basket-weave structures. Volume fraction of alpha(2) has been varied from 40 to 68 pct by changing the two-phase annealing temperature. It has been shown that the volume fraction and morphology of alpha(2) phase have a large influence on the fatigue crack propagation behavior. Fine colony structure shows no variation in the fatigue crack propagation rate with the alpha(2) volume fraction. On the other hand, equiaxed and basket-weave structures show the increased resistance to fatigue crack propagation as alpha(2) volume fraction decreases. Such increased resistance to crack growth is attributed primarily to tortuous crack paths, which result in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanism. Quantitative metallography and experimental crack closure measurements are presented to substantiate such interpretations.