Effects of morphology and volume fraction of α 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 α 2 morphology and volume fraction on the fatigue crack propagation behavior of a two-phase (α 2+β) 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 α 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 α 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 α 2 volume fraction. On the other hand, equiaxed and basket-weave structures show the increased resistance to fatigue crack propagation as α 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.