Fatigue crack growth behavior of beta-annealed Ti–6Al–2Sn–4Zr–xMo (x = 2, 4 and 6) alloys: Influence of microstructure and stress ratio

A good understanding of the fatigue crack growth (FCG) behavior is an important precondition for the ‘run-on’ fatigue life assessment of service components and the formulation of the inspection and repair schemes. In this investigation, a comparative study was undertaken to characterize the long crack fatigue crack growth behavior in relation to the alloy types, different lamellar microstructures and stress ratios in the case of three α + β titanium alloys with nominal compositions of Ti–6Al–2Sn–4Zr–xMo (Ti624x, x = 2, 4 and 6), an important alloy system for aero engine application.

The effects of microstructure and stress ratio on the fatigue crack growth behavior in the three beta-annealed Ti alloys were separated through carefully-designed experiment. The changes in fatigue crack growth rate caused by increasing stress ratio were much larger compared to those due to microstructure variation. The most important microstructural parameter was identified as the α/β colony size which in turn depends strongly on the Mo-content. The results showed that the damage tolerance properties gradually decreased from Ti6242 to Ti6246. An increased stress ratio was found to reduce the roughness-induced crack closure (RICC) both through larger crack opening levels and through heavier crack bifurcation reducing crack deflection. A prominent transition on the da/dN vs. ΔK plots was observed in the extra-coarse microstructures of Ti6242 and Ti6244 alloys tested under R = 0.1, due to, in the early stage of Paris-region, long and straight cleavage propagation under low stress ratio causing strong RICC effect and obviously reducing the fatigue crack growth rates.