On the origins of the dwell-fatigue lifetime variability in a Ti-6Al-4V forging

Over the past 50 years, premature aero-engine failures have occasionally been associated with titanium components subjected to dwell-fatigue loading. These infrequent events are linked to microstructural variability induced by prior thermo-mechanical processing. The presently reported study aimed at clarifying the consequences of heterogeneous microstructure characteristics in a Ti-6Al-4V forging on dwell-fatigue lifetimes. Extensive mechanical testing was combined with microstructure and texture characterization to highlight differences in behavior and clarify underlying causes. The competition between multiple failure mechanisms was evidenced. Microstructural characteristics and loading conditions favoring their prevalence were identified using complementary investigations of microstructure configurations at main crack initiation sites and analyses of crack nucleation kinetics. Collected data were then employed to perform a statistical analysis of the dwell-fatigue behavior sensitivity to microstructural and mechanical testing parameters. Lifetime predictions were derived and showed a good agreement with experimental measurements, indicating a reliable identification of most influential features. In particular, this study highlights the beneficial effect of high cooling rates on the dwell-fatigue resistance through mitigating microtexture intensity via the introduction of multiple crystallographic variants in transformed β regions.