Crack initiation mechanisms in Ti-6Al-4V subjected to cold dwell-fatigue, low-cycle fatigue and high-cycle fatigue loadings

Although Ti-6Al-4V is the most commonly used titanium alloy in the aerospace industry, the mechanisms governing crack initiation in the different fatigue regimes are not well understood yet. This situation partly pertains to a competition between multiple crack initiation mechanisms. In particular, applied loading conditions were identified as a key parameter governing the transition between mechanisms. The fatigue behavior of Ti-6Al-4V with a bi-modal microstructure was investigated in the present study using different waveforms, load ratios and frequencies to clarify this feature. A detailed characterization of the main crack initiation sites was carried out to identify microstructural configurations governing crack nucleation in low-cycle dwell-fatigue, low-cycle fatigue and high-cycle fatigue regimes. While lifetimes and fracture surfaces showed a good consistency with data from prior studies, a single microstructural configuration was found involved with the formation of crack initiation facets. All investigated cracks leading to specimen failure were nucleated along (0001) twist grain boundaries exhibiting similar features. Based on this finding, a criterion is proposed to identify candidates for crack initiation. This also demonstrates no major sensitivity of the critical microstructural configurations to environment, free surface, loading conditions and, as shown in previous studies, microstructure and composition. However, conventional fatigue failure results from one, or a few, surface or subsurface crack initiation sites while dwell-fatigue failure involves the formation of multiple internal cracks. This is accompanied by a significant dwell-fatigue life debit at peak stress magnitudes close to the yield strength. Features of microstructural configurations prone to crack nucleation are finally compared with data reported in existing literature to propose a mechanism of crack formation in Ti alloys.