The Role of Microstructure on High Cycle Fatigue Lifetime Variability in Ti-6Al-4V

The microstructural sources of fatigue lifetime variability were investigated for four different microstructural variations of Ti-6Al-4V. Specimens were tested at lower stresses to investigate the behavior in the HCF (high cycle fatigue) regime, which is characterized by lifetimes near or in excess of 106 cycles. Fractography and replication analyses confirmed that the lifetime was dominated by crack nucleation, and thus variations in the lifetime between individual test specimens are primarily attributed to variability in the time to nucleate a dominant crack.;Stereology was used to quantify key microstructural features for each tested specimen. These values were used as inputs for a series of microstructurally-based fuzzy logic neural network models. Using these models, virtual experiments were conducted to investigate the individual effect of each microstructural feature on the lifetime, an investigation that is impossible to conduct empirically because of the complex microstructure in these alloy systems. These virtual experiments demonstrated that colony size and alphalath thickness have the greatest effect on HCF lifetime of beta-processed Ti-6Al-4V alloys, and that colony size is more important that alpha lath thickness. For the a/beta -- processed microstructures, the volume fraction of primary alpha and the alpha lath thickness were shown to affect the lifetime, while the alpha p grain size was not.;Defect analyses of failed specimens indicated that damage accumulation is often localized during cyclic loading, with dislocation densities varying from one a lath to another. For all specimens, -type dislocations are seen and - type dislocations were observed only in regions of localized plastic strain. Investigation of site-specific TEM foils extracted from the crack nucleation region of a/beta -- processed specimens provided information about the nature and behavior of dislocations during the crack nucleation event. A comparison of short- and long- life specimens provides information about differences in the evolution of the dislocation structure prior to crack nucleation.;The potential of this combinatorial approach for future fatigue lifetime investigations is discussed. In particular, the project demonstrates that such an approach could be useful in developing a quantitative understanding of the role variations in microstructural features have on variations in HCF lifetime.