Effect of macrozones on fatigue crack initiation and propagation mechanisms in a forged ti-6Al-4V alloy under fully-reversed condition

In the present study, the mechanisms of fatigue crack initiation and propagation was investigated in a forged Ti-6Al-4V alloy with a bimodal microstructure through a combination of experiments and crystal plasticity finite element simulations. Transgranular crack initiations were observed on both basal and prismatic planes, in grains exhibiting high plastic activity. A fatigue indicator parameter based on the Tanaka-Mura model was found to correctly identify these critical grains. Intergranular crack initiations were found in pairs of grains sharing the same basal plane. An analysis of the grain boundary configuration suggested that these grains were separated by a (0001) tilt subgrain boundary as the c-axis was argued to belong to the boundary plane. Short crack propagation was found to be transgranular and crystallographic on either basal or prismatic plane depending on the microtextured regions they belong. Crack path was more serrated with high number of deflection and branching in regions favoring prismatic activity. However, no significant difference in terms of crack propagation rate was noticed between the different macrozones which was attributed to a relatively similar plastic activity in both regions. An analysis of the M-index finally suggested that the presence of a macrozone was not a necessary condition for crack initiation but appeared necessary for its propagation.