High-cycle fatigue of titanium-aluminum-vanadium: Investigation of mean stress sensitivity

An effort has been made to achieve a better understanding of the mechanisms of fatigue crack initiation and growth that are responsible for anomalous mean stress dependence of fatigue strength in Ti-6Al-4V alloy. The high cycle fatigue properties of Ti-6Al-4V with six different microstructure/texture combinations were investigated. Only material with lamellar microstructure exhibited linear Goodman relationship on the constant fatigue life diagram. Materials with coarse bimodal and equiaxed microstructures had pronounced mean stress sensitivity, with HCF strength at intermediate mean stresses being significantly lower than predicted by Goodman relationship. Cyclic strain tests in strain control mode did not reveal any significant differences in cyclic deformation behavior between the six investigated microstructures. Investigation of fatigue crack initiation process by replication technique showed that in most cases crack first initiates in the alpha grain, resulting in formation of flat alpha facet on the fracture surface. Results of fractography analysis suggest that cleavage plays a role in the fatigue crack initiation and early stages of crack propagation in these materials, and it is argued that crack initiates at the points of localized stress concentration due to the dislocations piling-up at the alpha grains unfavorably oriented for slip. This mechanism is accelerated in Ti-6Al-4V because of the planarity of slip which leads to abnormally low fatigue strength at intermediate mean stresses.