Effect of strain amplitude on low cycle fatigue and microstructure evolution in Ti-6Al-4V: A TKD and TEM characterization

Abstract

Low cycle fatigue (LCF) tests were carried out on mill annealed Ti-6Al-4V for 100 cycles at various strain amplitudes. The strain amplitudes were chosen such that, cyclic stress remains (i) completely in the elastic regime (0.006), (ii) onset of the yield point (0.008), and (iii) plastic regime (0.01). Cyclic stress softening was observed at higher strain amplitude (0.008 and 0.01), whereas no change in peak stress was observed for lower strain amplitude (0.006). Cyclic softening was observed mainly due to decrease in friction stresses with the number of cycles. The microstructural analysis using transmission electron microscope (TEM) and transmission Kikuchi diffraction (TKD) suggest the formation of slip band, hexagonal network and sub-grain boundary. The lower strain amplitude evidences the formation of slip bands and dislocation rearrangement, although the cyclic stress exhibits complete elastic behavior. In the case of higher strain amplitude, the development of defect structure and annihilation/rearrangement of dislocations occurs simultaneously. The cyclic deformation at higher strain amplitude favors the rearrangement of dislocation and formation of LABs or sub-grain boundary. The low angle boundaries appeared to originate from the alpha/beta interface and are propagated inside the a grains. It was observed that alpha/beta interface does not follow Burgers orientation relationship and acts as an obstacle for slip transfer resulting in LABs formation. Such microstructural development at a higher strain amplitude reduces the friction stress which results in cyclic softening.