Low cycle fatigue behavior and deformation mechanism of Ti-6Al-4V-0.55Fe alloy under the control of strain and stress amplitudes

The present study focuses on investigating the low cycle fatigue (LCF) behavior and cyclic deformation mechanism of Ti-6Al-4V-0.55Fe alloy under strain and stress control. Based on the results of this alloy under strain amplitudes and the corresponding stress amplitudes, the effect of tension-compression asymmetry (TCA) on fatigue behavior and microstructure evolution was systematically discussed with transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The results show that under strain control, TCA has little effect on LCF behavior due to the negligible compressive mean stress. And the deformation mechanism is mainly determined by planar slip. While under stress control, the accumulation of significant unidirectional ratcheting strain caused by TCA is observed, resulting in extra heterogeneous plastic deformation and more fatigue damage, responsible for the reduction in fatigue life. However, under low stress amplitude (<= 690 MPa), no ratcheting behavior occurs due to the constant strain amplitude (0.56%), which reduces the driving force of crack nucleation and prolongs fatigue life, compared with that under low strain amplitude (0.6%).