Low cycle fatigue behavior of commercial purity titanium in liquid nitrogen

The low cycle fatigue (LCF) behavior of commercial purity titanium was studied at 77 K. The results showed that the cyclic stress–strain curve of the material was above the monotonic stress–strain curve and cyclic stress amplitudes increased monotonically with cyclic strain amplitudes or number of cycles, indicating that cyclic hardening took place during the fatigue process. The curve of the plastic strain amplitude (εp) versus the number of cycles to fracture (Nf) at 77 K was above that at room temperature. This implied that the LCF resistance of commercial purity titanium at 77 K was superior to that at room temperature. It was found that the area fraction of twins increased with the number of cycles and cyclic strain amplitude and the highest area fraction of twins in fatigued specimens with different strain amplitudes approached the same value of about 80%. Moreover, the location of fatigue crack initiation and the mechanism of fatigue crack propagation changed markedly as the cyclic strain amplitude increased. At the cyclic strain amplitude of 1.4%, the fatigue cracks initiated either at the specimen surface or the subsurface and propagated with fatigue striations. However, at the cyclic strain amplitude of 6.6%, the fatigue cracks initiated only at the specimen surface and propagated in tearing ridges and microvoids on the fatigue fractured surface.