Very high-cycle fatigue behaviour of Ti-6Al-4V alloy under corrosive environment

Rotary-bending fatigue tests of Ti-6Al-4V alloy at very high-cycle regime were performed in air and 3.5% NaCl solution, respectively. The S-N curves exhibited the characteristic of horizontal asymptote (traditional fatigue limit) in laboratory air, while a lower fatigue limit was shown in 3.5% NaCl solution after around 3x10(8) cycles. The decrease of fatigue life was due to the generation of small surface defects in comparison with that in air. These defects were ascribed to the hydrogen-induced cracking and fatigue loading. The number of fatigue crack initiation sites decreases with increasing cycles. Only one crack origin site, dominated by slip mechanism, exists in each specimen at very high-cycle regime. The existing experimental S-N data of Ti-6Al-4V alloy from very high-cycle fatigue tests were also reviewed. It was shown that the fatigue life (R=-1) of Ti-6Al-4V alloy was predominantly influenced by the microstructure and not strongly sensitive to the loading mode and frequency (in the range from 52.5Hz to 20kHz) under air condition at room temperature.