THE EFFECT OF HYDROGEN ON THE HIGH CYCLE FATIGUE STRENGTH OF TI-6AL-4V

摘要

In order to investigate the effect of hydrogen on high cycle fatigue properties (N >10~7) of Ti-6Al-4V, test specimens of the alloy were electrochemically charged with hydrogen. The hydrogen concentration in the total volume of testing section without charging with hydrogen was 100 ～130 ppm, and that of the hydrogen-charged specimens was 2270～ 3060 ppm. Tension-compression fatigue tests were carried out at a stress ratio R of -1 in air at a room temperature. Data for hydrogen-charged specimens exhibited a decrease in fatigue resistance as compared to data for uncharged specimens. Further, the S-N plot for hydrogen-charged specimens did not show a definite fatigue limit even in the regime exceeding N =5x10~7 cycles. In all those specimens which had not been charged with hydrogen the fracture origin was at a facet at the specimen surface. On the other hand, in the hydrogen-charged specimens, the fracture origins exhibited a rough morphology as compared with the planar facet associated with uncharged specimens. In addition, in the hydrogen-charged specimens, some of the fracture origins were subsurface at a depth of depth ～20 μm. XRD analysis indicated that hydrogen charging led to a change in crystal structure in the surface layers of the specimens.