Abstract
To study the subcritical crack growth (SCG) behavior of Ti-6A1-4V alloy with 47 ppm internal hydrogen, sustained load tests were performed on fatigue precracked specimens in 202 kPa Ar, 101 kPa H <sub>2</sub> and 505 kPa H <sub>2</sub> at 20, 45, 70 and 95°C. The log crack velocity versus stress intensity factor curves had typical three-stage characteristics. The stage II crack growth velocity, (da/dt) <sub>II</sub> , did not change significantly with temperature. Evidence for hydrogen-induced strain localization was found in the specimens tested in Ar and H <sub>2</sub> environments. From the SCG curves and fractographic findings, a fracture mode transition was found at 505 kPa H <sub>2</sub> . As the applied initial stress intensity factor, K <sub>ini</sub> , was between 40 and 50 MPa m <sup>1/2</sup> , 41 < K <sub>th</sub> < 53 MPa m <sup>1/2</sup> , and (da/dt) <sub>II</sub> about 10 <sup>-5</sup> m/s, the fracture surface exhibited completely brittle features. This fracture mode was suggested to be induced by an autocatalytic process of hydride forming and cracking. For the specimens with K <sub>ini</sub> > 55 MPa m <sup>1/2</sup> , 70 < K <sub>th</sub> < 76 MPa m <sup>1/2</sup> , and (da/dt) <sub>II</sub> about 10 <sup>-8</sup> m/s, the fracture surface showed some isolated flat regions with micro-ridges. The transition of fracture mode is believed to be related to the relative size of plastic zone and the hydrogen affecting zone. © 1998 Elsevier Science S.A.