Hydrogen-induced subcritical crack growth in Ti–6Al–4V alloy

Abstract

To study the subcritical crack growth (SCG) behavior of Ti–6Al–4V alloy with 47 ppm internal hydrogen, sustained load tests were performed on fatigue precracked specimens in 202 kPa Ar, 101 kPa H 2 and 505 kPa H 2 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) II, did not change significantly with temperature. Evidence for hydrogen-induced strain localization was found in the specimens tested in Ar and H 2 environments. From the SCG curves and fractographic findings, a fracture mode transition was found at 505 kPa H 2. As the applied initial stress intensity factor, K ini , was between 40 and 50 MPa m 1/2, 41< K th <53 MPa m 1/2, and (da/dt) II about 10 −5 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 ini >55 MPa m 1/2, 70< K th <76 MPa m 1/2, and (da/dt) II about 10 −8 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.