The effects of environment and internal oxygen on fatigue crack propagation in Ti-6Al-4V

The role of environmental and internal oxygen on fatigue crack propagation is assessed for the titanium alloy Ti-6Al-4V at laboratory temperature. Growth rates for part through corner cracks are demonstrated to be highly dependent on the amount of oxygen present. Compared to data for conventional plate material at atmospheric conditions, tests conducted under a vacuum of 10(-6) torr illustrate a marked reduction in the rates of growth for the facet dominated phase at low values of Delta K. Intermediate vacuum levels and partial pressures of argon or hydrogen lead to growth rates that are similar to or slightly lower than the conventional atmospheric baseline. Significant growth rate accelerations are observed, however, for 6/4 material containing internal oxygen concentrations in excess of the standard 1700-2300 ppm. This acceleration is enhanced by the application of a high load ratio, R = 0.5. The implications with respect to current ideas on environmental interactions are discussed. (C) 1991 Elsevier Science S.A.