Fatigue crack growth surface reactions for titanium alloys exposed to water vapor

Abstract

Parallel fracture mechanics and surface chemistry studies have been carried out to develop understanding of environmentally assisted fatigue crack growth in titanium alloys. Room temperature crack growth response in water vapor was determined for annealed Ti-5wt.%Al2.5wt.%Sn alloy and for Ti-6wt.%Al-4wt.%V alloys in the solution-treated and solutions-treated plus overaged conditions as a function of water vapor pressure from 0.266 to 665 Pa at a frequency of 5 Hz and a load ratio of 0.1.Reference data were obtained in vacuum. The kinetics of reactions of water vapor and oxygen with fresh surfacesof Ti-5wt.%Al-2.5wt.%Sn alloy were measured at room temperature. The results showed that the reactions with water vapor and with oxygen are both rapid, with an initial sticking coefficient of 1.0 for water vapor and of 0.54–0.84 for oxygen. The reaction with water vapor produced a monolayer of oxide (TiO), whereas two to three layers of TiO resulted from the reaction with oxygen. Enhancement of fatigue crack growth by water vapor is believed to result from embrittlement by hydrogen that is produced by the reaction of water vapor with the fresh crack surfaces. The fatigue crack growth response was shown to conform to a model for transport-controlled crack growth.