Effect of strength on environment-assisted cracking of Ti-8V-6Cr-4Mo-4Zr-3Al in aqueous NaCl Part 1: Age hardening vs. work hardening

The objective of this research is to understand the effect of yield strength on the environment-assisted cracking (EAC) susceptibility of a beta Ti alloy, Ti-8V-6Cr-4Mo-4Zr-3Al (Beta-C), precracked and stressed actively in aqueous NaCl. Isothermal aging renders immune, beta-solution treated Beta-C susceptible to severe EAC coincident with strengthening by alpha precipitates. The threshold for EAC decreases to 40% of the air fracture toughness, subcritical crack growth rate approaches 100 mu m s(-1), and the crack path becomes fully intergranular for aging times in excess of several hours. In contrast, solution treated and cold-worked single-phase beta Beta-C retains immunity to EAC for similar high strengths. For several levels of cold work, Beta-C cracks by transgranular microvoid processes, with equal values of EAC threshold stress intensity and air fracture toughness. Cold work reduces the air fracture toughness of beta-phase Beta-C compared to values for beta/alpha microstructures at similar strengths. The strength increase due to alpha precipitation does not uniquely promote EAC susceptibility. (C) 1998 Published by Elsevier Science S.A. All rights reserved.