Yield Strength and Solution Composition Effects on Aqueous Environmental Crackingof Ti-8V-6Cr-4Zr-4Mo-3Al (Beta C)

摘要

A high-strength, a-precipitation hardened beta titanium alloy (Ti-8V-6Cr-4Zr-4Mo-3Al, STA Beta C) is prone to intergranular environmentally assisted cracking (EAC) in neutral aqueous NaCl given an occluded crack, active crack tip strain rate and electrode potential near free corrosion. The threshold stress intensity for EAC is reduced to about one-third of the plane strain fracture toughness and subcritical crack growth rates are 30 to 100 micrometers/sec. STA Beta C is immune to EAC with static loading or sufficiently cathodic polarization. Lower strength solution treated Beta C resists EAC in both pure NaCl and acidic sulfur-species-bearing chloride solutions, where fracture is by transgranular microvoid formation. These results are consistent with findings for STA Beta 21S and are qualitatively understood based on hydrogen environment embrittlement. EAC propagation in STA Beta C exhibits slow-rapid oscillation under rising crack mouth opening displacement but not during constant rate loading. This result implies that stress intensity factor and crack tip strain rate interactively govern da/dt.