Ti-6Al-4V titanium alloy is extensively employed in deep-sea structural applications owing to its excellent corrosion resistance, while its extra-low-interstitial (ELI) variant provides higher fracture toughness and is commonly presumed to exhibit even better stress corrosion cracking (SCC) resistance. In this work, displacement-rate-dependent fracture toughness (KQ) measurements and failure analysis were performed for compact tension specimens machined from an engineering Ti-6Al-4V ELI plate with different orientations, tested in air and 3.5 wt.% NaCl solution over displacement rates of 0.0012-1.2 mm/min. In air, KQ exhibits a pronounced loading-rate dependence, decreasing by more than 20% at low displacement rates relative to maximum rate, accompanied by quasi-cleavage features on the fracture surfaces indicative of hydrogen-assisted damage, likely arising from environmental or processing-related hydrogen uptake. In 3.5 wt.% NaCl solution, the minimum KQ within the low-rate regime (0.0012-0.12 mm/min) is 58 MPa·m0.5, comparable to values reported for conventional Ti-6Al-4V under similar conditions. The pronounced rate dependence and transition toward cleavage-like fracture reveal a strong coupling between loading kinetics and environmental degradation. This work demonstrates that enhanced intrinsic toughness does not necessarily translate into superior SCC resistance and establishes loading rate as a critical factor governing the environmental fracture of Ti-6Al-4V ELI under marine conditions.
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