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The requirement of a sharp notch or precrack to cause environmental crack initiation of metastable β-titanium alloys exposed to 0.6 M NaCl has been observed. The causal relationship has not been thoroughly examined, however. This paper seeks to explain the sharp notch requirement by examining notch acuity effects on a variety of parameters that affect HEAC susceptibility. These include the effects of a sharp notch on cation accumulation - hydrolysis - acidification, potential drop in solution and resulting hydrogen production, and localization of dynamic strain. It is shown that solution resistance down a sharp crack is two orders of magnitude larger for a fatigue precracked compact tension specimen than for a smooth bar. The potential drop down a sharp crack is severe enough to enable hydrogen production even when the applied potential is more positive than the reversible potential for hydrogen production. It also shown that a fatigue precrack results in an acidified crack tip chemistry (approximately pH 1) which is deleterious to HEAC resistance. The effects of a sharp notch on the interplay of mechanics, film rupture and hydrogen uptake are also examined. It is shown that the slip behavior at a sharp crack tip promotes localized film rupture and localized hydrogen uptake. Localization of hydrogen uptake may be critical for HEAC susceptibility in light of the large hydrogen concentration required to cause crack initiation (ca. 1000 wt. ppm) and the lack of significant hydrogen uptake on filmed surfaces.
Hydrogen environmentally assisted cracking (HEAC), notch radius, β-titanium alloys, crack tip solution acidification, ohmic loss, bare surface electrode kinetics, film rupture, hydrogen uptake, fatigue precrack
Postdoctoral Research Associate, Los Alamos National Laboratory, Los Alamos, NM
Associate Professor, University of Virginia, Charlottesville, VA