UNDERSTANDING THE POTENTIAL AND PH DEPENDENCY OF HIGH-STRENGTH BETA-TITANIUM ALLOY ENVIRONMENTAL CRACK-INITIATION

An explanation for the strong dependency of crack initiation of precra cked high-strength beta-titanium alloy's in room-temperature 0.6 M NaC l on applied potential and bulk-solution pH is presented. It is propos ed that environment-assisted cracking (EAC) susceptibility in neutral aqueous NaCl results from (1) film rupture due to plastic deformation at actively deformed crack tips, (2) accelerated dissolution of titani um, (3) crack tip acidification by hydrolysis of titanium ions, (4) cr ack tip potential excursions toward bare metal open-circuit potentials (OCPs) during film rupture due to large ohmic voltages in the crack s olution, (5) accelerated crack tip proton or water reduction concurren t with titanium dissolution, (6) bare surface-dominated hydrogen ingre ss into a fracture process zone, and (7) crack initiation by hydrogen embrittlement. Evidence for each of the above stages of the crack init iation scenario is presented, with emphasis on crack tip electrode kin etics and ohmic voltage calculations which govern process zone-control led hydrogen uptake. The seven stages are consistent with the strong d ependencies of crack initiation and growth in precracked high-strength beta-titanium alloys on (1) solution pH, (2) applied potential, and ( 3) strain rate, and they explain the ''apparent'' EAC resistance of sm ooth- and blunt-notch specimens. The latter lack both occluded crack t ip geometries to promote acidification and ohmic voltage drops below r eversible hydrogen, as well as localization of dynamic plastic strain. Hydrogen uptake is, subsequently, limited.