An electrochemical method has been employed to obtain the population distribution of minimum grain boundary Cr concentrations for sensitized AISI 304 stainless steel ($30400). The detailed information about the extent of Cr depletion provided by the distribution is compared to the average degree of sensitization (DOS) evaluated by the standard and a modified single loop-electrochemical potentiokinetic reactivation (SL-EPR) technique. The distribution of Cr levels on a grain by grain basis is shown to provide information about intergranular stress corrosion cracking (IGSCC) susceptibility that cannot be provided by EPR methods. Sensitized 304SS tested in an environment relevant to nuclear reactors has been shown to fail by IGSCC when the grain boundary Cr concentration is depleted to below a critical level of =13.5%. In a separate study, it has been shown that more than 23 % of the grain boundaries must be depleted to the relevant critical level in order to observe macroscopically brittle behavior. The present study combines these two criteria and defines a sensitized material's IGSCC susceptibility by revealing the extent of Cr depletion as well as the quantity of depleted grain boundaries.
The film rupture behavior on dynamically strained Ti-15 Mo-3 Nb-3 Al exposed to 0.6 M NaC1 has been examined by rapid data acquistion of anodic current transients. The anodic current transients resulted from dislocation intersection of the passive film, followed by film rupture, bare surface dissolution, and repassivation. The transient morphology during dynamic straining differs from that generated via conventional depassivation techniques (i.e., manual scratch and fractured thin film depassivation) which incorporate an electrode that does not experience active plastic straining following depassivation. During conventional depassivation testing, current transients increase relatively rapidly and decay with an approximately linear slope on the log i-log t plot. In contrast, the transients acquired during dynamic straining are characterized by a relatively slow current increase and a nonlinear current decay on the log i-log t plot. This nonlinear decay is not attributable to ohmic or capacitive effects. The difference between the anodic transient morphologies on dynamically strained and unstrained electrodes is attributed to the combination of many discrete dislocation intersections of the surface over a period which is much larger than the time required for repassivation of a single dislocation intersection. Additionally, atomic force microscopy revealed persistent slip on a limited number of slip planes, with slip offsets as large as 600 nm, which is consistent with the formation and emergence of superdislocations. Thus, film rupture results from surface intersection of a superdislocation comprised of individual dislocations which are spatially and temporally separated. Current transient modeling of superdislocation intersection agrees qualitatively with that observed experimentally. It is concluded that the repassivation behavior determined by conventional depassivation techniques may not be relevant for modeling of environmentally assisted cracking of dynamically strained electrodes in some cases. InfroductionRecent work has indicated that modern metastabletitanium alloys are susceptible to intergranular environmentally assisted cracking (EAC) in chloride environments. Film rupture has been shown to be a requirement for EAC of these alloys.4 Other evidence, such as a requirement for dynamic straining,1-3 a dependency on loading rate,2 and EAC immunity during static loading of 3-titani-* Electrochemical Society Active Member. * * Electrochemical Society Student Member. urn alloys"2 also suggest that film rupture plays a critical
The effects of electrochemically pre-dissolved hydrogen on room-temperature fracture initiation in Beta-C titanium (Ti-3Al-8V-6Cr-4Mo-4Zr wt pct) have been investigated using circumferentially notched tensile specimens. Finite element-based analysis of notch stress fields was used to define relationships between the local threshold stress for crack initiation vs total internal hydrogen concentration. The as-received, solution heat treated (ST, y.2 pct ϭ 865 MPa) and the ST ϩ peak-aged conditions (STA, y.2% pct ϭ 1260 MPa) were compared after defining the relationships between the fracture process zone hydrogen concentration, hydrogen-metal interactions (i.e., hydrostatic stress field occlusion, trapping, hydriding), and the resulting fracture initiation behavior of each. Solutionized ϩ peak-aged ( ϩ ␣) Beta-C fractured intergranularly above total hydrogen concentrations of ϳ1000 wt ppm. (5.1 at. pct). A fracture mode consistent with cleavage occurred at ϳ2100 wt ppm. (10.7 at. pct). Solutionized Beta-C resisted hydrogen-assisted cracking (e.g., did not crack intergranularly) but was not immune; cleavage cracking was provoked at ϳ4000 wt ppm. (20.4 at. pct). Coldworked ST Beta-C (CW, y.2 pct ϭ 1107 MPa) did not crack intergranularly; fracture initiation behavior was similar to the ST condition regardless of specimen orientation. This suggests that high yield strength alone does not account for the susceptibility to intergranular cracking observed in the STA  ϩ ␣ condition. Stroke-rate studies and X-ray diffraction investigation of H partitioning suggests that equilibrium hydriding and/or irreversible trapping does not singularly control intergranular fracture initiation of the STA condition. Fractographic evidence and finite element results show that a finite plastic zone exists prior to intergranular fracture of the STA condition. This suggests that a criterion for fracture that incorporates plastic strain and stress should be considered.
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