Decoupling the role of stress and corrosion in the intergranular cracking of noble-metal alloys

Intergranular stress-corrosion cracking (IGSCC) is a form of environmentally induced crack propagation causing premature failure of elemental metals and alloys. It is believed to require the simultaneous presence of tensile stress and corrosion; however, the exact nature of this synergy has eluded experimental identification. For noble metal alloys such as Ag-Au, IGSCC is a consequence of dealloying corrosion, forming a nanoporous gold layer that is believed to have the ability to transmit cracks into grain boundaries in un-dealloyed parent phase via a pure mechanical process. Here using atomic-scale techniques and statistical characterizations for this alloy system, we show that the separate roles of stress and anodic dissolution can be decoupled and that the apparent synergy exists owing to rapid time-dependent morphology changes at the dealloyed layer/parent phase interface. We discuss the applicability of our findings to the IGSCC of important engineering Fe- and Ni-based alloys in critical applications.

A method for site-specific and cryogenic specimen fabrication of liquid/solid interfaces for atom probe tomography

A site-specific, cryogenic, focused ion beam (FIB) method is presented for the preparation of atom probe tomography (APT) specimens from a frozen liquid/solid interface. As a practical example, the interface between water and a corroded boroaluminosilicate glass has been characterized by APT for the first time. The water/glass interface is preserved throughout specimen preparation by plunge freezing the corroding glass particles with the corrosion solution into slush nitrogen. Site-specific specimen preparation is enabled through a new approach to extract and mount a small volume of material using a cryogenically cooled FIB stage and micromanipulator. The prepared APT specimens are subsequently transferred from the FIB to APT under cryogenic and high-vacuum conditions using a novel FIB/APT transfer shuttle and home-built environmental transfer hub attached to the APT system. Particular focus is given to the technical methods for specimen fabrication under cryogenic conditions. Persistent challenges are discussed in addition to future opportunities for this new specimen preparation method.

Examinations of oxidation and sulfidation of grain boundaries in alloy 600 exposed to simulated pressurized water reactor primary water

High-resolution characterizations of intergranular attack in alloy 600 (Ni-17Cr-9Fe) exposed to 325°C simulated pressurized water reactor primary water have been conducted using a combination of scanning electron microscopy, NanoSIMS, analytical transmission electron microscopy, and atom probe tomography. The intergranular attack exhibited a two-stage microstructure that consisted of continuous corrosion/oxidation to a depth of ~200 nm from the surface followed by discrete Cr-rich sulfides to a further depth of ~500 nm. The continuous oxidation region contained primarily nanocrystalline MO-structure oxide particles and ended at Ni-rich, Cr-depleted grain boundaries with spaced CrS precipitates. Three-dimensional characterization of the sulfidized region using site-specific atom probe tomography revealed extraordinary grain boundary composition changes, including total depletion of Cr across a several nm wide dealloyed zone as a result of grain boundary migration.