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.

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.

Investigation of interfacial strength in nacre-mimicking tungsten heavy alloys for nuclear fusion applications.

Tungsten heavy alloys have been proposed as plasma facing material components in nuclear fusion reactors and require experimental investigation in their confirmation. For this purpose, a 90W-7Ni-3Fe alloy has been selected and microstructurally manipulated to present a multiphase brick-and-mortar structure of W-phase 'bricks' surrounded by a ductile 'mortar'. This work draws inspiration from nature to artificially imitate the extraordinary combination of strength and stiffness exhibited by mollusks and produce a nacre-mimicking metal matrix composite capable of withstanding the extremely hostile environment of the reactor interior and maintaining structural integrity. The underlying mechanisms behind this integrity have been probed through high-resolution structural and chemical characterization techniques and have revealed chemically diffuse phase boundaries exhibiting unexpected lattice coherency. These features have been attributed to an increase in the energy required for interfacial decohesion in these systems and the simultaneous expression of high strength and toughness in tungsten heavy alloys.

Stoichiometry and valence measurements of niobium oxides using electron energy-loss spectroscopy.

Qualitative and quantitative electron energy-loss spectroscopy analyses have been performed on niobium and stable niobium oxides (NbO, NbO(2) and Nb(2)O(5)). At integration windows (Delta) greater than 75 eV, k-factor analysis can be used to distinguish between the stoichiometry of the three oxides within 5.7% error. As seen in other metal oxides, with increasing oxidation state the metal ionization edges shift to higher energies relative to the O-K edge. Normalized M(2,3) white-line intensities show a strong correlation with 4d occupancy for each compound. The data are in correspondence with that observed in the literature for 4d transition metals using normalized L(2,3) white lines. Lastly, a distinctive energy-loss near-edge, structure of the O-K edge was observed for each oxide, which could be used as a fingerprint for analysis of unknowns.

Scanning electron microscopic analysis of the mineralization of type I collagen via a polymer-induced liquid-precursor (PILP) process.

We have put forth the hypothesis that collagen is mineralized during bone formation by means of a polymer-induced liquid-precursor (PILP) process, in which a liquid-phase mineral precursor could be drawn into the gaps and grooves of the collagen fibrils by capillary action, and upon solidification, leave the collagenous matrix embedded with nanoscopic crystallites of hydroxyapatite. This hypothesis is based upon our observations of capillarity seen for liquid-phase mineral precursors generated with calcium carbonate. Here, we demonstrate proof-of-concept of this mechanism by mineralizing Cellagen sponges (type I reconstituted bovine collagen) in the presence of a liquid-precursor phase to calcium carbonate. Scanning electron microscopy (SEM) was used to examine the mineralized collagen, which in combination with selective etching studies, revealed the extent to which the mineral phase infiltrated the collagenous matrix. A roughly periodic array of disk-like crystals was found to be embedded within the collagen fibers, demonstrating that the mineral phase spans across the diameter of the fibers. Some of the morphological features of the mineralized fibers in our in vitro model system are similar to those seen in natural bone (albeit of a different mineral phase), lending support to our hypothesis that these non-equilibrium morphologies might be generated by a PILP process. SEM provides a different perspective on the morphology of bone, and has been useful here for examining the extent of mineralization in composite structures generated via the PILP process. However, further investigation is needed to examine the nanostructural arrangement of the crystallites embedded within the collagenous matrix.