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To understand and model performance in supercritical CO2 (sCO2) for high-efficiency, concentrating solar power (CSP) and fossil energy power cycles, reaction rates are compared at 750°C in 0.1 MPa CO2 and 30 MPa sCO2 as well as laboratory air as a baseline on structural materials such as Ni-based alloy 625. Due to the thin reaction products formed even after 5000 h, scanning transmission electron microscopy was used to study the Cr-rich surface oxide scale. The scales formed in CO2 and sCO2 had a much finer grain size with more voids observed in CO2. However, the observations on alloy 625 were complicated by Mo and Nb-rich precipitates in the adjacent substrate and Al internal oxidation. To simplify the system, a binary Ni-22Cr alloy was exposed for 1000 h in similar environments. After exposure in sCO2, there was an indication of carbon segregation detected on the Cr2O3 grain boundaries. After exposure in air, metallic Ni precipitates were observed in the scale that were not observed in the scale formed on alloy 625. The scale formed in air on a second Ni-22Cr model alloy with Mn and Si additions did not contain Ni precipitates, suggesting caution when drawing conclusions from model alloys. [ABSTRACT FROM AUTHOR]

Initial experimental work at 700°-800 °C is in progress to develop a lifetime model for supercritical CO2 (sCO2) compatibility for a 30-year lifetime of a >700 °C concentrated solar power system. Nickel-based alloys 282, 740H and 625 and Fe-based alloy 25 are being evaluated in 500-h cycles at 1 and 300 bar, and 10-h cycles in 1 bar industrial grade CO2. The alloys showed similar low rates of oxidation in 1 and 300 bar CO2 in 500-h cycles at 750 °C. However, in 10-h cycles, alloy 25 showed accelerated attack at 700° and 750 °C. Transmission electron microscopy scale cross-sections on alloy 25 after 1000 h at 700 °C in sCO2 and in air only showed a small row of carbides beneath the scale in the former environment. Similar characterisation was performed on alloys 625 and 282 after sCO2 exposure at 750 °C. [ABSTRACT FROM AUTHOR]

To fabricate oxide dispersion strengthened bond coatings, commercial Co-30wt-%Ni-20Cr-8Al-0•4Y powder was milled with 2% additions of Al2O3, Y2O3 or Y2O3 + HfO2. Low-pressure plasma sprayed, free-standing specimens were oxidised in air + 10%H2O at 1100 °C both isothermally (100 h) and in 500, 1−h cycles. Dry air cyclic testing conducted at both ORNL and FZJ showed remarkably similar results. In general, the water vapour addition caused more scale spallation. Two LPPS specimens without oxide additions were tested for comparison. The specimens with 2%Al2O3 addition exhibited the best behaviour as the powder already contained 0•4%Y. Additions of 2%Y2O3 and especially 1%Y2O3 + 1%HfO2 resulted in over-doping as evidenced by high mass gains and the formation of Y- and Hf-rich pegs. Scanning transmission electron microscopy of the isothermal specimens showed no Hf and/or Y segregation to the alumina scale grain boundaries in the over-doped specimens. [ABSTRACT FROM AUTHOR]

The thermochemical compatibility of potential replacement fuel rod cladding materials for zirconium alloys in light water reactors was assessed. Considered were the family of FeCrAl alloys, Nb-1%Zr (similar to PWC-11), and a hybrid SiC-composite with a metallic barrier layer. The niobium alloy was also seen as requiring an oxidation protective layer, and a diffusion silicide was investigated. Metallic barrier layers for the SiC-composite reviewed included an FeCrAl alloy, Nb-1%Zr, and chromium. Thermochemical calculations were performed to determine oxidation behavior of the materials in steam and for hybrid SiC-composites to determine possible interactions between the metallic layer and SiC. In addition, experimental exposures of SiC-FeCrAl alloy reaction couples at 673, 1073, and 1273 K for 168 h in an inert atmosphere were made, and microanalysis was performed. Whereas all materials were determined to oxidize under higher oxygen partial pressures in the steam environment, these varied by material. The computed and experimental results indicate the formation of liquid phase eutectic in the FeCrAl-SiC system at the higher temperatures. [ABSTRACT FROM AUTHOR]

Preliminary research has demonstrated the ability to utilise novel scan strategies in the electron beam melting (EBM) process to establish control of crystallographic texture within Inconel 718 deposits. Conventional EBM scan strategies and process parameters yield coarse columnar grains aligned parallel to the build direction. Through varying process parameters such as beam power, beam velocity, beam focus and scan strategy, the behaviour of the electron beam can be manipulated from a line source to a point source. The net effect of these variations is that the resulting crystallographic texture is controlled in a manner to produce either epitaxial deposits or fully equiaxed deposits. This research demonstrates the ability to change the crystallographic texture on the macroscale indicating that EBM technology can be used to create complex geometric components with both site-specific microstructures and material properties. [ABSTRACT FROM AUTHOR]

Small angle neutron scattering (SANS) and scanning transmission electron microscopy (STEM) were used to study film formation by magnesium alloys AZ31B (Mg-3Al-1Zn base) and ZE10A (Elektron 717. E717: Mg-1Zn + Nd, Zr) in H2O and D20 with and without 1 or 5 wt% NaCl. No SANS scattering changes were observed after 24 h D2O or H2O exposures compared with as-received (unreacted) alloy, consistent with relatively dense MgO-base film formation. However, exposure to 5 wt% NaCl resulted in accelerated corrosion, with resultant SANS scattering changes detected. The SANS data indicated both particle and rough surface (internal and external) scattering, but with no preferential size features. The films formed in 5 wt% NaCl consisted of a thin, inner MgO-base layer, and a nano-porous and filamentous Mg(OH)2 outer region tens of microns thick. Chlorine was detected extending to the inner MgO-base film region, with segregation of select alloying elements also observed in the inner MgO, but not the outer Mg(OH)2. Modeling of the SANS data suggested that the outer Mg(OH)2 films had very high surface areas, consistent with loss of film protectiveness. Implications for the NaCl corrosion mechanism, and the potential utility of SANS for Mg corrosion, are discussed. [ABSTRACT FROM AUTHOR]

Three oxide dispersion strengthened (ODS) Fe-12 wt-%Cr-5 wt-%Al (12 at-%Cr-9·7 at-%Al) alloys were mechanically alloyed with different oxide additions: (1) Y2O3, (2) Y2O3+HfO2 and (3) Y2O3+ZrO2. The as extruded microstructure was characterised including the oxide particle composition and size distribution. The 700°C Pb-Li compatibility was evaluated for a fusion energy application and the steam oxidation resistance at 1200°C and higher temperatures was evaluated for a nuclear accident tolerant fuel cladding application. The alloy prepared with only a Y2O3 addition contained some low Al regions and did not perform well, but provided a baseline for comparison. The other alloys contained sufficient Cr and Al to form a protective LiAlO2 surface oxide, which inhibits dissolution in isothermal Pb-Li. Also, a protective α-Al2O3 scale formed in steam oxidation at 1200°C, similar to the scale formed in dry air at 1200°C. For the alloy with HfO2, the scale remained protective to 1475°C in steam. [ABSTRACT FROM AUTHOR]

Alternative light water reactor fuel cladding materials are being investigated to replace Zircaloy for enhanced accident tolerance, which involves oxidation resistance to steam environments at ≥1200°C for short times. As chromia-forming alloys and Ni-containing alloys are both undesirable for this application, the focus has been on FeCrAl, although NiAl was used to evaluate the effect of steam oxidation at 1600°C for this study. For commercial and model FeCrAlY alloys, a critical Cr-Al composition was identified for 1 bar isothermal steam (100% H2O) oxidation resistance at 1200°C, which differed for exposures in Ar-50%H2O at the same temperature. Alloys with lower Cr and Al contents were not able to form a protective alumina scale under these conditions. To simulate the accident scenario, exposures were also conducted in steam with the temperature rising 5°C min−1 to 1500°C for the most oxidation resistant alloys. Using thermogravimetry, the maximum use temperature for candidate alloys was determined for different Cr and Al contents. Minor additions such as Y and Ti appeared to be beneficial for oxidation resistance. Similar to prior studies, alumina scales formed in air and in steam appeared to have only subtle differences in microstructure. [ABSTRACT FROM AUTHOR]

An isotopic tracer study of the film growth mechanism for pure magnesium, AZ31B, and ZE10A (Elektron 717, E717) magnesium alloys in water at room temperature was performed. A series of individual and sequential exposures were conducted in both H2 18O and D216O, with isotopic tracer profiles obtained using secondary ion mass spectrometry (SIMS). The water-formed films consisted primarily of partially hydrated MgO. The SIMS sputter depth profiles indicate that H and D penetrated throughout the film and into the underlying metal, particularly for the Zr- and Nd-containing E717 alloy. Film growth for the UHP Mg involved aspects of both metal outward diffusion and oxygen/hydrogen inward diffusion. In contrast, the film on the Al-containing AZ31B alloy grew primarily by inward oxygen and inward hydrogen diffusion. The 18O and D profiles for the film formed on E717 were the most complex, with the 180 data most consistent with inward lattice oxygen diffusion, but the D data suggests inward, short-circuit diffusion through the film. It is speculated that preferential inward short circuit hydrogen transport may have been aided by the presence of nano Zn2Zr3 particles throughout the E717 film. Such hydrogen penetration may have implications from both a corrosion resistance and hydrogen storage perspective. [ABSTRACT FROM AUTHOR]

The films formed on ultrahigh purity Mg, Elektron 717 (ZE10A), and AZ31B in water at room temperature were characterized by TEM, XPS, and SIMS. The films consisted primarily of MgO, with surface regions also containing Mg(OH)2 and MgCO3. SIMS suggested H throughout the films and into the underlying metal. Segregation of Zn to the metal/film interface and Al in the film was observed for AZ31B. Similar Zn film segregation was also detected for Elektron 717, along with Nd at the alloy/film interface and nano-size Zn2Zr3 precipitates throughout the film. Implications of these findings on film growth are discussed. [ABSTRACT FROM AUTHOR]

Although expensive, Ni-based superalloys are of interest for the ultrasupercritical steam program because of their good creep and oxidation resistance at temperatures above 700 °C. As the effect of steam oxidation on the alloy mechanical properties is unknown, creep specimens of alloy CCA617, 740, and 230 were pre-oxidized for 2000 and 4000 h in steam at 800 °C before testing in air at the same temperature. Compared with as fabricated material, exposure in steam decreased the creep properties of alloy CCA617, had less of an effect on alloy 740, and did not affect alloy 230. Testing of a specimen repolished after steam exposure as well as microstructural observations indicate that the oxidation affected zone at the specimen surface is not responsible for the properties degradation. Surprisingly, a similar time anneal in an inert environment resulted in a drastic decrease of creep rupture life and an increase in the creep rate and elongation at rupture. Transmission electron microscopy analysis revealed that the mechanical properties decrease for alloy CCA617 is related to the absence of γ′ precipitates within the grains. [ABSTRACT FROM AUTHOR]

The pitting corrosion of Al-Cu thin film alloys was investigated using samples that were heat treated in air to form through-thickness Al2Cu particles within an Al-0.5% Cu matrix. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) analysis revealed Cu-rich regions 250-800 nm in lateral extent near the metal/oxide interface. Following exposure that generated pitting corrosion, secondary electron, secondary ion, and AFM images showed pits with size and density similar to those of the Cu-rich regions. The role of the Cu-rich regions is addressed. [ABSTRACT FROM AUTHOR]

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011. [ABSTRACT FROM PUBLISHER]