Larry R Walker

Assessment of thermal nitridation to protect metal bipolar plates in polymer electrolyte membrane fuel cells

Thermally (gas) nitrided Nb-30Ti-20W wt % exhibited excellent corrosion resistance and maintained good electrical conductivity in 80°C H 2 SO 4 environments designed to simulate proton exchange membrane (PEM) fuel cell bipolar plate conditions. This result supports proof of principle for thermally nitrided alloys as candidate bipolar plate materials, although such Nb-base alloys are too costly for most PEM fuel cell applications. Investigation of the corrosion resistance of a series of relatively inexpensive, thermally nitrided model Ni-X base alloys designed to form Cr 2 N, CrN, NbN, TiN, or VN surface layers have identified nitrided Ni-Cr base alloys as showing corrosion resistance nearly equal to that of the nitrided Nb-30Ti-20W.

Oxidation resistance and mechanical properties of Laves phase reinforced Cr in-situ composites

Abstract Two-phase Cr(X)–Cr2X (X=Nb, Ta) in-situ composites are of interest for high-temperature applications due to their high melting points and potential for high-temperature strength. A six cycle, 120 h, 1100°C cyclic oxidation screening test was used to evaluate potential for high-temperature oxidation resistance of several Cr(X)–Cr2X in-situ composites. Alloys based on the Cr–Ta system near the Cr(Ta)–Cr2Ta eutectic exhibited superior oxidation resistance compared to corresponding alloys based on the Cr–Nb system. The binary Cr–Ta alloys were also found to exhibit a moderate degree of room-temperature fracture toughness, in the range of 9–10 MPa√m. It was concluded that the Cr(Ta)–Cr2Ta alloys are a promising base for future high-temperature intermetallic alloy development efforts.

Exposure of Ceramics and Ceramic Matrix Composites in Simulated and Actual Combustor Environments

A high-temperature, high-pressure, tube furnace has been used to evaluate the long term stability of different monolithic ceramic and ceramic matrix composite materials in a simulated combustor environment. All of the tests have been run at 150 psia, 1204 degrees C, and 15% steam in incremental 500 h runs. The major advantage of this system is the high sample throughput; >20 samples can be exposed in each tube at the same time under similar exposure conditions. Microstructural evaluations of the samples were conducted after each 500 h exposure to characterize the extent of surface damage, to calculate surface recession rates, and to determine degradation mechanisms for the different materials. The validity of this exposure rig for simulating real combustor environments was established by comparing materials exposed in the test rig and combustor liner materials exposed for similar times in an actual gas turbine combustor under commercial operating conditions.

Solid state Si diffusion and joint formation involving aluminum brazing sheet

Abstract The paper provides information about the use of in situ determined diffusion coefficients of silicon for modeling of a brazed joint formation, i.e. formation of the equilibrium surface of a molten Al+ x Si alloy at the onset of solidification in the joint. Diffusion coefficients of Si were determined (within both the joint and/or residue zone) to analyze its migration across the clad–core interface of an Al brazing sheet, including both the period prior to reaching brazing temperature range, and the peak brazing temperature range. Subsequently, diffusion coefficients were used to predict the joint formation during brazing. Migration of silicon is not uniform along the clad–core interface during brazing and depends, in addition to material characteristics and process parameters, on the vicinity of the joint zone. It is argued that, due to these alterations, the joint formation modeling must be performed by using in situ determined diffusion coefficients. The diffusion coefficients determined directly from electron probe microanalysis (EPMA) scans at different locations along the cladding sheet and within the joint zone differ between each other and when compared to the literature data. This variation influences the outcome of the residue formation modeling; hence the joint formation modeling may be affected. The relation between these phenomena is briefly discussed and quantitative data regarding diffusion coefficients, and in particular an approach to utilization of these data in modeling of joint formation, are provided.

Characterization of the breakaway al content in alumina-forming alloys

Abstract Residual Al contents were measured in Fe- and Ni-base alloys in order to improve lifetime prediction models based on the consumption of the Al reservoir. Measurements were made using electron probe microanalysis (EPMA) on both foil (50–125 μm thickness) and plate (0.5–2mm) material. For FeCrAl, NiCrAl and Fe3Al plate specimens, significant Al concentration gradients were measured. An Al gradient is not considered in current reservoir type models. Residual Al contents at the onset of breakaway oxidation were somewhat lower than previously reported for several FeCrAl alloys and significantly higher (»10at.%) for Fe3Al and Fe–Al alloys. The implications of these results for performance and lifetime predictions are discussed.

MICROANALYSIS OF ALKALI-ACTIVATED FLY ASH-CH PASTES

Abstract Samples of a Class F fly ash and calcium hydroxide (CH) hydrated in pH 13.2 sodium hydroxide solution were analyzed using backscattered electron, scanning Auger, and X-ray microanalysis. The Class F fly ash, composed mainly of aluminosilicate glass and silica, was reacted for 8, 14, and 78 days at various temperatures. These samples represent both long-term and early-age stages of hydration. Results show that a hydrate product with calcium to silica ratio near 1.4 and katoite are formed. X-ray and scanning Auger microanalysis show evidence of the formation of hydrate product on the surface of both fly ash and CH particles at early ages. This finding suggests a new mechanism to explain prior data that shows that the hydration rates increase with increasing CH–ash content in the starting mixture.

Oxidation behaviour of cast Ni–Cr alloys in steam at 800°C

Abstract To evaluate the steam oxidation resistance of cast Ni base alloy candidates for advanced steam turbine casings, laboratory experiments were conducted at 800°C. Alloys ranged from weaker, solid solution strengthened alloys 230 and 625 to stronger, precipitation strengthened alloys 105, 263 and 740, which are more difficult to fabricate and join. In general, these Ni–Cr based alloys exhibit low mass gains and form thin, protective Cr rich external oxides in 17 bar steam or 1 bar air. However, Al and Ti in these alloys internally oxidise in all cases. After 5000 h exposures, the average and maximum internal oxide penetration depths were measured, and the values were ranked based on the alloy Al+Ti contents. The middle range of Al+Ti compositions investigated, such as for alloys 617, 263, 282 and 740, showed the deepest penetrations. Further characterisation of the reaction products by electron microprobe showed a complex behaviour including significant Ti incorporation into the scale formed in both steam and air, and Ti rich oxide at both the gas and metal interfaces. Based on the Al and Ti contents, the internal oxidation observed in these alloys in steam was atypical.

On the Loss of Protective Scale Formation in Creep-Resistant, Alumina-Forming Austenitic Stainless Steels at 900°C in Air

A family of creep-resistant, Al2O3-forming austenitic (AFA) stainless steels was recently developed. The alloys exhibit excellent oxidation resistance up to ∼800°C, but are susceptible to internal attack of Al at higher temperatures. In the present work, higher levels of Ni, Cr, Al, and Nb additions were found to correlate with improved oxidation behavior at 900°C in air. The alloys generally appeared to be initially capable of external Al2O3 scale formation, with a subsequent transition to internal attack of Al (internal oxidation and internal nitridation) that is dependent on alloy composition. Compositional profiles at the alloy/scale interface suggest that the transition to internal oxidation is preceded by subsurface depletion of Al in the lower-Al compositions. In higher Al-containing compositions, NiAl second-phase precipitates act as an Al reservoir, and Al depletion may not be a key factor. Alloy design directions to increase the upper-temperature limit of protective Al2O3 scale formation in these alloys are discussed.

Microanalytical and Computational Analysis of Class F Fly Ash

A Class F fly ash has been analyzed using synchrotron x-ray diffraction (XKD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) to determine the composition, phase distribution, particle size, and particle morphology. XRD showed the existence of mullite, quartz, Fe 2 O 3 (hematite or maghemite), and CaSO 4 (anhydrite), with the possibility of calcium hydroxide (CH), which was also confirmed using TGA. SEM and x-ray microanalysis indicate four particle types: 1) a mixed aluminosilicate (A-S) phase with variable amounts of calcium, iron, and magnesium; 2) quartz or glassy silica; 3) FeO, Fe 2 O 3 , or an MgO-FeO phase; and 4) Ca(OH) 2 mixed with CaSO 4 . A combination of image analysis and XRD was used to identify and quantify the phase fraction of the major components in the system, including aluminosilicate glass and silica. These were used to construct segmented microstructures suitable for use in multi-scale microstructural simulations. Microanaysis also suggests that the A-S glassy portion of the ash may actually be two separately identifiable glasses or at least bounded by the compositions AS 3 and A 2 S 3 .

Nitrogen impurity gettering in oxide dispersion ductilized chromium

Work by Scruggs in the 1960s demonstrated that tensile ductility could be achieved at room temperature in powder metallurgically-produced Cr alloyed with MgO. During consolidation, much of the MgO converted to the MgCr{sub 2}O{sub 4} spinel phase, which was hypothesized to getter nitrogen from the Cr, rendering it ductile. We have duplicated this effect, achieving room temperature tensile elongations of 4% for hot-pressed Cr-6MgO-(0-1)Ti (wt.%) and 10% for hot-pressed and extruded Cr-6MgO-0.75Ti. Direct incorporation of nitrogen into the MgCr{sub 2}O{sub 4} phase was not detected; however, impurities, particularly nitrogen and sulfur, were observed to segregate to and/or precipitate at interfaces between the MgO/MgCr{sub 2}O{sub 4} phases and the Cr matrix. Exploratory studies of other non-spinel forming oxide dispersions (La{sub 2}O{sub 3}, TiO{sub 2} and Y{sub 2}O{sub 3}) showed a similar pattern of impurity segregation/precipitation, suggesting that there is nothing unique about spinel dispersions in Cr with regards to impurities. However, none of these other dispersions resulted in similar levels of tensile elongation.