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Cast Fe-Ni-Cr chromia-forming austenitic stainless steels with Ni levels up to 45 wt.% are used at high temperatures in a wide range of industrial applications that demand microstructural stability, corrosion resistance, and creep strength. Although alumina scales offer better corrosion protection at these temperatures, designing cast austenitic alloys that form a stable alumina scale and achieve creep strength comparable to existing cast chromia-forming alloys is challenging. This work outlines the development of cast Fe-Ni-Cr-Al austenitic stainless steels containing about 25 wt.% Ni with good creep strength and the ability to form a protective alumina scale for use at temperatures up to 800–850°C in H2O-, S-, and C-containing environments. Creep properties of the best alloy were comparable to that of HK-type cast chromia-forming alloys along with improved oxidation resistance typical of alumina-forming alloys. Challenges in the design of cast alloys and a potential path to increasing the temperature capability are discussed.

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 s...

There is a need for next-generation, high-performance power electronic packages and systems utilizing wide-band-gap devices to operate at high temperatures in automotive and electricity transmission applications. Sn-3.5Ag solder is a candidate for use in such packages with potential maximum operating temperatures of about 200°C. However, there is a need to understand the thermal cycling reliability of Sn-3.5Ag solders subject to such high-temperature operating conditions. The results of a study on the damage evolution occurring in large-area Sn-3.5Ag solder joints between silicon dies and direct bonded copper substrates with Au/Ni-P metallization subject to thermal cycling between 200°C and 5°C are presented in this paper. Interface structure evolution and damage accumulation were followed using high-resolution X-ray radiography, cross-sectional optical and scanning electron microscopies, and X-ray microanalysis in these joints for up to 3000 thermal cycles. Optical and scanning electron microscopy results showed that the stresses introduced by the thermal cycling result in cracking and delamination at the copper–intermetallic compound interface. X-ray microanalysis showed that stresses due to thermal cycling resulted in physical cracking and breakdown of the Ni-P barrier layer, facilitating Cu-Sn interdiffusion. This interdiffusion resulted in the formation of Cu-Sn intermetallic compounds underneath the Ni-P layer, subsequently leading to delamination between the Ni-rich layer and Cu-Sn intermetallic compounds.

Eutectic Pb –Li is a leading candidate for current fusion blanket concepts as a coolant. However, there is very little data about the compatibility of most materials with Pb–Li above 500°C where the dissolution rate of many conventional alloys increases rapidly. Current work is beginning to assess Pb–Li compatibility from 500 to 800°C using isothermal capsule experiments. Aluminide coatings hold some promise in protecting conventional Fe-base alloys at 600 – 700°C. However, there is a significant initial Al loss that has not been clearly explained. Furthermore, the reaction product with coated materials is LiAlO2 rather than Al2O3 at 600° and 700°C. Even when pre-oxidized to form α-Al2O3, an alumina layer on FeCrAl transformed to LiAlO2 at 700° and 800°C. At 500°C, the preformed oxide partially transformed from alumina and some Li was detected in the oxide layer.

A high-purity Fe–20Cr and commercial type 430 ferritic stainless steel were exposed at 700 and 800 °C in dry air and air with 10% water vapor (wet air) and characterized by SEM, XRD, STEM, SIMS, and EPMA. The Fe–20Cr alloy formed a fast growing Fe-rich oxide scale at 700 °C in wet air after 24 h exposure, but formed a thin chromia scale at 700 °C in dry air and at 800 °C in both dry air and wet air. In contrast, thin spinel + chromia base scales with a discontinuous silica subscale were formed on 430 stainless steel under all conditions studied. Extensive void formation was observed at the alloy–oxide interface for the Fe–20Cr in both dry and wet conditions, but not for the 430 stainless steel. The Fe–20Cr alloy was found to exhibit a greater relative extent of subsurface Cr depletion than the 430 stainless steel, despite the former’s higher Cr content. Depletion of Cr in the Fe–20Cr after 24 h exposure was also greater at 700 °C than 800 °C. The relative differences in oxidation behavior are discussed in terms of the coarse alloy grain size of the high-purity Fe–20Cr material, and the effects of Mn, Si, and C on the oxide scale formed on the 430 stainless steel.

A vacuum-tight transfer stage is described, that enables air or moisture sensitive specimens to be safely transferred from a protective environment, like that in a dry box, to a scanning electron microscope without exposing the sample to ambient conditions. The transfer stage concept described here is, in fact, applicable to the protected transfer of hygroscopic or air-sensitive samples to any vacuum-based analysis equipment where the transfer device opens automatically under vacuum to reveal the sample. This device concept is also applicable to the transfer of samples under vacuum from one glove box to another where the transfer chamber can be easily opened manually to reveal the sample. An example of one application of the device is provided by a controlled exposure study of stabilized lithium metal particles (SLMP) at various stages of shelf life. We found that a reaction coating formed on “fresh” SLMP after it was exposed to 100 Pa air for as short as 15 m. Such a reaction to air exposure was not observed on “aged” SLMP with a thickened carbonate surface coating after 4 and 6 months of shelf life. The case study of the surface coating on SLMP clearly demonstrated the excellent vacuum performance of the novel transfer stage.

Intercritically austempered ductile irons hold promise for applications requiring fatigue durability, excellent castability, low production energy requirements, reduced greenhouse gas emissions, and excellent machinability. In the present study, four different ductile iron alloys, containing manganese and nickel as the primary austenite-stabilizing elements, were heat treated to obtain different quantities of austenite in the final microstructure. This article reports the microstructures and phases present in these alloys. Furthermore, lattice strains and diffraction elastic constants in various crystallographic directions and the transformation characteristics of the austenite were determined as a function of applied stress using in situ loading during neutron diffraction at the second generation Neutron Residual Stress Facility at the High Flux Isotope Reactor at Oak Ridge National Laboratory.

Thermal stability is a requirement for the deployment of thermoelectric (TE) materials in the application of power generation. Lead–antimony–silver–tellurium (LAST) is an emerging material with promising TE properties. This study focuses on the thermal stability of a LAST sample with composition Ag0.86Pb19SbTe20 fabricated from a cast ingot. Using a customized heating stage, the morphology of the LAST particles was studied via scanning electron microscopy (SEM) in situ, between room temperature and 848 K. The material was found to be stable below 823 K. The inclusion phase, which was antimony-rich, had a lower thermal stability than the PbTe-rich matrix. SEM findings were consistent with the results of a thermogravimetric analysis. The results from an in situ heating study suggest that protective environments should be used during the processing and application of LAST materials.

A family of alumina-forming austenitic (AFA) stainless steels is under development for use in aggressive oxidizing conditions from ~600–900 °C. These alloys exhibit promising mechanical properties but oxidation resistance in air with water vapor environments is currently limited to ~800 °C due to a transition from external protective alumina scale formation to internal oxidation of aluminum with increasing temperature. The oxidation behavior of a series of AFA alloys was systematically studied as a function of Cr, Si, Al, C, and B additions in an effort to provide a basis to increase the upper-temperature oxidation limit. Oxidation exposures were conducted in air with 10% water vapor environments from 800–1000 °C, with post oxidation characterization of the 900 °C exposed samples by electron probe microanalysis (EPMA), scanning and transmission electron microscopy, and photo-stimulated luminescence spectroscopy (PSLS). Increased levels of Al, C, and B additions were found to increase the upper-temperature oxidation limit in air with water vapor to between 950 and 1000 °C. These findings are discussed in terms of alloy microstructure and possible gettering of hydrogen from water vapor at second phase carbide and boride precipitates.

Tensile, fatigue, fracture toughness, and creep experiments were performed on a commercially available magnesium-aluminum alloy (AM60) after three processing treatments: (1) as-THIXOMOLDED (as-molded), (2) THIXOMOLDED then thermomechanically processed (TTMP), and (3) THIXOMOLDED then TTMP then annealed (annealed). The TTMP procedure resulted in a significantly reduced grain size and a tensile yield strength greater than twice that of the as-molded material without a debit in elongation to failure (e f ). The as-molded material exhibited the lowest strength, while the annealed material exhibited an intermediate strength but the highest e f (>1 pct). The TTMP and annealed materials exhibited fracture toughness values almost twice that of the as-molded material. The as-molded material exhibited the lowest fatigue threshold values and the lowest fatigue resistance. The annealed material exhibited the greatest fatigue resistance, and this was suggested to be related to its balance of tensile strength and ductility. The fatigue lives of each material were similar at both room temperature (RT) and 423 K (150 °C). The tensile-creep behavior was evaluated for applied stresses ranging between 20 and 75 MPa and temperatures between 373 and 473 K (100 and 200 °C). During both the fatigue and creep experiments, cracking preferentially occurred at grain boundaries. Overall, the results indicate that thermomechanical processing of AM60 dramatically improves the tensile, fracture toughness, and fatigue behavior, making this alloy attractive for structural applications. The reduced creep resistance after thermomechanical processing offers an opportunity for further research and development.

We report observations of the nature and spatial distribution of sulfur species on a sulfated Ba-based commercial lean NOx trap (LNT) catalyst. The monolithic catalyst was sulfated in a bench flow reactor during 60/5-s NOx-storage/reduction cycling to achieve a total sulfur loading of 3.4 g L−1 of catalyst. Washcoat composition, structure and sulfur distribution were analyzed with electron probe microanalysis, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction. The most significant washcoat elements of catalytic relevance were Pt, Pd, Rh, Ba, Ce, Zr, Mg, Al, and these were present mainly in four distinct domains: (i) Mg/Al mixed oxide with Pt, Ce; (ii) Al oxide with Rh, Pd; (iii) Ce/Zr mixed oxide with Pt, Pd, Ba (high Ba content); (iv) Ce/Zr mixed oxide with Pt, Pd, Ba (low Ba content). Sulfur was present in the form of sulfates that decreased in concentration along the LNT axis from front to back. Barium showed the highest sulfur affinity leading to a plug-like axial progression of its sulfation. The sulfation of Al, Mg/Al, and Ce/Zr oxides was less vigorous with a more axially dispersed and less penetrating front.

Short-term sulfidation–oxidation exposures were conducted under high pS2 and low pO2 conditions for TiCrAl and FeCrAl alloys at 600 and 800 °C. Low mass gains and submicron Al-and Ti-rich oxide scales were formed on TiCrAl at 600 °C, while high mass gains and FeS-based scale formation were observed for FeCrAl. Based on the good behavior of TiCrAl, third-element effect additions of Cr are not inherently detrimental under sulfidation–oxidation conditions. Rather, differences in the mechanistic action of the third-element addition of Cr between FeCrAl and TiCrAl alloys and its relevance to low oxygen potential sulfidation–oxidation environments were the key factors in determining whether or not a protective alumina scale was established.

A family of creep-resistant austenitic stainless steels based on alumina (Al2O3) scale formation (AFA alloys) for superior high-temperature oxidation resistance was recently identified. Excellent oxidation behavior was observed at 650 and 700 °C in air with 10% water vapor. However, particularly at 800 °C, the presence of water vapor greatly increased the tendency for internal oxidation of Al. Water vapor also enhanced subscale Al depletion in some AFA alloys relative to dry air exposure. Increased levels of Nb additions were found to significantly improve oxidation resistance, as were reactive element additions of Hf and Y. Computational thermodynamic calculations of the austenitic matrix phase composition and the volume fraction of MC, B2-NiAl, and Fe2Nb base Laves phase precipitates were used to interpret oxidation behavior in terms of two-phase oxidation theory, reservoir effect, and the third-element effect of Cr. Of particular interest was the enrichment of Cr in the austenitic matrix phase by additions of Nb, which aided the establishment and maintenance of alumina. Higher levels of Nb additions also increased the volume fraction of B2-NiAl precipitates, which served as an Al reservoir during long-term oxidation. Implications of these findings for the design of AFA alloys with increased upper use temperature limits are discussed.

The impact of different types of diesel particulate matter (PM) and different sampling conditions on the wall deposition and early soot cake build-up within diesel particulate filters has been investigated. The measurements were made possible by a newly developed diesel exhaust filtration analysis system in which in-situ diesel exhaust filtration can be reproduced within small cordierite wafer disks, which are essentially thin sections of a diesel particulate filter wall. The different types of PM were generated from selected engine operating conditions of a single-cylinder heavy-duty diesel engine. Two filtration velocities 4 and 8 cm/s were used to investigate PM deep-bed filtration processes. The loaded wafers were then analysed in a thermal mass analyser that measures the soluble organic fraction as well as soot and sulphate fractions of the PM. In addition, the soot residing in the wall of the wafer was examined under an optical microscope illuminated with ultraviolet light and a variable pressure scanning electron microscope to determine the bulk soot penetration depth for each loading condition. It was found that a higher filtration velocity results in a higher wall loading with approximately the same penetration depth into the wall. PM characteristics impacted both wall loading and soot cake layer characteristics. Results from imaging analysis indicate that the soot penetration depth into the wall was affected more by PM characteristics (which changes with engine operating conditions) than by filtration velocity.

In order to develop an improved oxidation-limited lifetime model, experiments are being conducted to assess the critical factors that affect the rate of Al loss from Fe-base alumina-forming alloys during cyclic oxidation. Both wrought and dispersion strengthened ferritic and intermetallic Fe–Al±Cr alloys are being investigated at exposure temperatures from 1100–1300°C. Higher temperatures, thinner specimens and cycle frequency are used to accelerate degradation during oxidation. However, the effects of these experimental choices often have not been critically assessed in prior work. Electron probe microanalysis has been used to measure the residual Al content under various conditions and at various stages of life. For 0.5–2mm thick specimens, lifetime repeatedly showed a linear relationship to thickness. Increasing the oxidation temperature appears to be the most viable strategy to accelerate the evaluation of oxidation resistance for comparing and developing alloys.

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.

Ternary interdiffusion coefficients in the γ-Ni and y'-Ni 3 Al phases of the Ni-Al-Pt system at 1150°C were experimentally determined by applying a Boltzmann-Matano method at a common composition in the time-invariant diffusion paths for a given two couples. The variation of the main-term interdiffusion coefficients of Al, ?Ni AlAl , with increasing of Pt content was different for the y and y' phases. In y ?Ni AlAl generally increased with increasing Pt content, but it decreased with increase in Pt in y' phase. By contrast, the main-term coefficients for Pt, ? Ni PtPt apparently increased monotonically with increasing Pt content in both phases, but there was little dependency on Al content. The cross-term interdiffusion coefficients, ? Ni AlPt and ? Ni PtAl , were negative in both y and y', showing that the chemical interaction between Al and Pt in these phases is negative. ? Ni AlPt values were larger than those of ? Ni PtAl ; however, there was no clear dependence of ? Ni AlPt and ? NiPtAl on Pt and Al contents within the composition ranges studied. The cross-term interdiffusion coefficients were comparable to the main-term coefficients in y and within an order of magnitude of each other in y'. The significance of these findings are discussed and quantitatively assessed.

Aluminide coatings made by chemical vapor deposition on ferritic (Fe-9Cr-1Mo) and austenitic (Type 304L) substrates are being evaluated in humid air at 650°–800 °C. A humid air environment is used to identify coating failure during exposure, as uncoated substrates experience rapid oxidation at these temperatures. One goal of this work was to demonstrate the potential benefits and problems with alumina-forming coatings of two different thicknesses. The higher exposure temperatures were selected to accelerate the degradation of the coating by interdiffusion with the substrate. Another goal is to develop a lifetime model based on the results. The critical Al content of the coating at which failure occurs is a key parameter needed to complete the model, and a coating failure after ∼10 kh at 700 °C provides some information.

Gas nitridation has shown excellent promise to form dense, electrically conductive and corrosion-resistant Cr-nitride surface layers on Ni-Cr base alloys for use as proton exchange membrane fuel cell (PEMFC) bipolar plates. Due to the high cost of nickel, Fe-base bipolar plate alloys are needed to meet the cost targets for many PEMFC applications. Unfortunately, nitridation of Fe-base stainless steel alloys typically leads to internal Cr-nitride precipitation rather than the desired protective surface nitride layer formation, due to the high permeability of nitrogen in these alloys. This paper reports the finding that it is possible to form a continuous, protective Cr-nitride (CrN and Cr{sub 2}N) surface layer through nitridation of Fe-base stainless steel alloys. The key to form a protective Cr-nitride surface layer was found to be the initial formation of oxide during nitridation, which prevented the internal nitridation typically observed for these alloys, and resulted in external Cr-nitride layer formation. The addition of V to the alloy, which resulted in the initial formation of V{sub 2}O{sub 3}-Cr{sub 2}O{sub 3}, was found to enhance this effect, by making the initially formed oxide more amenable to subsequent nitridation. The Cr-nitride surface layer formed on model V-modified Fe-27Cr alloys exhibited excellent corrosion resistancemore » and low interfacial contact resistance under simulated PEMFC bipolar plate conditions.« less

A high-purity CVD β-SiC showed a relatively low corrosion rate in deoxygenated supercritical water at 500°C. The corrosion rate was lower than that previously reported for CVD SiC in 360°C water and much lower than that reported for sintered and reaction-bonded SiC. The present study confirmed that CVD SiC was preferentially attacked at the grain boundaries. Analytical examinations did not reveal the presence of a measurable oxide scale. As a result, it is believed that corrosion of the high-purity SiC occurred via hydrolysis to hydrated silica species at the surface that were rapidly dissolved into the supercritical water.

The relationships between nitridation, microstructure and corrosion were studied for a thermally nitrided model Ni–50Cr (wt.%) alloy to gain insight into the protection of metallic bipolar plates in proton exchange membrane fuel cells (PEMFCs). Surface layers containing CrN, Cr2N or ternary Cr–Ni–N π phase nitride were formed, dependent on the nitridation conditions. Studies in aerated pH 3 sulfuric acid at 80 °C indicated that the best corrosion resistance was exhibited by CrN + Cr2N surface layers, which were more corrosion resistant than Ni–50Cr metal, Cr metal, and nitrided Cr. X-ray photoelectron spectroscopy (XPS) analyses indicated oxygen enrichment of the polarized surfaces in the form of Cr-oxide and Cr-oxynitride species. Such oxygen enrichment does not appear to result in detrimental increases in interfacial contact resistance/electrical properties of the surface.

The goal of this paper was to determine if NiO-forming alloys are a viable alternative to Cr2O3-forming alloys for solid-oxide fuel-cell (SOFC) metallic interconnects. The oxide-scale growth kinetics and electrical properties of a series of Li- and Y2O3-alloyed, NiO-forming Ni-base alloys and La-, Mn-, and Ti-alloyed Fe–18Cr–9W and Fe–25Cr base ferritic Cr2O3-forming alloys were evaluated. The addition of Y2O3 and Li reduced the NiO scale growth rate and increased its electrical conductivity. The area-specific-resistance (ASR) values were comparable to those of the best (lowest ASR) ferritic alloys examined. Oxidation of the ferritic alloys at 800°C in air and air+10% H2O (water vapor) indicated that Mn additions resulted in faster oxidation kinetics/thicker oxide scales, but also lower oxide scale ASRs. Relative in-cell performance in model SOFC stacks operated at 850°C indicated a 60–80% reduction in ASR by Ni+Y2O3, Ni+Y2O3, Li, and Fe–25Cr+La,Mn,Ti interconnects over those made from a baseline, commercial Cr2O3-forming alloy. Collectively, these results indicate that NiO-forming alloys show potential for use as metallic interconnects.

The computational heat and mass transfer modeling approach presented in this paper emphasizes the influence of undercoolings on dendrite structure formations of the alpha phase crystals inherent to advanced phases of an aluminum brazing netshape manufacturing sequence. In the first segment of this work, the empirical evidence involving the outcome of the solidification process and its kinetics was presented. In this paper, simulation of the alpha phase crystal pattern formation is corroborated with empirical findings obtained by utilizing an AA4343/AA3003 brazing sheet exposed to controlled atmosphere brazing (CAB) in ultra-high purity nitrogen.

This work provides empirical evidence needed for an in depth phenomenological study of dendrite growth phenomena during brazing of aluminum alloys in form of composite brazing sheets. The main objective of this study was (1) a collection of experimental evidence associated with heat and mass transfer modeling of the Al + Si solid solution dendrite macro morphology evolution inherent to joint formation during brazing, and (2) the dendrite growth kinetics analysis. The isothermal dwell and the quench that follow the clad molten aluminum binary alloy surface-tension-driven flow into the joint at the peak brazing temperature upon melting lead to the solidification of the metal micro layer and joint formation. Before, during and after the isothermal dwell a significant reduction of Si content in the melt is found. So, a subsequent dissolution may affect the interface zone between the molten clad and substrate. α-phase dendrite assemblies imbedded in an irregular eutectic in the joint zone are the main morphological features of the solidification microstructures. The major characteristic of the phenomenon is a sensitivity of the dendrite pattern selection and dendrite population on brazing process parameters, in particular on the temperature during the dwell.

The addition of Fe to Cr–Cr2Ta eutectic alloys increased ambient fracture toughness from a maximum of 12–14 to 18–21 MPa m1/2. The elevated-temperature yield strength was depressed, but still reached >250 MPa at 1000 °C. The Fe additions also resulted in a potentially beneficial change in oxidation mechanism from Cr2N subscale formation to internal oxidation.

Early work indicated that Ru reduced the ductile-to-brittle transition temperature of Cr even at small concentrations. To evaluate the potential of Ru as a beneficial alloying element in the two-phase Cr–TaCr2 alloys, a number of Cr–8 at.%Ta alloys with 0–10 at.%Ru were prepared, and their microstructures and mechanical properties were evaluated. The Ru addition was found to move the eutectic point of Cr–TaCr2 to a higher Ta level, as compared to the binary Cr–Ta system. Ru partitioned preferentially in the Laves phase over in the Cr matrix, with a partitioning ratio of 2–5:1, depending on the Ru addition level. Ru was also found to mainly occupy the Cr site in the TaCr2 Laves phase. The hardness of both the primary Cr matrix and the eutectic microconstituent increased slightly with the increase in Ru addition. At a higher Ru addition level (i.e. 10 at.%), the hardness of the primary Cr matrix increased significantly, probably due to the precipitation of extremely fine Laves-phase precipitates. The Ru addition did not noticeably affect the fracture toughness of the two-phase alloys.

A simple method is described for enriching the surface aluminum concentration of carbon steel. The process of weld overlaying steel substrate with pure aluminum wire results in an aluminum concentration of ∼15–17 at.% (8–9 wt.%) for depths of ∼2 mm. The aluminum-enriched surface shows an improved oxidation and sulfidation resistance. The paper gives details of the process along with microstructural analysis and oxidation resistance improvement results.

A sputtered {gamma} (TiAl)+Laves TiAlCr coating formed an adherent Al{sub 2}O{sub 3} scale on SN 282 Si{sub 3}N{sub 4} when oxidized. A Ti-N rich layer was formed at the TiAlCr-Si{sub 3}N{sub 4} interface, which effectively resulted in beneficial local Al enrichment underneath the growing Al{sub 2}O{sub 3} scale, rather than detrimental Al loss to the substrate.

An effective diffusion barrier between the superalloy and aluminide coating would reduce coating degradation by lowering the rate of Al loss to the substrate by interdiffusion, and by inhibiting diffusion of substrate elements (such as Cr, Re, Ta, W) into the coating, which degrade its corrosion and oxidation resistance. In this preliminary study, sputtered Hf–Ni and Hf–Pt were evaluated as potential high-temperature diffusion barrier layers beneath NiAl or NiPtAl coatings. A Ni 3 Hf-type layer formed during aluminizing of Ni–Hf thin films, and appeared to result in reduced β-NiAl depletion during oxidation testing at 1150 °C. Some Hf-containing simple aluminide coatings also showed very good oxidation resistance. However, the Ni–Hf phase disrupted coating microstructure and displayed limited high-temperature stability. Heat treatment of Hf and Pt thin films formed a compound similar to HfPt 3 -type, which subsequently dissolved during aluminizing.

A series of single-phase Cr(X), single phase Cr 2X Laves phase, and two-phase Cr(X) + Cr2 X alloys (X = Nb or Ta) were thermally nitrided for 24 hr at 1100°C in N2--4H2 and then oxidized for 2 hr at 1100°C in air. The Cr(X) phase nitrided to form Cr2N while the Cr2X phases nitrided to form a complex local mixture of Cr2N/Cr and CrNbN/CrTaN, Cr3Nb3N/Cr3Ta3N phases depending on the depth in the nitrided zone. The Ta only slightly increased the isothermal oxidation rate of nitrided Cr(Ta) and Cr2Ta-reinforced Cr alloys, compared with nitrided, unalloyed Cr. Further, the nitrided two-phase alloys Cr--9.5Ta and Cr--20Ta exhibited improved Cr2O3 scale adherence relative to nitrided unalloyed Cr and Cr--1Ta. In contrast, Nb was detrimental to the oxidation resistance of the nitrided Cr(Nb) and Cr2Nb-reinforced Cr alloys, resulting in the formation of nonprotective Cr--Nb oxides rather than continuous Cr2O3. A phenomenological explanation for these effects based on phase chemistry and microstructural distribution is presented. Implications of these results for understanding the oxidation behavior of developmental high-temperature, Laves-strengthened Cr alloys, as well as possible applications as oxidation and wear-resistant coatings are discussed.

The use of intermetallic alloy precursors is explored as a new means to synthesize complex transition and refractory metal nitrides, carbides, and related phases. The conditions under which model single-phase Cr3Pt and two-phase Cr3Pt-dispersed Cr alloys form Cr3PtN antiperovskite when thermally nitrided were studied. Phenomenological experiments suggest that the key variable to achieving single-phase Cr3PtN surface layers is the Cr3Pt phase composition. In two-phase β-Cr−Cr3Pt alloys, the formation of single-phase Cr3PtN at Cr3Pt precipitates by in-place internal nitridation was found to be a strong function of the size of the Cr3Pt dispersion in the microstructure. Nanoscale Cr3Pt dispersions were readily converted to near single-phase Cr3PtN, whereas nitridation of coarse Cr3Pt particles resulted in a cellular or discontinuous-type reaction to form a lath mixture of Cr3PtN and a more Cr-rich Cr3Pt or β-Cr. The potential for using such external/internal oxidation phenomena as a synthesis approach to lay...

Preferential thermal nitridation was used to form a pin-hole defect free CrN/Cr2N surface on a model Ni–Cr alloy. Excellent corrosion resistance and negligible contact resistance increase was observed over a 4100 h exposure in 80 °C sulfuric acid and when used as a metallic bipolar plate in a 1000 h proton exchange membrane (PEM) fuel cell test.

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.

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.

The microstructural aspects of dross formation in industrial Al re-melt furnaces for several different alloy compositions have been evaluated. The Al alloys used in this study included 1350 (no Mg), 3004 (low Mg), and 5182 (high Mg). Dross specimens were collected directly from the different Al alloy melts in industrial furnaces using a consistent sampling protocol in order to compare the microstructure and phase development of the dross as a function of melt temperature and composition. The results showed that the sequence of phases formed during the re-melt process was the same for all the alloys examined; amorphous-Al2O3 forms first followed by either α-Al2O3+AlN (for non-Mg-containing alloys) or cubic MgO, then MgAl2O4, and lastly α-Al2O3 (for low- or high-Mg content). The formation of MgAl2O4 is associated with accelerated oxidation rates (known as breakaway oxidation) and this reaction proceeds until the Mg is depleted at the molten surface. At this point, aluminum oxidation is predominant a...

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.

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.

The oxidation and nitridation behavior of Cr(Nb) solid solution and Cr2Nb Laves phase was studied individually and in combination at 1100°C and 950°C in dry air, humid air, and N2–4H2 for alloys containing high or low levels of oxygen and sulfur impurities. At 1100°C, humid-air exposure of alloys in which the Cr2Nb phase was the matrix resulted in greatly increased subscale nitridation as compared with exposure in dry air. At 950°C, little difference between dry vs. humid-air exposures was observed; however, greatly increased subscale nitridation was observed in the Cr2Nb-matrix eutectic alloy when the levels of oxygen and sulfur impurities in the alloys were reduced. In contrast, unalloyed Cr and Cr(Nb) solid-solution alloys were relatively insensitive to water vapor or impurity oxygen/sulfur effects under the conditions studied, although the addition of Nb as a solute in Cr did increase the extent of subscale nitridation. Possible synergistic effects between sulfur impurities and water vapor in the Cr2Nb alloys and the possibility for similar effects in other intermetallic systems are discussed.

Single crystal platelets of AlN were successfully grown on 6H-SiC(0001) by a novel technique designed to suppress SiC decomposition, promote two-dimensional growth, and eliminate cracking in the AlN. X-ray diffractometry and synchrotron white beam X-ray topography demonstrate that the final AlN single crystal is of high structural quality.

The oxidation of SiC-fiber-reinforced SiC composites at high temperature (1200°C) and elevated total water vapor pressures (up to 1.5 atm) was studied. Substantial degradation of SiC composites was observed under these exposure conditions, particularly when BN (used as a fiber coating) was a composite constituent. Besides accelerated SiC recession due to oxidation, the reactions of oxygen and H 2 O with boron and between boria and SiO 2 led to extensive borosilicate glass formation as well as some volatile H x B y O z products. The extent of these reactions and the rate of overall degradation were compared for different as-processed structures by exposing composites fabricated by processes such as chemical-vapor- and silicon melt-infiltration. Microstructural and compositional factors such as the amount of interconnected porosity and total boron content in the composite had important influences on the rate of degradation. Based on the present results, the overall oxidation behavior of the different composites at high water vapor pressures can be explained based on the concurrent surface reactions involving the formation of volatile reaction products, borosilicate glass, and silica.

A new synthesis route, based on internal oxidation reactions in multiphase alloys, is proposed for the controlled production of near-surface, complex ceramic-ceramic or ceramic-metallic composite structures. Using this approach, a microdispersion of a complex nitride perovskite, Cr3PtN, was formed in Cr2N or Cr(Pt) by internal nitridation of a two-phase Cr(Pt) + Cr3Pt precursor alloy. A framework for use of this phenomenon to synthesize island micro- (and potentially meso- or nano-) composite functional surface structures is presented.

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.

The oxidation behavior of the Cr(X) solid solution and intermetallic Cr2X Laves phases (X = Nb, Ta) was studied individually and in combination at 1,100°C in humid air. The two-phase Cr(Ta) + Cr2Ta alloys exhibited superior oxidation resistance to either the single-phase Cr(Ta) or the single-phase Cr2Ta alloys. However, the two-phase Cr(Nb) + Cr2Nb alloys exhibited lower oxidation resistance than either the single-phase Cr(Nb) or Cr2Nb alloys. The two-phase Cr(Ta) + Cr2Ta alloys were also found to exhibit superior oxidation resistance to the two-phase Cr(Nb) + Cr2Nb alloys. This was hypothesized to be related to the lower amount of the Cr2X phase in the Cr(Ta)–Cr2Ta eutectic and to the lower solubility of Ta in the Cr(X) phase, relative to the Cr–Nb system. Both of these factors resulted in significantly less oxidation and nitridation in the Cr(Ta) + Cr2Ta alloys.

A new family of Cr(Ta)-Cr2Ta intermetallic alloys based on Cr-(6-10)Ta (at%) is currently under development for structural use in oxidizing environments in the 1,000–1,300°C temperature range. These alloys show excellent strength and creep resistance and good oxidation resistance at high temperatures in air. Oxidation resistance comparable to commercial reactive element doped chromia-forming alloys and creep resistance comparable to single-crystal superalloys have been demonstrated. To date, only modest room-temperature fracture toughness (in the 11–12 MPa m½ range) has been achieved. Preliminary results of a promising approach to improve room-temperature fracture toughness via ductilization of Cr with MgO additions are discussed.

This report provides an overview of several candidate technologies for cladding nickel-based corrosion protection layers onto high-temperature structural alloys. The report also provides a brief overview of the welding and weld performance issues associated with joining nickel-clad nickel-based alloys. From the available techniques, two cladding technologies were selected for initial evaluation. The first technique is a line-of-sight method that would be useful for cladding large structures such as vessel interiors or large piping. The line-of-sight method is a laser-based surface cladding technique in which a high-purity nickel powder mixed into a polymer binder is first sprayed onto the surface, baked, and then rapidly melted using a high-power laser. The second technique is a vapor phase technique based on the nickel-carbonyl process that is suitable for cladding inaccessible surfaces such as the interior surfaces of heat exchangers. An initial evaluation for performed on the quality of nickel claddings processed using the two selected cladding techniques.

The Capstone C65 Microturbine primary surface recuperator (PSR) core has been manufactured from Haynes alloy HR-120 since 2005 (Microturbine is a registered trademark of Capstone Turbine Corporation; Haynes and HR-120 are trademarks of Haynes International, Inc.). When exposed to the harsh operating environment of the microturbine PSR, HR-120 forms a protective oxide scale that is resistant to the effects of the water vapor present in the exhaust gas. Long-term accelerated microturbine testing with samples in a modified PSR with a removable aft dome is ongoing at an elevated turbine exit temperature (TET) ∼100°F higher than normal operation. The elevated TET test engine is operated at steady-state conditions, and the engine is shut down at predetermined intervals for sample removal. Material characterization of the elevated TET samples has been carried out by Capstone Turbine Corporation in collaboration with Oak Ridge National Laboratory. The surface oxide scale formation and associated alloy compositional changes have been evaluated for elevated TET samples with operating lives ranging from ∼1800 h to ∼26,500 h. In addition, field-operated HR-120 recuperators have been sectioned and samples have been evaluated for operating lives ranging from ∼5500 h to ∼18,000 h. Results from the microstructural and compositional analyses of both the long-term steady-state elevated TET HR-120 samples and the field-operated HR-120 recuperator samples will be presented and compared.

The spatially resolved electrical transport properties have been studied on the surface of optimally-doped superconducting Ba(Fe1-xCox)2As2 single crystal by using a four-probe scanning tunneling microscopy. While some non-uniform contrast appears near the edge of the cleaved crystal, the scanning electron microscopy (SEM) reveals mostly uniform contrast. For the regions that showed uniform SEM contrast, a sharp superconducting transition at TC = 22.1 K has been observed with a transition width TC = 0.2 K. In the non-uniform contrast region, TC is found to vary between 19.6 and 22.2 K with TC from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co concentration remains 7.72% in the uniform region, but changes between 7.38% and 7.62% in the non-uniform region. Thus the variations of superconductivity are associated with local compositional change.