Peter F. Tortorelli

A review of recent developments in Fe3Al-based alloys

Fe{sub 3}Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfidation resistance. However limited room temperature ductility ({lt}5%) and a sharp drop in strength above 600 {degree}C have limited their consideration for use as structural materials. Recent improvements in tensile properties, especially improvements in ductility produced through control of composition and microstructure, and advances in the understanding of environmental embrittlement in intermetallics, including iron aluminides, have resulted in renewed interest in this system for structural applications. The purpose of this paper is to summarize recent developments concerning Fe{sub 3}Al-based aluminides, including alloy development efforts and environmental embrittlement studies. This report will concentrate on literature published since about 1980, and will review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.

Critical factors affecting the high-temperature corrosion performance of iron aluminides

Abstract Iron aluminides are known to exhibit good-to-excellent corrosion resistance in a number of high-temperature environments. Under most conditions, this resistance derives from the establishment and maintenance of a sound and adherent alumina layer. Consequently the performance of iron aluminides under different aggressive high-temperature can be related to fundamental factors that affect the development, adhesion, and durability (lifetime) of protective alumina. Overall corrosion resistance depends not only on thermodynamic stability of the reaction product in a particular environment and its growth kinetics, but also on scale integrity and morphology, the chemical and physical nature of the oxide–metal interface, alloy strength, and the specific composition of the iron aluminide. Despite the multiplicity of factors, a relatively simple lifetime prediction model based on aluminum consumption can be used to evaluate the influences of changes in material parameters and to determine approaches to improving high-temperature corrosion resistance of iron aluminides.

Evaluation of CFCC liners with EBC after field testing in a gas turbine

Abstract Under the Ceramic Stationary Gas Turbine (CSGT) Program sponsored by the U.S. Department of Energy (DOE), a team led by Solar Turbines Incorporated has successfully designed engines, utilizing silicon carbide/silicon carbide (SiC/SiC) continuous fiber-reinforced ceramic composite (CFCC) combustor liners. Their potential for low NO x and CO emissions was demonstrated in eight field-engine tests for a total duration of more than 35,000 h. In the first four field tests, the durability of the liners was limited primarily by the long-term stability of SiC in the high steam environment of the gas turbine combustor. Consequently, the need for an environmental barrier coating (EBC) to meet the 30,000-h life goal was recognized. An EBC developed under the National Aeronautics and Space Administration high speed civil transport, enabling propulsion materials program was improved and optimized under the CSGT program and applied on the SiC/SiC liners by United Technologies Research Center (UTRC) from the fifth field test onwards. The evaluation of the EBC on SiC/SiC liners after the fifth field test with 13,937-h at Texaco, Bakersfield, CA, USA is presented in this paper.

High-Temperature Oxidation Behavior of ODS-Fe3Al

The high-temperature oxidation behavior of an oxide dispersion-strengthened (ODS) Fe3Al alloy has been studied during isothermal and cyclic exposures in oxygen and air over the temperature range 1000 to 1300°C. Compared to commercially available ODS–FeCrAl alloys, it exhibited very similar short-term rates of oxidation at 1000 and 1100°C, but at higher temperatures the oxidation rate increased because of increased scale spallation. Over the entire temperature range, the oxide scale formed was α-Al2O3, with the morphological features typical of reactive-element doping and was similar to those formed on the ODS–FeCrAl alloys. Although initially this scale appeared to be extremely adherent to the Fe3Al substrate, an undulating metal–oxide interface formed with increasing time and temperature, which led to cracking of the scale in the vicinity of surface undulations accompanied by a loss of small fragments of the full-scale thickness. In some instances, the surface undulations appeared to have resulted from gross outward local extrusion of the alloy substrate. Similar features developd on the FeCrAl alloys, but they were typically much smaller after a given oxidation exposure. The ODS–Fe3Al alloy has a significantly larger coefficient of thermal expansion (CTE) than typical FeCrAl alloys (approximately 1.5 times at 900°C) and this appears to be the major reason for the greater tendency for scale spallation. The stress generated by the CTE mismatch was apparently sufficient to lead to buckling and limited loss of scale at temperatures up to 1100°C, with an increasing amount of substrate deformation at 1200°C and above. This deformation led to increased scale spallation by producing an out-of-plane stress distribution, resulting in cracking or shearing of the oxide.

Effect of Cycle Frequency on High-Temperature Oxidation Behavior of Alumina-Forming Alloys

Cycle frequency affects both high-temperature oxidation behavior and the method in which the cyclic test is conducted. Several issues are discussed using examples taken from results for Ni-base and Fe-base, alumina-forming alloys. For alloys that form adherent scales, cycle frequency has little effect on results over extended test times ( ≥500 hr). When an alloy forms a less adherent scale, reducing the cycle time often has the expected effect of increasing the mass loss per unit exposure time; however, the opposite effect is observed in other cases. Low-frequency cycle experiments can be conducted with specimens contained in alumina crucibles. This has the important benefit of collecting the spalled oxide and measuring the “total” mass gain, equivalent to the metal wastage. However, higher-frequency-cyclic tests cannot be performed with crucibles because of the large thermal mass and thermal-shock problems of alumina crucibles. The test method and cycle frequency ultimately have a strong effect on lifetime predictions.

Effects of alloy additions on the microstructure and properties of CrCr2Nb alloys

Abstract Alloying additions of Ni, Co, Fe, Al and Re at levels up to 16 at.% were added to Cr Cr2Nb alloys containing 5.6–17 at.% Nb for the study of their microstructure, oxidation behavior, and mechanical properties. These alloys contain patches of primary Cr-rich solid solution surrounded by the eutectic structure having Cr2Nb and Cr(∼6% Nb) phases. The supersaturated Cr-rich solid solution in the cast alloys precipitated out secondary Cr2Nb (Laves-phase) particles upon annealing at and above 900 °C. The studies by TEM and electron microprobe analysis indicated that the transition elements of Fe, Co, and Ni partition strongly in the Cr2Nb-type Laves phase whereas rhenium and aluminum were only moderately enriched in the Laves phase. The Cr Cr2Nb alloys with ≤ 12% Nb exhibited considerable plastic deformation under compression tests. The yield strength of the alloys depended strongly on the volume fraction of the hard Laves phase. The oxidation resistance also increased with this volume fraction. Among the alloying additions, rhenium was the only element that substantially hardened the Cr Cr2Nb alloys at room temperature and 1000 °C. The hardening behavior is discussed in terms of partitioning and sublattice occupation of the alloying elements as well as altering the volume fracture of the hard Laves phase. Multilayer oxide products formed on these alloys upon exposure to air of an elevated temperature. Little beneficial effect of any of the alloying additions on oxidation behavior at 950 °C was found.

Evaluation of iron-aluminide CVD coatings for high temperature corrosion protection

Abstract Chemical vapor deposited (CVD) Fe-Al coatings are being investigated to address fundamental issues concerning aluminide coating performance and lifetime. By using a well-controlled laboratory CVD procedure, the coatings are uniform in composition, purity and microstructure. A typical ferritic steel, Fe-9Cr-1Mo, and an austenitic stainless steel, 304L (nominally Fe-18Cr-9Ni), were coated to examine differences in the two types of substrates. For both substrates, the as-deposited coating consisted of a thin (<5 μm), Al-rich layer above a thicker (30–50 μm), lower Al content layer. To follow-up on initial results, which showed good coating performance in air+10vol.%H2O and H2S-H2O-H2-Ar, cyclic tests were performed in both environments at 800°C and more detailed characterization of the isothermally exposed coatings was conducted. During 2–5, 25h cycles at 800°C in H2S-H2O-H2-Ar, CVD coatings on both substrates showed progressively more attack during each cycle. However, in 1h cycles at 800°C in air + 10vol.%H2O, the coatings showed excellent performance, similar to cast Fe-(15–20at.%)Al specimens. The uncoated alloys were significantly attacked during all of these tests. Thermal expansion measurements show Al additions up to 20at% have little effect on the mean expansion of ferritic alloys but the higher thermal expansion of austenitic steels may be a better match with Fe3Al coatings.

Corrosion and compatibility considerations of liquid metals for fusion reactor applications

Abstract The current understanding of corrosion and environmental effects on the integrity and mechanical properties of structural materials used with liquid metals in fusion reactors is reviewed. Corrosion processes in liquid lithium systems are examined and their influence on material degradation is discussed. Compatibility considerations that might arise from use of molten lead, bismuth, lead-bismuth, or lead-lithium are reviewed relative to the possible use of these liquids in fusion reactors.

Corrosion of ferrous alloys exposed to thermally convective Pb-17 at. % Li

Abstract A type 316 stainless steel thermal convection loop with type 316 stainless steel coupons and a Fe-9Cr-1Mo steel loop containing Fe-12Cr-1MoVW steel specimens circulated molten Pb-17 at% Li at a maximum temperature of 500°C. Specimens were exposed for more than 6000 h. Mass loss and surface characterization data were compared for these two alloys. At any particular exposure time, the corrosion of type 316 stainless steel by Pb-17 at% Li was more severe, and of a different type than that of similarly exposed Fe-12Cr-1MoVW steel. The austenitic alloy suffered nonuniform penetration and dissolution by the lead-lithium, whereas the Fe-12Cr-1MoVW steel tended to be more uniformly corroded. The presence of a ferritic layer on the type 316 stainless steel, and its susceptibility to spalling during specimen cleaning, were shown to be important in evaluating the data for this steel and in comparing corrosion losses for the two types of alloys. A model for the nonuniform penetration of type 316 stainless steel by Pb-17 at% Li was suggested.

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.