Metal dusting of binary iron aluminium alloys at 600 °C

In this work experiments on metal dusting of binary iron aluminium alloys with 15, 26 and 40 at.% Al were performed in strongly carburising CO‐H₂‐H₂O gas mixtures at 600 °C. The mass gain kinetics was measured using thermogravimetric analysis (TGA). The carburised samples were characterised by means of light optical microscopy (LOM), scanning electron microscopy (SEM), X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). It was found that the mass gain kinetics depends on the CO content of the gas mixtures and on the Al content of the alloys. With decreasing carbon activity the carburisation reaction kinetics decreases and the onset of metal dusting is retarded for increasing time periods. With increasing Al content of the alloys the carburisation reaction is slower and metal dusting sets on at later times. The samples were not pre‐treated for the formation of a protective oxide scale. By X‐ray Photoelectron Spectroscopy (XPS) analyses of the carburised iron aluminium samples it was found that the formation of Al₂O₃ layers has taken place in the CO‐H₂‐H₂O gas atmospheres. Needle‐ or plate‐like κ‐phase (Fe₃AlC_x) precipitates close to the surface of the carburised Fe‐15Al sample were detected by means of XRD and LOM. The coke on top of the carburised samples mainly consists of filamentous carbon with metal particles at their tips.     

Microprocesses of coke formation in metal dusting

The microprocess of coke formation during metal dusting on iron in a carburizing atmosphere with medium and extremely high carbon activities as well as the influence of sulphur have been studied down to the nanometer scale using high resolution electron microscopy (HREM) and analytical electron microscopic techniques (AEM). While for medium carbon activities the metal dusting proceeds via a formation, disintegration and further decomposition of a metastable carbide Fe₃C into Fe and C, the additional formation of the carbide Fe₅C₂ and the stabilization of carbides in the coke region have been observed for extremely high carbon activities. If sulphur is present in the atmosphere metal dusting takes place solely in the S-free surface areas. Furthermore, sulphur deposited from the atmosphere will suppress the nucleation of graphite in the coke. In addition, the results reveal that, irrespective of the degree of the carbon activity, there is a fundamental initial reaction micromechanism of metal dusting characterized by a vertically oriented deposition of graphite lattice planes with respect to the original surface of the substrate and with free ends affecting the decomposition of the carbides and thus forming a coke of carbon and iron, or of carbide particles, depending on the carbon activity.     

Coke formation and metal dusting of electroplated Ni3Al-CeO2-based coatings in CO-H2-H2O

Coke formation and metal dusting of electrodeposited pure, 5 μm CeO₂-dispersed, and 9-15 nm CeCO₂-dispersed Ni₃Al coatings were investigated in CO-H₂-H₂O at 650 °C for a period of 500 h. All Ni₃Al coatings showed the inferior long-term resistance to coke formation and metal dusting to the Fe-Ni-Cr alloy due to failure to form a continuous Al₂O₃ scale. CeO₂-dispersed Ni₃Al coatings, especially 9-15 nm CeCO₂-dispersed coatings, exhibited more severe coke formation and metal dusting than the pure Ni₃Al coating. The detrimental effect of CeO₂ is believed to be caused by the enhanced formation of NiO/Ni crystals on the coating surfaces or at the grain boundaries, which catalysed the carbon deposition and promoted the carbon attack on Ni₃Al coatings.     

Influence of Cr on the Oxidation of Fe3Al and Ni3Al at 500°C

The influence of chromium on the mechanical properties of the aluminides Fe₃Al and Ni₃Al has been studied extensively. In order to evaluate the role of Cr during the early stages of oxidation, Fe₃Al and Ni₃Al containing 2 and 4 at.% Cr were oxidized in dry air at 500°C for 6, 50, and 100 hr. The oxide scale on Fe₃Al consists of a layer of Fe₂O₃ mixed with FeAl₂O₄ on top of a continuous layer of (Al, Cr)₂O₃. Ni₃Al is covered with a mixed layer of (Al, Cr)₂O₃ and NiO/NiAl₂O₄ underneath a layer of NiO/NiAl₂O₄. Moreover, Cr induces the nucleation and growth of Fe₂O₃ and NiO particles at the oxide surface of Fe₃Al and Ni₃Al, respectively. This is due to enhanced cationic diffusion through the Cr-modified oxides. As a conclusion, additions of Cr up to 4 at.% are detrimental to the oxidation behavior of both aluminides at 500°C.     

A new semiconductor Al2Fe3Si3 with complex crystal structure

Al₂Fe₃Si₃, a new semiconductor with complex triclinic structure was synthesized by arc melting and spark plasma sintering, followed by heat treatment. The nominal compositions of samples have been changed to compensate Al evaporation during synthesis process, and single Al₂Fe₃Si₃ phase has been obtained with the nominal composition of Al: Fe: Si = 26: 37: 37 (6 at.% Al excess against stoichiometry). In this study, we measured the sound velocity, thermal expansion coefficient, Vickers hardness, fracture toughness, electrical conductivity, Seebeck coefficient, and thermal conductivity of the new semiconductor Al₂Fe₃Si₃. The Al₂Fe₃Si₃ sample displayed positive Seebeck coefficient from 300 to 850 K, with a maximum Seebeck coefficient of 110 μV/K at 430 K. The Debye temperature of Al₂Fe₃Si₃ was 640 K, which was similar to or higher than those of other Al, Fe, Si based thermoelectric materials, but the lattice thermal conductivity was lower, 4–5 W/mK, due to the complex crystal structure of Al₂Fe₃Si₃. The maximum ZT value was 0.06 at 580 K.     

Oxidation Behavior of Ni3Al and Fe3Al: II. Early Stage of Oxide Growth

Oxidation treatments of Ni₃Al and Fe₃Al were performed at room temperature in 0.2 atm O₂ for 5 min and at 300 and 500°C in air for 5 min, and 6, 50, 100, and 200 hr. The oxides were analyzed by XPS, AES, and SEM. A model explaining the initial stages of oxide formation is suggested. At room temperature and 300°C, islands of Al₂O₃ and NiO combined with NiAl₂O₄ formed on Ni₃Al. At 500°C, the Ni oxides grow laterally and cover the Al₂O₃ islands. Islands of Al₂O₃ and Fe₂O₃ mixed with Fe(Fe, Al)₂O₄ formed on Fe₃Al at room temperature. At 300 and 500°C the scale is composed of an outer layer rich in Fe oxides and an inner layer rich in Al oxides. During long time exposure, islands of Fe₂O₃ and Fe(Fe, Al)₂O₄ formed at the surface by diffusion of Fe cations through the alumina layer. The oxide growth on Fe₃Al reaches a steady-state regime after formation of the continuous alumina layer. At 300°C, the oxide formed on Fe₃Al is thicker than on Ni₃Al, whereas it is reverse at 500°C.     

Recent advances in understanding metal dusting: A review

Recent experimental investigations have widened the understanding of metal dusting significantly. Microscopic observations have been used to dissect dusting mechanisms. Iron dusts by growing a cementite surface scale, which catalyses graphite nucleation and growth. The resulting volume expansion leads to cementite disintegration. Cementite formation on iron can be suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster, because carbon diffusion is more rapid in α‐Fe than in Fe₃C. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the dusting mechanism of austenitic materials like high alloy Cr–Ni steels and Ni base materials is one of graphite nucleation and growth within the near surface metal. In all alloys examined, both ferritic and austenitic, the principal mass transfer process is inward diffusion of carbon. Alloying iron with nickel leads to a transformation from one mechanism with carbide formation to the other without. Copper alloying in nickel and high nickel content stainless steels strongly suppresses graphite nucleation, as does also an intermetallic Ni–Sn phase, thereby reducing greatly the overall dusting rate. A surface layer of intermetallic Ni–Sn Fe‐base materials facilitates the formation of a Fe₃SnC surface scale which also prevents coking and metal dusting. Current understanding of the roles of temperature, gas composition and surface oxides on dusting rates are summarised. Finally, protection against metal dusting by coatings is discussed in terms of their effects on catalysis of carbon deposition, and on protective oxide formation.     

The high-temperature strength of some Fe3Al alloys

The high-temperature strength of a Fe–25%Al–2%Nb alloy in both a solutionised state and a precipitated state has been determined for temperatures up to 900 °C and compared with that of a solution hardened Fe₃Al alloy. Some comparisons of these materials with previously reported Fe₃Al-based materials are also made. The Fe–25%Al–2%Nb material is capable of retaining good strength to temperatures above 800 °C, but rapid coarsening of the Laves precipitate particles at higher temperatures leads to strength loss at such temperatures. The presence of stable dispersed particles at the intermediate temperatures means that good strength can be retained to very low strain rates.     

Effect of pre‐oxidation on coke formation and metal dusting of electroplated Ni3Al–CeO2‐based coatings in CO–H2–H2O

Pre‐oxidation was introduced to improve the resistance of electroplated pure, 5 µm CeO₂‐dispersed, and 9–15 nm CeO₂‐dispersed Ni₃Al coatings to coke formation and metal dusting in 24.4%CO–73.3%H₂–2.3%H₂O at 650 °C. Coke formation and metal dusting of pre‐oxidized Ni₃Al‐based coatings were retarded up to 200 h owing to a thin Al₂O₃ scale induced during pre‐oxidation. The long‐term effectiveness of pre‐oxidation nonetheless depended on the integrity of Al₂O₃ scale. The pure Ni₃Al coating suffered severe spallation after pre‐oxidation and thereby showed the worst long‐term resistance. Two pre‐treated 9–15 nm CeO₂‐dispersed Ni₃Al coatings exhibited the best long‐term resistance to carbon attack because nano‐CeO₂ particles maintained a full coverage of Al₂O₃ scale on the coatings. Two 5 µm CeO₂‐dispersed Ni₃Al coatings showed significant spallation after pre‐oxidation because of an overdoping effect and experienced coke formation and metal dusting during long‐term exposure.     

Formation and stability of refractory metal diborides in an Fe3Al matrix

The feasibility of forming refractory diboride particles in an Fe₃Al matrix by conventional casting techniques is examined; the microstructural stability of such particles upon subsequent exposure to elevated temperatures is discussed. Four different alloys were cast (Fe–Al–Ti–B, Fe–Al–Zr–B, Fe–Al–Nb–B, and Fe–Al–Ta–B); the nominal composition in each case was intended to yield stoichiometric Fe₃Al with 10 vol% of the stoichiometric refractory diboride. As-cast, annealed and heat treated alloys were examined using optical microscopy, X-ray diffraction, differential thermal analysis and scanning electron microscopy in conjunction with energy dispersive X-ray (EDX) spectroscopy to understand the formation and thermal stability of refractory diborides (TiB₂, ZrB₂, TaB₂, NbB₂) in Fe₃Al. In all four cases, refractory diborides formed; the Fe–Al–Ti–B and Fe–Al–Zr–B alloys yielded a two-phase microstructure of Fe₃Al and the diboride. The Ti and Zr diboride reinforcements were stable and showed minimal coarsening following high temperature exposure. The Nb- and Ta-containing quaternary alloys, in addition to the respective diborides, included a eutectic reaction at temperatures between 1150°C and 1200°C that results in the formation of an Fe-rich phase thought to be Fe₂B.     

Orientation relationship between a ferritic matrix and κ-phase (Fe3AlCx) precipitates formed during metal dusting of Fe-15Al

An Fe-15 at.% Al alloy has been exposed to a strongly carburising CO–H₂–H₂O gas mixture under metal dusting conditions. Cementite (Fe₃C) was detected to be present only in the coke which is a reaction product of the high-temperature corrosion metal dusting. Needle- or plate-like κ-phase (Fe₃AlC_x) precipitates close to the surface were identified by means of electron back-scattered diffraction (EBSD). EBSD analyses of several precipitates and neighboured matrix areas indicate a Nishiyama–Wassermann orientation relationship between the κ-phase precipitates and the -Fe(Al) matrix.     

Interface electron structure of Fe3Al/TiC composites

1 Introduction Recently a few studies[1?3] have been carried out to utilize carbides or borides as reinforcements in iron aluminides, thus improving their mechanical properties. Additionally, an intermetallic matrix reinforced with ceramic phase might mak…     

Structural and mechanical properties of Fe–Al compounds: An atomistic study by EAM simulation

The structural properties, formation enthalpies, and mechanical properties of Fe–Al compounds (FeAl, Fe₂Al, Fe₃Al, FeAl₂, FeAl₃ and Fe₂Al₅) are studied by using embedded-atom method (EAM) which is acquired by Mobius lattice inversion. The potential is transferrable and therefore does well for studying different Fe–Al compounds. The calculated lattice parameters and cohesive energies of Fe–Al compounds agree with the experimental and some EAM results. According to elastic constants restrictions, all the six Fe–Al compounds are mechanically stable. The calculated bulk moduli of the compounds increase with the increasing Fe concentration. Furthermore, results showed that FeAl, Fe₃Al, FeAl₃, FeAl₂, Fe₂Al₅ have lower ratios of shear modulus to bulk modulus and Fe₂Al has higher ratio.     

Simultaneous synthesis and consolidation of nanocrystalline Fe2Al5 and Fe2Al5-Al2O3 by pulsed current activated sintering and their mechanical properties

Nanopowders of Fe, Al and Fe₂O₃ are fabricated by high energy ball milling. Using the pulsed current activated sintering method, the densification of nanocrystalline Fe₂Al₅ and Al₂O₃ reinforced Fe₂Al₅ composites were simultaneously synthesized and consolidated within two minutes from mechanically activated powders. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostuctured materials. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. As nanomaterials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid to the application of nanomaterials. Not only the hardness but also the fracture toughness of the Fe₂Al₅-Al₂O₃ composite was higher than that of monolithic Fe₂Al₅ due to the addition of the hard phase of Al₂O₃ and the crack deflection by Al₂O₃.     

Effect of severe plastic deformation on the structure and low-temperature internal friction of Fe3Al and (Fe,Cr)3Al

Internal friction (IF) in severely deformed Fe-26Al and Fe-26Al-5Cr (at. %) alloys based on the Fe₃Al intermetallic compound has been studied in a temperature range from ?180 to 650°C. A group of low-temperature IF maxima have been detected in the temperature range from ?130 to 100°C at a frequency of 1.4–2.2 kHz. A comparative analysis of the revealed peaks has been carried out for Fe₃Al both with and without chromium. The thermal stability of some IF maxima which form a group of low-temperature peaks in the severely deformed state has been studied. The activation energies for the maxima detected have been evaluated. An assumption has been made about the dislocation-related mechanism of the peak origin, some of which appear to be of the Hasiguti type. Within the framework of this assumption, the height and stability of some IF maxima in the Fe-26Al-5Cr alloy, as compared to those in the Fe-26Al alloy, can be explained by increasing mobility of dislocations (dislocation segments) due to alloying with chromium.     

Surface Microstructure of Fe3Al After Severe Plastic Deformation

Nanocrystals were produced on the surface of Fe₃Al intermetallic compound by a severe plastic deformation technology—surface mechanical attrition. The phase and grain evolutions were characterized through x-ray diffraction technology, transmission electron microscopy, and Mossbauer spectroscopy. The results show that surface grains are refined to 15 nm in average size after 60 min of attrition. The surface grains present inhomogeneity due to the nonuniform plastic deformation when the attrition time is less than 15 min. Large quantities of dislocations and tangles of dislocations are observed in those larger grains. Al₈Cr₅ phase forms on the surface of Fe₃Al samples which have undergone attrition for less than 15 min. When the attrition time increases, the Al₈Cr₅ phase becomes disordered and dissolves into the matrix. In addition, disordering of Fe₃Al occurs only in a very thin area during severe plastic deformation.     

Room Temperature Mechanical Properties of Fe3Al Intermetallic Alloys with Li and Ni Additions

This work analyzes the effect of very small additions of Li and Ni on the mechanical properties of the Fe₇₂Al₂₈ intermetallic alloy, close to the Fe₃Al ratio. Hardness and tensile strength at room temperature were analyzed for the cast Fe₃Al intermetallic alloys as a function of Li and Ni additions. For this purpose, high-purity raw materials were melted in an open induction furnace into a SiC crucible and the liquid alloys were poured into sand moulds to directly get tensile test-shaped specimens to minimize machining. Mechanical tests were carried out in the as-cast condition as well as after a homogenization heat treatment at 400 °C for 120 h. The obtained microstructure was characterized by XRD and SEM. Results show an increase in ductility, particularly when Li was added.     

Solid-state phase stabilities of relevance to metal dusting in coal gasifier environments

Thermochemical calculations involving some typical mixed-gas environments and temperatures of gasifiers were used to examine iron-based phase stabilities to assess the potential for metal dusting in coal gasification systems. The results indicated that Fe₃C is only stable under conditions when certain reactions are suppressed and that FeO and Fe_0.877S tend to form instead of the carbide. Assuming Fe₃C is a necessary step in the metal dusting of steels, there are therefore numerous coal gasification systems where this type of carbon-related degradation will not occur, particularly under conditions associated with higher oxygen and sulfur activities.     

α‐Fe layer formation during metal dusting of iron in CO‐H2‐H2O gas mixtures

The formation of an α‐Fe layer between cementite and graphite was observed and investigated during metal dusting of iron in CO‐H₂‐H₂O gas mixtures at both 600°C and 700°C. The condition to form this phenomenon is determined by the gas composition which depends on temperature. The iron layer formation was observed for CO content less than 1 % at 600°C and less than 5 % at 700°C. With increasing CO contents, no α‐Fe layer was detected at the cementite/graphite interface by optical microscopy. In this case cementite directly contacts with the coke layer. The morphologies of the coke formed in the gas mixtures with low CO contents were also analysed. Three morphologies of graphite have been identified with 1 % CO at 600°C: filamentous carbon, bulk dense graphite with columnar structure, and graphite particle clusters with many fine iron containing particles embedded inside. At 700°C with 5 % CO the coke mainly consists of graphite particle clusters with some filamentous carbon at the early stage of reaction. Coke analysis by X‐ray diffraction shows that both α‐Fe and Fe₃C are present in the coke. The mechanism of α‐Fe accumulation between cementite and graphite is discussed in this paper.     

Metal dusting of ferritic Fe‐Al‐M‐C (M = Ti,V, Nb,Ta) alloys in CO‐H2‐H2O gas mixtures at 650 °C

Iron aluminides are known for their resistance to high temperature oxidation and sulphidation. Only little information is available about carburisation and metal dusting of Fe‐Al alloys. Metal dusting experiments with Fe‐15Al and Fe‐15Al‐2M‐1C alloys (in at.%) with M = Ti, V, Nb, or Ta were conducted at 650°C in CO‐H₂‐H₂O gas mixtures with the carbon activity a_c = 28. The kinetics of the carbon transfer was measured using thermogravimetric analysis (TGA). It is shown that the mass gain kinetics decreases by adding the alloying elements Nb, Ta, V, or Ti with C. Alloying with titanium and carbon leads to the most significant decreasing effect. The metallographic cross section observation showed a general metal wastage for Fe‐15Al, but local pitting for the Fe‐15Al‐2Nb‐1C and Fe‐15Al‐2Ta‐1C alloys. For the Fe‐15Al‐2V‐1C and Fe‐15Al‐2Ti‐1C alloys no significant attack was observed. Needle‐ or plate‐like Fe₃AlC_x precipitates were detected in the carburised samples. The existence of this ternary carbide with perovskite structure was predicted by thermodynamic calculations using the software Thermo‐Calc. The morphology of graphite on the surface was analysed by scanning electron microscopy (SEM). Mainly fine filaments with iron containing particles were detected. Cementite was detected in the coke layer by X‐ray diffraction analysis (XRD).