Papers by Keyword: Iron Aluminide

Abstract: In the present work, samples of a binary intermetallic alloy (Fe₃Al) with 26at.%Al were submitted to electrochemical corrosion evaluation in a 0.5M H₂SO₄ solution containing naturally dissolved oxygen. The corrosion resistance was evaluated by applying linear polarization, electrochemical impedance spectroscopy and potentiodynamic polarization at 22 and 35°C. The results obtained revealed that in both conditions the material exhibits active-passive behavior. Heating to 35°C did not alter the passivity characteristics of the alloy (passivation range and passive current density), but elevated the corrosion current density and the critical current density. The polarization resistance of the samples was also decreased with temperature, following the trend observed for the corrosion current density.

Abstract: The influence of Spark Plasma Sintering / Field Assisted Sintering Technology applying pulsed direct current up to the root-mean-square current densities of 129 A/cm² on the interfacial reactions in Al - Fe - Al stacks was investigated at temperatures between 500°C and 600°C. Independently of the current density and current direction, thin Al₁₃Fe₄ and wide Al₅Fe₂ phases were detected in the diffusion couples. The Al₅Fe₂ phase consisted of columnar grains having a {001}-fiber texture. Al₁₃Fe₄ was found in the form of discontinuous spots at the Al/Al₅Fe₂ interface. The interface between Al₅Fe₂ and Fe was highly fringed. The layer growth kinetics of Al₅Fe₂ was parabolic. The growth rate was strongly enhanced in the SPS/FAST experiments as compared to the conventional diffusion experiments, independently, on the current direction. It is suggested that the enhanced growth rates are a result of temperature gradients existing in a typical Spark Plasma Sintering device. Possible effects of thermomigration and electromigration are discussed.

Abstract: High-temperature creep of a Fe3Al-type iron aluminide alloyed by niobium and different additions of carbon was studied in the temperature range from 600 to 800 °C. The alloys contained (atomic %) (i) 27.6 Al, 1.15 Nb, 0.19 C and (ii) 27.1 Al, 1.11Nb, 0.76 C (Fe balance). Creep tests were performed in compression at constant load with stepwise loading. Stress exponent and activation energy of the creep rate were determined. Creep resistance of the low-carbon alloy is better at lower temperatures, while the opposite is true at temperature of 800 °C.

Abstract: Pressureless reactive sintering production of iron aluminides is always connected with high porosity of the product. Previous research showed that silicon reduces the porosity significantly. In this work, the effect of alloying elements (Cu, Ni) on the reactive sintering behaviour and on the porosity of Fe-Al and Fe-Al-Si alloys was studied. Microstructure, phase composition, mechanical and tribological properties were studied as functions of alloy composition.

Abstract: In this work, we employed the Mössbauer spectroscopy in a study of point defect formation in intermetallic phases of the B2 structure from the Fe-Al system as a function of Al concentration. We present the values of the 57Fe isomer shift and quadruple splitting for the components describing the point defect in the local environment of a Mössbauer nuclide. The concentration of the Fe vacancies and Fe atoms substituting Al (Fe-AS) are determined. The results shown that an increase in Al content causes an increase in vacancy and Fe-AS

Abstract: In this work, Mössbauer spectroscopy and X-ray powder diffraction (XRD) are used in a study of point defect formation in intermetallic phases of the B2 structure of the Fe-Al system as a function of Al concentration. The results are compared with the concentrations of point defect determined from positron annihilation data. In the Mössbauer effect, two types of samples are investigated: Fe-Al alloys with few additives obtained by induction melting and Al-rich metallic powders produced by the self-decomposition method and intensive grinding of high energy in the electro-magneto-mechanical mill. The work presents the values of the 57Fe isomer shift and quadruple splitting for the components describing the point defect in the local environment of a Mössbauer nuclide. The concentration of the Fe vacancies and Fe atoms substituting Al (Fe-AS) are determined. The results show that an increase in Al content causes an increase in vacancy and Fe-AS concentration.

Abstract: Multi-walled carbon nanotubes (MWNTs)-Fe3Al was prepared by spark plasma sintering. Microstructural investigations show that MWNT remains in the composites. The magnetic properties were investigated with alternating gradient force magnetometer, which shows that the composites still display good soft magnetic property. The MWNTs-Fe3Al composites have a similar magnetic hysteresis loops to that of Fe3Al, indicating the good soft magnetic properties of the composite. Excellent magnetic property implies that MWNT-Fe3Al composites may also have significant potential for applications in electronic-magnetic nanodevice fields.

Abstract: The steels with chromium contents between 9 and 12%wt are used for power plants with advanced steam conditions. These steels possess good creep properties similar to the 9% Cr steels as well as good creep and good oxidation resistance at temperatures between 500-600°C. In the last years efforts have been made to develop coatings for protection against oxidation in order to allow operation of steam turbines at 650°C. In this study, Al-Hf protective coatings were deposited by CVD-FBR on the ferritic steel HCM-12A followed by a diffusion heat treatment, and were shown to be protective at 650°C under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure. The morphology and composition of the coatings were characterized by techniques, including scanning electron microscopy (SEM), electron probe microanalysis, and Xray diffraction (XRD). The results showed a substantial increase of steam oxidation protection afforded by Al-Hf coating deposited by the CVD-FBR process.

Abstract: Slurry iron aluminide coatings are very resistant to steam oxidation at 600-650º C. These coatings can be used to protect new generation Ultra Super Critical (USC) steam power plant ferritic/martensitic steel components. The microstructure of the initially deposited coating changes as a function of time, mainly due to coating-substrate interdiffusion, going from mostly Fe2Al5 to FeAl, causing the precipitation of AlN in those substrates containing a minimum content of N and moreover, developing Kirkendall porosity at the coating-substrate interface. Steam oxidation at 650º C causes the formation of a protective thin layer of hexagonal χ-Al2O3 phase along with some α- and γ-Al2O3 after the first few hours of exposure. However, despite the relatively low temperature, and after several thousands hours the protective layer was mostly composed of α-Al2O3. A study of the evolution of the microstructure of slurry aluminide coatings deposited on P92 and exposed to steam at 650º C has been carried out by scanning and transmission electron microscopy and X ray diffraction.

Abstract: A novel super-hydrophobic film was prepared by myristic acid (CH3(CH2)12COOH) chemically adsorbed onto the polyethyleneimine (PEI) coated Fe3Al-type intermetallic wafer. The film character and structure were probed with contact angle measurement, scanning electron microscopy (SEM) and atomic force microscope (AFM). The results suggest that the structure of the film is similar to lotus and the seawater contact angle is larger than 150◦. Moreover, the corrosion resistances of untreated and modified samples in seawater were investigated by electrochemical impedance spectroscopy (EIS). Experimental results show that the corrosion rate of Fe3Al-type intermetallic with super-hydrophobic surface decreases dramatically because of its special microstructure.