Sulfidation properties of an Fe-26.6 at.% Cr alloy at temperatures of 973–1173 K in H2S-H2 atmospheres at sulfur Pressures 104–10−6 Pa

An investigation is reported on the sulfidation properties of an Fe-26.6 at. % Cr alloy at 973, 1073, and 1173 K in H₂S-H₂ atmospheres at sulfur pressures 10⁴ 10^–6 Pa. The sulfidation kinetics when plotted according to a parabolic relationship usually exhibited an early slow transient period before onset of parabolic kinetics. Scales contained up to three layers. A triplex (CrFe)S_x/(CrFe)₃S₄/-(FeCr)S_x scale was formed at high sulfur pressures (range I), a single-phase (FeCr)S_x or a duplex (CrFe)S_x/(FeCr)S scale at intermediate sulfur pressures (range II), and a single-phase (CrFe)S_x scale at low sulfur pressures (range III). These pressure ranges at 973 K were: range I = 10^–2Pa, 10^–2 > (range II) 10^–5 Pa, and range III .     

Sulfidation mechanism of an Fe-26.6 at.% Cr alloy at temperatures 973–1173 K in H2S-H2 atmospheres at sulfur pressures 104–10−6 Pa

An investigation has been carried out to establish the sulfidation mechanism of an Fe-26.6 at. % Cr alloy at 973, 1073, and 1173 K in H₂S-H₂ atmospheres at sulfur pressures 10⁴ .     

Sulfidation properties of nickel-20 wt.% molybdenum alloy in hydrogen-hydrogen sulfide atmospheres at 700°C

The sulfidation kinetics and morphological development of the reaction products for a Ni-20wt.% Mo alloy exposed at 700° C to H₂-H₂S atm at sulfur pressures in the range 1×10^–11 to 2×10^–2 atm are reported. At Ps₂5×10^–11 atm, the reaction product was Mo₂S₃ which grew as an external scale by parabolic kinetics. For 1×10^–1024×10^–10 atm, simultaneous internal precipitation and external growth of MoS₂ occurred by linear kinetics. An external duplex scale was formed at sulfur pressures 2×10^–822×10^–2 atm in which the inner layer was a two-phase mixture of MoS₂ and nickel sulfide, and the outer layer contained solid nickel sulfides and a liquid Ni-Mo sulfide phase. Catastrophic linear kinetics occurred under these latter condition.     

Hydrogen permeation in Fe-40 at.% Al alloy at different temperatures

The electrochemical permeation technique for studying transport and trapping of hydrogen in Fe-40 at.% Al alloy at temperatures of 5, 25, 45 and 65 °C was used in the paper. The influence of temperature on the effective hydrogen diffusion coefficient, hydrogen permeation rate and hydrogen solubility was determined. The activation energy of hydrogen diffusion in iron aluminide in the studied temperature range was also determined.     

Sulfidation of manganese in H2S-H2 atmospheres at temperatures between 1073 and 1273 K

The sulfidation behavior of Mn in H₂S-H₂ atmospheres, atm, maintained at a total pressure of 1 atm was investigated at temperatures between 1073 and 1273 K. The reaction kinetics obeyed the parabolic rate law; the parabolic rate constant was proportional to where n=6.1±0.3. A value of 21,000±2000 cal/mole was estimated for the activation energy of the growth of the sulfide scale under constant sulfur pressure. Polycrystalline columnar -MnS scales were formed which exhibited texture with preferred (111) orientation. The parabolic rate constants at different sulfur pressures from this and previous investigations were used to evaluate the manganese self-diffusion coefficient in -MnS at .     

Sulfidation of manganese in sulfur vapor at low pressure and temperatures between 873 and 1273 K

An investigation was carried out on the sulfidation kinetics of manganese plates by thermogravimetry in pure sulfur vapor at low pressures, 4.5×10^–5< p_S2 (atm)<7.2×10^–4, at temperatures between 973 and 1173 K. The reaction kinetics were parabolic and the sulfidation rate constant at 1073 K was proportional to where n = 6.4±0.8. The activation energy for the sulfidation reaction at atm was 25,000±3000 cal·mol^–1. The -MnS developed a preferred orientation texture, and grain growth was observed even though the metal exhibited no preferred orientation.     

The effect of aluminum ion implantation on the oxidation of an Fe-6 at.% Al alloy at 1173 K

Samples of an Fe-6 at.% Al alloy were implanted with doses of 5×10¹⁶–3×10¹⁷ Al⁺ ions/cm² at 100 kV, and subsequently oxidized at 1173 K in O₂ gas at 27 kPa. By comparison with the behavior of unimplanted specimens, no effect on oxidation was observed if the dose of implant was less than 1×10¹⁷ Al⁺ ions/cm². At doses of 1×10¹⁷ Al⁺ ions/cm² or higher, a change in the oxide scaling morphology was effected from a duplex -Fe₂O₃--Al₂O₃ scale to an -Al₂O₃-enriched film with a dispersion of -Fe₂O₃ nodules. Oxygen uptake by the high-dose samples was due principally to nodule formation and weight gains after 45 hr were lower by 50–70% as compared with the unimplanted material. Nucleation and growth of the nodules occurred only within the first 5 hr of reaction, but their presence acted as an upper limit on the benefits provided by implantation of aluminum. Nodule formation was shown to be characteristic of the oxidation properties of the alloy material, not an extraneous feature introduced by the implantation process itself. Surface preparation prior to implantation had little effect on oxidation: high-dose electropolished samples behaved similarly to mechanically polished samples when implanted to comparable doses.     

Oxidation of a quaternary two-phase Cu–40Ni–17.5Cr–2.5Al alloy at 973–1073 K in 101 kPa O2

Oxidation of a quaternary two-phase Cu–40Ni–17.5Cr–2.5Al (at.%) alloy was investigated at 973–1073 K in 101 kPa O₂. The alloy is composed of two phases. One light phase with lower Cr content forms the matrix of the alloy, and the other medium gray phase richer in Cr is presented in the form of continuous islands. At 973 and 1073 K, the kinetic curves for the present alloy deviate evidently from the parabolic rate law. They show a large mass gain in initial stage, and then their oxidation rates decrease evidently with time until they become very small up to 24 h. Cross sectional morphologies show the present alloy is able to form continuous external scales of chromia over the alloy surface with a gradual decrease in the oxidation rate. However, the previous studies showed that a ternary two-phase Cu–40Ni–20Cr alloy is unable to form protective external scales of chromia over the alloy surface, but is able to form a thin and very irregularly continuous layer of chromia at the top of the mixed internal oxidation region. Therefore, substituting Cr in Cu–40Ni–20Cr alloy with 2.5 at.% Al is able to decrease the critical content required to form Cr oxide and help to form continuous external scales of chromia under lower Cr content in two-phase alloys.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

A new sol-gel process for preparing Ba（Mg1／3Nb2／3）O3 nanopowders

The stacking fault energies of Ti-46Al-8.5Nb-0.2W alloy at 298 K and 1273 K were determined. The principle for the determination of the stacking fault energies is based on the fact that the stacking fault energy and the elastic interaction energy acting on the dissociated partial dislocations are equal. After the compress deformations with the strain of 0.2% at 298 K and 1273 K, and water quench to maintain the dislocation structures deformed at 1273 K, the dissociation distances between two partial dislocations were determined by weak beam transmission electron microscopy (WBTEM) technique. Based on these dissociation distances and the corresponding calculation method, the stacking fault energies were determined to be 77-81 mJ/m2 at 298 K and to be 57-60mJ/m2 at 1273 K respectively.     

Sulfidation properties of an Fe-23.4Cr-18.6Al alloy at temperatures 1073 and 1173 K in H2S-H2 atmospheres of sulfur at pressures 104-10−5 Pa

An investigation is reported on the sulfidation properties of an Fe-23.4Cr-18.6Al(at.%) alloy at 1073 and 1173 K in H₂S-H₂ atmospheres, 10⁴ > P_{S 2} 10⁵Pa. The sulfidation kinetics exhibited an early transient period before onset of parabolic kinetics. Values of the parabolic sulfidation rate constants increased by as much as 10⁵ from their smallest values at low sulfur pressures, P_{S 2} 10^–4 Pa, to maximum values at sulfur pressures P_{S 2} 10² Pa. Multilayered scales were formed, the number and types of layers dependent on sulfur pressure. A fully developed scale at sulfur pressures P_{S 2} > 10^–3 Pa.     

Influence of electroplating conditions on magnetic properties of Fe-36wt.% Ni alloy film

A Fe-Ni soft magnetic film was prepared in sulphate solution by electroplating. The influences of the molar ratio of n [Fe2+ ]/ n [Ni2+ ], current density, bath temperature, pH and L-ascorbic acid concentration on magnetic properties of Fe-Ni alloy film were investigated. The results show that the saturated flux density(BS) of the film increases initially and decreases after it reaches the specific value with the increase of n [Fe2+ ]/ n [Ni2+ ] molar ratio, current density, bath temperature and pH. However, the relationship between L-ascorbic acid concentration and B S keeps linear. It is observed that the coercive force( Hc) is enhanced with the increase of n [Fe2+ ]/ n [Ni2+ ] molar ratio, current density and pH. By comparison, when the bath temperature increases, Hc always decreases. With the increase of L-ascorbic acid concentration, the coercive force increases initially and then decreases.     

Oxidation properties of Co-Mn alloys at temperatures from 1273 to 1473 K

The oxidation behavior of Co alloys containing up to 23.0 % Mn was investigated at temperatures of 1273–1473 K in an oxygen pressure range of 10–10⁵ Pa using thermogravimetry, electron-probe microanalysis, and optical microscopy.Oxidation followed a parabolic rate law, except in the early reaction stage. For the growth of the (Co, Mn)O scale, parabolic rate constants increased with increasing manganese content, whereas with further additions of manganese the rate constants decreased due to an intermediate layer of (Co, Mn)₃O₄. This critical manganese content increased with increasing temperature. The scale structures were classified into three groups: group I, a single-layer scale; group II, an inner-outer double layer; and group III, an inner-intermediate-outer triple layer. The inner layer consisted of (Co, Mn)O; the outer layer was a mixture of (Co, Mn)O and (Co, Mn)₃O₄; the intermediate layer was composed of (Co, Mn)₃O₄. Cation concentration profiles were measured across the external scales with single- or multilayer structures. Diffusional analysis indicated that manganese moved 1.25 times faster than did cobalt in the (Co, Mn)O scale. Activation energies for oxidation are 118–126 kJ/moles for groups I and II, and for the alloys in group III they are 290–367 kJ/moles. Parabolic rate constants and scale structures are discussed on the basis of the Co-Mn-O phase diagram.     

Internal friction, dilatometric and calorimetric study of anelasticity in Fe-13 at.% Ga and Fe-8 at.% Al-3 at.% Ga alloys

This paper investigates the structural transitions associated with different cooling rates from a high temperature disordered state and the effect of substitution of Ga atoms by Al atoms in Fe-Ga binary alloys on the ordering processes. Two iron-based low carbon (about 0.04 at.% C) alloys Fe-13 at.% Ga and Fe-8 at.% Al-3 at.% Ga are studied. Internal friction, dilatometric and calorimetric tests are carried out to check ordering in these alloys and contribution of structural defects to relaxation spectrum. Several thermally activated internal friction peaks have been observed and their activation parameters evaluated by means of temperature and frequency dependent internal friction tests using forced vibration. For most of these peaks physical mechanisms are proposed. Apart from these thermally activated relaxation peaks, a structural, frequency independent relaxation takes place at 250-300 °C. Dilatometric and DSC curves show the appearance of a contraction effect in the same temperature range. This effect was studied in alloys cooled down with different cooling rates. We believe that the frequency independent internal friction peak (denoted as the P3 peak in this paper) and peaks at dilatometric and DSC curves are controlled by the same structural mechanism and therefore the activation energy for this anelastic mechanism is derived from DSC data.     

Effect of ambient air pressure on the oxidation kinetics of Fe-6 at.% Si alloy

Surface oxidation of Fe-6 at.% Si alloy was investigated during annealing in ambient air of various pressures with simultaneous isothermal resistivity registrations. Measurements have been done in the temperature range 500-540 °C. Chemical and phase compositions of the samples were analyzed using X-ray photoelectron spectroscopy, conversion electron Mössbauer spectroscopy (CEMS), transmission Mössbauer spectroscopy (TMS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Phase analysis showed that during isothermal resistivity measurement in a low pressure air 100 mbar a protective film of hematite α-Fe₂O₃ was formed on the surface of FeSi substrate. By decreasing pressure to 10⁻² mbar the time dependence of the resistivity exhibits an increase due to the transformation of hematite to magnetite Fe₃O₄. The activation energy for this transformation is 115 ± 5 kJ/mol. By regressive increasing the pressure back from 10⁻² to 100 mbar a non-protective oxide scale of hematite + magnetite was formed. The results were interpreted in the light of the iron-oxygen phase diagram.     

Mechanical properties of injection molded Fe-6% Ni-0.4% C steels with varying Mo contents of 0.5 to 2%

In our previous studies, sintered and heat-treated alloy steels (Fe−6Ni−0.5Mo−0.4C (mass%)) produced by a MIM process showed excellent mechanical properties of 2000 MPa tensile strength and 5% elongation. This was attributed to the solid solution strengthening and the mezzo-heterogenous microstruture, which consisted of martensite or retained austenite (Ni and Mo rich phases) surrounded by a network of tempered martensite. This study has been performed to clarify the effect of Mo on the mezzo-heterogeneous microstructure and the mechanical properties of MIM processed and sintered alloy steels (Fe−6Ni−0.4C) with varying Mo content (0.5–2 mass%). The tensile properties of the heat-treated steels with added 2 mass% Mo were lower than those of the steels with added 0.5 mass% Mo. The reduction in the tensile properties, particularly the appearance of large pores formed at the original location of Mo power through the transient liquid phase formation and the low hardness of the matrix, was due to the low sintered density. By using mechanically milled fine Ni and Mo powders, the heat-treated steel (Fe−6Ni−2Mo−0.4C) showed excellent properties, including tensile strength of 1800 MPa and ductility of 2.2% elongation. This article is based on a presentation made in the symposium “The 3rd KIM-JIM Joint Symposium on Advanced Powder Materials”, held at Korea University, Seoul, Korea, October 26–27, 2001 under auspices of The Korean Institute of Metals and Materials and The Japan Institute of Metals.     

In situ measurement of Young’s modulus of FeO scale formed on pure iron at 973–1273 K by acoustic resonance method

The Young’s modulus of FeO scales formed on a pure iron bar was measured at temperatures between 973 and 1273 K. The FeO was formed with charging 4.2 × 10⁻⁴ mol of oxygen gas five times into a reaction chamber. During the growth of FeO, a sinusoidal vibration signal was applied to the bar. The vibration force and the acceleration of the oxidised iron bar were measured in situ in order to obtain resonance frequencies. The Young’s modulus of FeO, as estimated at oxidation temperatures of 973, 1173, and 1273 K, was 98.6, 75.9, and 83.4 GPa, respectively.