Hot hardness of (Fe,Cr)3C and (Fe,Cr)7C3 carbides

Hot hardness was measured on the primary carbides, (Fe, Cr)₃C and (Fe, Cr)₇C₃, in unidirectionally solidified iron-carbon-chromium hypereutectic alloys with chromium more than 4.8 wt %. The hardness-temperature relation was represented by two Ito-Shishokin formulae,H _v =A(— BT), and thus was drawn by two lines on a semilogarithmic graph. The inflection temperature where the two lines intersected was found at 730 to 860 K for (Fe, Cr)₃C carbide containing 0 to 14 wt % Cr, increasing with an increase in the chromium concentration in the carbide, and at about 910 K for (Fe, Cr)₇C₃ carbide containing 36 to 76 wt % Cr. With increasing chromium concentration in each carbide, the hardness of the carbide increased and the thermal softening coefficients decreased. The effect of chromium on the hardness, the inflection temperature and the thermal softening coefficients was more pronounced for (Fe, Cr)₃C carbide than for (Fe, Cr)₇C₃ carbide. Each of the thermal softening coefficients,B ₁(T<T _t),B ₂(T>T _t), the inflection temperature,T _t, room-temperature hardness,H _v(T _RT), and the hardness atT _t,H _v(T _t), related linearly to the chromium concentration in the carbides, and hence the hot hardness of the carbides could be expressed as functions of temperature and chromium concentration in the carbides. The relationships betweenH _v(T _RT) andH _v(T _t) and between the thermal softening coefficient,B ₂, and the activation energy for creep,Q _c(kJ mol^–1), were represented by the following equations:H _v(T _t)0.7H _v(T _RT),B ₂=1.26/Q _c.     

(Fe,Cr)7C3/Fe surface gradient composite: Microstructure,microhardness, and wear resistance

In this study, the (Fe,Cr)₇C₃/Fe surface gradient composite was produced by in situ synthesis process with subsequent heat treatment. According to the results of thermal analysis, the as-cast specimen was subjected to heat treatment at 1180 °C for 3 h in argon atmosphere. The phase composition, microstructure, microhardness, and wear resistance of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers hardness tester, and wear resistance tester, respectively. The XRD results show that α-Fe and (Fe,Cr)₇C₃ are the predominant crystalline phases in the composite obtained. The volume fraction of (Fe,Cr)₇C₃ particulates has a gradient distribution from the surface to the matrix, and the morphology of (Fe,Cr)₇C₃ particulates changes considerably. A dense ceramic layer is formed on the upper surface of (Fe,Cr)₇C₃/Fe surface gradient composite with a volume fraction of 90%. The microhardness of the dense ceramic layer is 1484 HV_0.1, and its relative wear resistance is five times higher than that of the iron matrix.     

90°coupling in (Fe/Cr/Fe)AFM/Cr/Fe system epitaxially grown on GaAs(0 0 1)

Single-crystalline (Fe/Cr/Fe)_AFM/Cr/Fe structures were epitaxially grown on atomically flat GaAs(0 0 1). Choosing the same thickness of the antiferromagnetically (AFM) coupled Fe layers in the bottom (Fe/Cr/Fe)_AFM structure, their net magnetization is balanced to zero, in particular up to a spin-flop transition when the field is applied along the [1 1 0] direction. For the Cr thicknesses at which the top Fe layer is weakly magnetically coupled to the bottom (Fe/Cr/Fe)_AFM structure, at low fields, the magneto-optical Kerr effect and/or SQUID signal from the sample corresponds to the top Fe layer only. An influence of the Cr spin structure on the magnetization reversal in the Fe layer is reported. In particular, a strong increase of coercivity (by a factor of 12) is found at low temperatures. A 90° coupling is detected which affects the minor loops measured along the [−1 1 0] and [1 0 0] directions.     

Superplasticity of Fe3Al(Cr)

Abstract

The superplasticity of an Fe₃Al based intermetallic alloy with 3 at.-% chromium has been investigated in the strain rate range 10^-5-10^-2 s^-1 at test temperatures between 700 and 900°C. The composition of the iron aluminide was Fe–28Al–3Cr (at.-%) with additions of titanium and carbon. After thermomechanical processing the material possessed a coarse grained microstructure with an average grain size of 55 ± 10 μm. Strain rate exponents of 0·33≤m≤0.42 were recorded at strain rates of approximately 10^-5-10^-3 s^-1 in the temperature range 750-900°C. Superplastic elongations of 350% and more were achieved. From thermal activation analysis of superplastic flow, an activation energy of 185 ± 10 kJ mol^-1 was derived. This value is comparable to activation energies of superplastic flow in Fe₃Al(Ti) alloys. However, in unalloyed Fe₃Al the activation energy is higher, ~ 263 kJ mol^-1. Optical microscopy showed grain refinement to ~ 30 ± 5 μm in size in superplastically strained tensile specimens. Transmission electron microscopy gave evidence of the formation of subgrains of 0·3–0·5 μm in size. Superplasticity in this iron aluminide is mainly attributed to viscous dislocation glide, controlled by solute drag in the transformed B2 lattice at the deformation temperatures. During superplastic deformation, subgrain formation and grain refinement in the gauge length were revealed. From this it is concluded that dynamic recrystallisation makes an important contribution to the deformation mechanism of superplastic flow in this material.     

Understanding the solidification and microstructure evolution during CSC-MIG welding of Fe–Cr–B-based alloy

The present is a study of the solidification and microstructure of Fe–28.2%Cr–3.8%B–1.5%Si–1.5%Mn (wt.%) alloy deposited onto a 1020 plain carbon steel substrate using the controlled short-circuit metal inert gas welding process. The as-solidified alloy was a metal matrix composite with a hypereutectic microstructure. Thermodynamic calculation based on the Scheil–Gulliver model showed that a primary (Cr,Fe)₂B phase formed first during solidification, followed by an eutectic formation of the (Cr,Fe)₂B phase and a body-centered cubic Fe-based solid solution matrix, which contained Cr, Mn and Si. Microstructure analysis confirmed the formation of these phases and showed that the shape of the (Cr,Fe)₂B phase was irregular plate. As the welding heat input increased, the weld dilution increased and thus the volume fraction of the (Cr,Fe)₂B plates decreased while other microstructural characteristics were similar.     

Effect of chromium content on electrical resistivity and tensile properties of Ti–Fe–Cr alloys

Abstract

The effects of substituting chromium for iron and the use of low cost ferrochromium alloys in the production of β-Ti–Fe–Cr alloys have been studied with respect to phase constitution, stability, and mechanical properties, in solution treated and quenched states using resistivity, hardness measurement, X-ray diffraction, and tensile testing. Resistivity at room and liquid nitrogen temperatures, and hardness decreased while the ratio of resistivity at liquid nitrogen temperature to that at room temperature increased with increases in chromium content. Alloys of Ti–Fe–Cr, with almost the same electron per atom value, with higher chromium content have smaller volume fractions of athermal omega than alloys with higher iron content. There is less solution hardening in the former alloys than in the latter alloys. Tensile strength decreased with increases in chromium content, while elongation and reduction in area significantly increased. The balance between tensile strength and ductility (elongation and reduction in area) improved in the alloys with added chromium as a substitute for iron. Therefore, no negative influences of ferrochromium alloying on mechanical properties was observed in this study.     

Structure and mechanical properties of rapidly solidified Al-(Fe,Cr) and Al-Mg-(Fe,Cr) alloys

For Al-(Fe,Cr) and Al-Mg-(Fe,Cr), by employing a combination of X-ray diffraction, metallography and transmission electron microscopy, detailed understanding of microstructural transformations that occur during rapid solidification and consolidation has been achieved. A major decrease in the solid solubility extension of Fe in an Al-Mg system with an increase of Mg concentration has been found. The decrease in solubility of Fe results in the reduction of strength and hardness of the Al-Mg-Fe alloys in comparison with Al-Mg-Cr alloys.     

Strengthening of σ phase in a Fe20Cr9Ni cast austenite stainless steel

The strengthening mechanism of σ phase in a Fe20Cr9Ni cast austenite stainless steel used for primary coolant pipes of nuclear power plants has been investigated. The yield and ultimate tensile strengths of aged specimens increased comparing with those of the unaged ones. It was found that the increase of strengths is due to the hard and brittle (σ + γ₂) structure which decomposed from α phase in the steel. Fracture surfaces of specimens after in situ tensile test showed that the inhibition of (σ + γ₂) structure on the dislocation movements was more significant than ferrite although cracks started predominately at σ/γ₂ interfaces. The (σ + γ₂) structure behaves like a fiber reinforced composite material.     

Mechanical properties of Cr, Mn, Fe, Co, and Ni modified titanium trialuminides

To increase gas turbine efficiency, new high-temperature-resistant and lightweight materials are needed. Titanium trialuminides are an interesting option, but they are currently too brittle at room temperature to be used as turbine blades. Brittleness of TiAl₃ can be reduced by alloying with several transition metal elements. The effect of ternary additions of Cr, Mn, Fe, Co, or Ni at various stoichiometries (3 to 9 at.%) to the intermetallic TiAl₃ is studied by metallography, X-ray diffraction and three-point-bend testing. These methods indicate that the 6 at.% Cr or Co additions give the highest flexural strength with only a small trade-off in failure strain when compared to the 9 at.% additions of chromium and manganese.     

Structural and electrical conductivity studies on (M,V)-TiO2 (M=Al, Cr, Fe) rutile solid solutions at high temperature

In this study, M _x Ti_0.91–1.82x V_0.09+0.82x O₂ (0x0.1; M=Al, Cr, Fe) solid solutions have been synthesized by ceramic method and characterized by X-ray diffraction and electron microscopy. The results indicate that rutile solid solutions stability depends on the trivalent cation (M). Solid solutions are stable at 1000 °C, but only when M=Cr is a single phase with rutile structure obtained at 1400 °C. The electrical behavior of the system studied corresponds to extrinsic p-type semiconduction and electrical conductivity is related to the stability of rutile solid solutions.     

Auger electron spectroscopy analysis of interface roughness of Fe/Cr bilayers

In this work we investigate the surface and interfacial properties of Fe/Cr and Cr/Fe bilayers before and after annealing using Auger electron spectroscopy (AES). The roughness of the interface is also determined with the X-ray reflection method. The fitted values of inelastic mean free path λ_{Cr in Fe} reproduce the calculated value for Cr in Fe well, whereas the values of λ_{Fe in Cr} are significantly larger than the calculated ones, suggesting mutual segregation of atoms during growth. The low-energy range Auger spectra demonstrated that the MNN lines of Cr covered with Fe and Fe covered with Cr disappear after the deposition of 1 nm overlayer, this being an indication of continuous deposited film, but not excluding mixing at interfaces. The results of X-ray reflectometry measurements, which give the values of Fe/Cr and Cr/Fe roughness, are in accordance with this observation. The LMM Auger spectra of annealed samples showed that at the largest applied temperature, Cr diffuses into Fe, but the reverse effect of Fe diffusion into Cr is not observed.     

Coprecipitation of Pu(IV) with Fe(III) and Cr(III) hydroxides from nitrate-acetate solutions under hydrothermal conditions simulating deep disposal of liquid radioactive wastes

Precipitation of Fe(III), Cr(III), Ni(II), and Mn(II) from nitrate-acetate solutions and coprecipitation of Pu(IV) with Fe(III) and Cr(III) were studied. The degree of precipitation of 80–95% is attained for Fe(III) at 95–200°C and pH>0.5–0.6, and for Cr(III), at T=95°C and pH≥4.0 or T=200°C and pH≥1.0. The phase composition of the precipitates formed by thermal hydrolysis of iron nitrate in model solutions was analyzed. Depending on pH and temperature, the solid phase contains various modifications of Fe₂O₃, FeOOH, and amorphous phases. Noticeable coprecipitation of plutonium from nitrate-acetate solutions is observed at pH≥4, and it is incorporated in the precipitate only at formation of FeOOH. No coprecipitation of Pu(IV) with Fe₂O₃ was found. Under the given experimental conditions, plutonium in aqueous solutions occurs in the oxidation state +4 forming monoacetate (or, probably, hydroxo acetate) complexes.     

Crystal morphology of ternary compound (Cr,Fe)5Si3 obtained by in situ chemical vapour deposition

Ternary compound crystals of (Cr, Fe)₅Si₃ were obtained on a quartz substrate by the in situ CVD process using in situ reaction of the stainless steel 410 powder, Si₂Cl₆ and hydrogen, and its crystal morphology was examined in some detail. Crystals with various interesting morphologies, such as spiral, conical, rose-like, seaweed-like, globefish-like, etc., were obtained. The most commonly observed growth habits of the crystals were spiral, conical and seaweed-like crystals.     

Formation of boride layers at the Fe–25% Cr alloy–boron interface

Two boride layers based on the FeB and Fe₂B compounds are formed at the interface between a Fe–25% Cr alloy and boron at 850–950 °C and reaction times up to 12 h. The characteristic feature of both layers is a pronounced texture. Each of two boride layers is compositionally two-phase. The outer layer consists of the (Fe,Cr)B and (Cr,Fe)B phases. The inner layer comprises the (Fe,Cr)₂B and (Cr,Fe)₂B phases. The diffusional layer-growth kinetics are close to parabolic and can alternatively be described by a system of two non-linear differential equations, also producing a fairly good fit to the experimental data. Annealing of a borided Fe–Cr sample in the absence of boriding media results in the disappearance of the (Fe,Cr)B–(Cr,Fe)B layer, with the (Fe,Cr)B phase disappearing first. Microhardness values are 21.0 GPa for the outer layer, 18.0 GPa for the inner layer and 1.35 GPa for the alloy base. The abrasive wear resistance of the (Fe,Cr)B–(Cr,Fe)B layer, found from mass loss measurements, is more than 150 times greater than that of the alloy base.     

Fabrication and oxidation resistance of nanocrystalline Fe10Cr alloy

Nanocrystalline (nc) and microcrystalline (mc) Fe10Cr alloys were prepared by high energy ball-milling followed by compaction and sintering, and then oxidized in air for 52 h at 400 °C. The oxidation resistance of nc Fe10Cr alloy as determined by measuring the weight gain after regular time intervals was compared with that of the mc alloy of same chemical composition (also prepared by the same fabrication route and oxidized under identical conditions). Oxidation resistance of nc Fe10Cr alloy was found to be in excess of an order of magnitude superior than that of mc Fe10Cr alloy. This article also presents results of secondary ion mass spectrometry (SIMS) of oxidized samples of nc and mc Fe–Cr alloys, evidencing the formation of a more protective oxide scale in the nc alloy.     

Division of character zones and elements distribution of Fe3Al/Cr–Ni alloy fusion bonded joint

Abstract

A Fe₃Al/Cr–Ni alloy fusion bonded joint was divided into four character zones of a homogeneous mixture zone, a partial mixture zone, a partially fused zone and a heat affected zone. The microstructures, elements distribution and phase constitutions of the various character zones were analysed via metalloscope, SEM, electron probe microanalysis and X-ray diffraction. The results indicated that the microstructures were dissimilar in the different character zones. A 0·04–0·05 mm austenite rich band existed in the partial mixture zone. The diffusion of Fe, Al, Cr, Ni and C mainly occurred in fusion zone where Cr and Ni diffused into Fe₃Al to substitute some Fe on α ₁, α ₂, and β sublattices to form substitutional solid solution. The phase constitutions of Fe₃Al/Cr–Ni joint were Fe₃Al, γ-Fe, FeAl, NiAl, an unidentified Fe–C compound and an Fe–Cr–C compound (Cr₉Fe)₇C₃.     

Effect of strain rate on microstructures and mechanical properties of Fe–18Cr–8Ni steel

The effect of strain rate on deformation microstructures and mechanical properties of Fe–18Cr–8Ni austenitic stainless steel was investigated at strain rates of from 10^?3 to 100?s^?1. The results indicated that the deformation mechanism of steel changes from transformation induced plasticity (TRIP) to TRIP?+?twinning induced plasticity (TWIP) effect when the strain rate is increased from 10^?3 to 100?s^?1. The yield strength of steel increases gradually with strain rate increased, while the tensile strength and elongation first decreases and then increases slowly. The changes in tensile strength and elongation are due to the change of deformation mechanism with the strain rate increased.     

Sol-gel synthesis and characterisation of (Nd,Cr) co-doped BiFeO3 nanoparticles

Bi_0.95Nd_0.05Fe_1–xCr_xO₃ (x = 0, 0.01, 0.03, 0.05) samples are synthesised by the sol-gel method. The variation in structure, magnetisation, electrical and photocatalytic properties by Cr doping at Fe site in Bi_0.95Nd_0.05FeO₃ samples is analysed. X-ray diffraction pattern confirms the formation of rhombohedral structure in all the samples. The crystallite size is calculated using the Scherrer relation and found to be in nanometre range. Kubelka-Munk theory is used to determine the direct band gap of the samples from reflectance spectra. The saturation magnetisation is found to enhance the concentration of chromium. Arrott-Belov-Kouvel plot confirms the ferromagnetic nature in the samples. The leakage mechanisms are studied to understand the influence of Cr concentration on the BFO. A good correlation exists between the leakage current and ferroelectric behaviour. Photocatalytic tests show degradation of methylene blue dye in the presence of H₂O₂. A drastic decrease in photocatalytic activity is observed with the concentration of Cr.     

First principles study on the structural properties and electronic structure of X2B (X = Cr, Mn, Fe, Co, Ni, Mo and W) compounds

First principles calculations are performed to study the stability, electronic and structural properties of X₂B (X = Cr, Mn, Fe, Co, Ni, Mo and W). The calculated cohesive energy and formation enthalpy of these compounds both have negative values, which indicate that they are thermodynamically stable structures. The ground states of Cr₂B and Mn₂B are anti-ferromagnetic; Fe₂B and Co₂B are ferromagnetic; Ni₂B, Mo₂B and W₂B are paramagnetic. The calculated local magnetic of Fe₂B is 1.962μ_B/Fe, and for Co₂B is 1.182μ_B/Co. They are comparable to the values of Fe₃B (1.97μ_B/Fe) and Co₃B (1.18μ_B/Co), but smaller than pure Fe and Co. The observed magnetic behaviors of X₂B compounds can be explained by Stoner’s model. Two main peaks are observed in the calculated PDOS (partial density of states) of these compounds (P1 and P2). P1 is caused by strong covalent X–B bonds and P2 is attributed to metallic X–X bonds.     

Thermomagnetic studies of (Co0.93Fe0.07)75−x Cr x Si15B10 metglas

Thermomagnetization measurements have been carried out on the Co-Fe-Cr-Si-B metglas up to 7.5 at% Cr. For the Co₇₀Fe₅Si₁₅Si₁₀ metglas, application of magnetic field has been found to influence the process of crystallization. Thermal annealing of this alloy at temperaturesT _c<T _a<T _cr leads to a decrease in the value ofT _c due to chemical short-range ordering (CSRO) and topological short-range ordering (TSRO). Introduction of chromium into the above alloy reduces the saturation magnetization and Curie temperature of the alloy at the rate of 5.55 emu gm^–1 and 41 K per at% Cr, respectively. These are explained due to the antiparallel coupling of the 3 d electrons of iron and cobalt atoms with those of chromium and to a deterioration in the strength of ferromagnetic exchange interactions between Co-Co, Co-Fe and Fe-Fe magnetic pairs.