The influence of primary precipitates on the tensile strength of unidirectionally solidified (Fe,Cr)-(Cr,Fe)7C3 in-situ grown composites containing 30 wt % Cr

The influence is investigated of metal dendrites and primary carbide needles on the tensile strength of the (Fe, Cr)-(Cr, Fe)₇C₃ insitu grown composite cont-aining 30 wt % Cr. It is found that these irregularities in the aligned structure diminish the tensile strength of thein-situ composite both at room temperature and at 900°C. The maxima in the tensile strength versus composition curves occur at different compositions for these temperatures. A possible explanation of this behaviour is given.     

Formation mechanism of in-situ V8C7 and V3B4 particles in iron matrix composites by vacuum sintering

The in-situ V₈C₇-V₃B₄/Fe45 composites were fabricated by vacuum sintering using ferrovanadium, carbon black and Fe45 alloy powders as raw materials. The microstructure, phase structure and hardness of sintered Fe45+x wt.% (FeV+C) composite specimens (x = 0, x = 5, x = 10, x = 15, x = 20) were investigated. The results show that the type and shape of in-situ borides and carbides depend on the content of (FeV+C) mixed powders. When x = 5, the composite consists of α-(Fe, Cr, V) solid solution, strip M₇C₃ carbides and block M₂B borides. When x = 10, the V₃B₄ particles initiate inside the M₂B borides by in-situ precipitation, and the excess vanadium reacts with carbon to in-situ form V₈C₇ particles. When x≥15, the M₂B completely transform into V₃B₄. Furthermore, the V₈C₇ and V₃B₄ particles coarsened and tended to be spheroidal and rectangle shape respectively. The V₃B₄ particles coarsen possibly by contacting and merging of the small V₃B₄ particles. However, the V₈C₇ particles coarsen possibly by the Ostwald ripening. The hardness of the composites increases firstly and then decreases with the increase of (FeV+C) mixed powders. The Fe45+10 wt.% (FeV+C) composite has the highest hardness value (54.2 HRC), which is due to the fine boride and carbide reinforcement and little pore defect.     

Laser in-situ synthesis of mixed carbide coating on steel

A 2.5 KW Nd:YAG laser was employed to modify the surface of a AISI 1010 steel deposited with a precursor powder mixture of Fe, Ti, Cr and C. In-situ formation of TiC and chromium carbides [M₇C₃ (M = Fe, Cr) and Cr₇C₃] was observed as function of laser processing power at constant scan speed. Although TiC was present in all the samples, the chromium carbides were absent in samples processed at certain laser powers. Corresponding to this behavior, variation in mechanical properties of the coating was observed. The hardness and wear properties of the samples without chromium carbides was inferior in comparison to samples with both TiC and chromium carbides.     

Corrosion in H2/H2S mixtures of a Ni77Cr16Fe7 alloy previously covered with an external Cr2O3 layer

A thermodynamic and kinetic study of sulphidization of Ni₇₇Cr₁₆Fe₇ alloys — previously covered with a Cr₂O₃3 layer — was made in a H₂/H₂S mixture, under a wide range of sulphur pressures (10^–4 atm <p(H₂S) < 10^–1 atm) and at temperatures between 700 and 1000° C. The mechanism for sulphur penetration through the external chromium oxide and the competition between Cr₂O₃ reduction and CrS formation were studied with the help of thermogravimetry and analytical techniques. The essential role of chromium carbides appears to be to modify the distribution of sulphur in the structure and to lower the critical energy for sulphide formation. Two growth laws were found to apply for external layer formation, one for the industrial form of the alloy and the other for a purer synthetic form. The alteration in mechanical properties of the external sulphidized zone were used to show the initial softening effect of chromium depletion, then the hardening effect of sulphide precipitation.     

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.     

Electrical Properties and Ferromagnetism in (Ga,Cr)As

MBE-grown (Ga,Cr)As has interesting electric and magnetic properties. Ga_1–x Cr _x As with x = 0.1 exhibits short-range ferromagnetic behavior at low temperatures. This is manifest in several anomalous properties: magnetization does not scale with B/T; fitting M(B) requires a model of distributed magnetic cluster or polarons; and inverse susceptibility is nonliner in T (non-Curie–Weiss) at low fields. At room temperature, the conductivity is activated and Hall measurements yield a hole concentration of 10²⁰ cm^–3, indicating that chromium acts as an acceptor similar to Mn in GaAs. For decreasing temperature, the conductivity decreases by eight orders of magnitude and follows exp(1/T ^1/2).     

Relationship between microstructure,hardness and corrosion resistance in 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons

The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M₇C₃ with stoichiometry as (Cr_3.37, Fe_3.63)C₃, (Cr_4.75, Fe_2.25)C₃ and (Cr_5.55, Fe_1.45)C₃, respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M₇C₃ in 20 wt.%Cr iron, whereas both M₂₃C₆ and M₇C₃ secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.     

(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.     

Compositional range,thermal stability,hardness and electrical resistivity of amorphous alloys in Al-Si (or Ge)-transition metal systems

An amorphous single phase was found to be formed in wide compositional ranges in rapidly solidified Al-Si-transition metal (M) and Al-Ge-M alloys. The compositional ranges are in the range from 12 to 53 at. % Si or Ge and 8 to 23% M and Al-Si-Co and Al-Ge-Fe alloys have the widest glass-formation ranges. Because the interaction between aluminium and silicon or germanium atoms is thought to be repulsive from the immiscible equilibrium phase diagrams, the glass formation is probably due to an attractive interaction of M-Si (or Ge) and Al-M pairs. Hardness, H _v, and crystallization temperature, T _x, increase with increasing M content and the highest values reach 1120 DPN and 715 K, while the change with silicon or germanium content is much smaller for H _v and is hardly seen for T _x. Additionally, the H _v and T _x have maximum values for Al-Si (or Ge)-M (M=Cr, Mn or Fe), decrease with the decrease and increase in the group number of M element and are the lowest for Al-Si (or Ge)-Ni alloys. The compositional dependence is interpreted under the assumption that T _x and H _v of the aluminium-based amorphous alloys are mainly dominated by the attractive interaction of M-(Si or Ge) and Al-M pairs. Room-temperature resistivity, _RT, increases in the range of 220 to 1940 cm with increasing silicon or germanium and M contents. The change in _RT with the group number of M elements shows a maximum phenomenon for manganese. It has thus been clarified that the characteristics of the Al-Si-M and Al-Ge-M amorphous alloys have the different compositional dependence as compared with those for conventional metalmetalloid amorphous alloys, probably because of the unusual interaction among the constituent elements.     

Effects of Mo on microstructure of as-cast 28 wt.% Cr–2.6 wt.% C–(0–10) wt.% Mo irons

Microstructures of as-cast 28 wt.% Cr–2.6 wt.% C irons containing (0–10) wt.% Mo with the Cr/C ratio of about 10 were studied and related to hardness. The experimental irons were cast into dry sand molds. Microstructural investigation was performed by light microscopy, X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. It was found that the iron with about 10 wt.% Mo was eutectic/peritectic, whereas the others with less Mo content were hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. Mo addition promoted the formation of M₂₃C₆ and M₆C. At 1 wt.% Mo, multiple eutectic carbides including M₇C₃, M₂₃C₆ and M₆C were observed. M₂₃C₆ existed as a transition zone between eutectic M₇C₃ and M₆C, indicating a carbide transition as M₇C₃(M_2.3C) → M₂₃C₆(M_3.8C) → M₆C. At 6 wt.% Mo, multiple eutectic carbides including M₇C₃ and M₂₃C₆ were observed together with fine cellular/lamellar M₆C aggregates. In the iron with 10 wt.% Mo, only eutectic/peritectic M₂₃C₆ and M₆C were found without M₇C₃. Mo distribution to all carbides has been determined to be increased from ca. 0.4 to 0.7 in mass fraction as the Mo content in the irons was increased. On the other hand, Cr distribution to all carbides is quite constant as ca. 0.6 in mass fraction. Mo addition increased Vickers macro-hardness of the irons from 495 up to 674 HV₃₀. High Mo content as solid-solution in the matrix and the formation of M₆C or M₂₃C₆ aggregates were the main reasons for hardness increase, indicating potentially improved wear performance of the irons with Mo addition.     

Transformation of (Cr,M)7C3-type carbides during nitriding of chromium alloyed steels

Thein situ rearrangement of (Cr, M)₇C₃-type carbides has been observed during ion nitriding of a commercial chromium-carbon alloyed steel (Z160CDV12). The mass balance concerning nitrogen, carbon and substitutional elements proves that carbides undergo a total transformation into substitutional (Cr, M) N-type nitrides with a simultaneous release ofα-iron from the very nitride phase. A detailed transmission electron microscopy microstructural study confirms the analytical results. There are no crystallographic relations between the carbides and their corresponding transformation products. The rejection of carbon from the carbides into the ferrite matrix leads to the precipitation of a network of cementite along the prior austenite grain boundaries.     

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.     

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.     

The formation of chromium carbide during chromizing

The carbides formed during the chromizing of various types of carbon and chromium steels are considered in terms of the ternary phase diagram. A correlation is found between the carbide or carbides formed and a diffusion couple model. In low chromium, high carbon steels, an intermediate layer is formed which seems to be a (Fe,Cr)₃C cementite phase. The carbide which is formed on low carbon constructional steels depends on the detailed carbon and chromium profiles. Data found in the literature support the present interpretation.     

Transition from upper to lower bainite in Fe–C–Cr steel

Abstract

An analytical evaluation of transition temperature from upper to lower bainite in Fe – 0·38C – 0·93Cr (wt-%) steel was carried out. Calculations were based on the model constructed by Takahashi and Bhadeshia, which involves a comparison between the time t_θ needed to precipitate cementite within the bainitic ferrite plates with the time t_θ required to decarburise supersaturated ferrite plates. It was found that the distribution of lath widths, shown by histograms, of the bainitic ferrite varies with isothermal transformation temperatures and holding times. The transition between upper and lower bainite is found to occur over a narrow range of temperatures (350 – 410°C) and depends on the thickness of bainitic ferrite laths and the volume fraction of precipitated cementite. On comparing t _d and t_θ it was found that a transition temperature from upper to lower bainite reaction L _S of about 350°C could be predicted if the thickness of bainitic ferrite laths is set as w _o = 0·1 μm and the volume fraction of cementite set as ξ = 0·01. Calculated differences in the relative behaviour of t _d and t_θ revealed the occurrence of upper and lower bainite in steel Fe – 0·38C – 0·93Cr consistent with the results of transmission electron microscopy investigation.     

Chromium carbide laser-beam surface-alloying treatment on stainless steel

In order to improve the resistance to wear, oxidation and corrosion of a stainless steel die, chromium carbide surface-alloying treatment was carried out on a 12 % Cr stainless steel using a CO₂ laser. Cr₃C₂ powder slurry was coated on the stainless steel and then a 3 kW CO₂ laser beam was used to irradiate the specimen. The thickness of surface-alloyed layer was about 0.3 mm and the chromium concentration was about 40 % throughout the alloyed-region. Large amounts of Cr₃C₂ and Cr₇C₃ were also distributed in this alloyed layer. From the results of the isothermal oxidation test at 960 °C for 100 h, it was found that the surface-alloying treatment improved the oxidation resistance by about 100 times due to the distribution of chromium carbides and the increase in the chromium concentration. The results of the cyclic oxidation test revealed that the oxidation layer was very stable on the chromium carbide surface-alloyed region, while it scaled off very easily from the substrate region due to porous oxidation products. The microhardness was about 1100 H_v due to the dispersion and precipitation of chromium carbides in addition to the martensitic structure in the surface-alloyed region. The microhardness did not decrease much, despite heating at 960 °C for 100 h. The chromium carbide surface-alloying treatment improved the wear-resistance greatly, and the results of the wear-resistance test were very consistent with those of the microhardness test.     

DC magnetization studies in Nb/Fe superconducting multilayers

The behavior of superconducting transition temperature T_C in superconducting/ferromagnetic (S/F) multilayers as a function of different layer thicknesses and for varying magnetic moment μ_B of the F layer atoms is studied. Multilayer structures consist of five bilayers of constant superconducting Nb layer thickness of 400 Å and Fe of 6 and 24 Å each. The analysis of the magnetization data revealed that for t_Fe=6 Å, the Fe layer is non-magnetic. The interpretation of the observed T_C behavior is attributed to the change of the interaction of the cooper pairs with this layer at the onset of ferromagnetism for t_Fe=6 Å. The hysteresis curve recorded under isothermal conditions at 4.5 K for [Nb (400 Å)/Fe (6 Å)]₅ multilayers shows the usual M–H hysteresis behavior which is typical of a hard type-II superconductor exhibiting an irreversibility field H_irr of 3.5 kOe with substantial pinning at lower field. In addition, [Nb(400 Å)/Fe(6 Å)]₅ multilayer displays anomalous behavior in the form of paramagnetic peak in the superconducting state just below the transition temperature T_C=6.25 K.     

Viscosity and supercooling of Fe-Cr (≤40 at % Cr) melts

We have studied the composition dependences of the kinematic viscosity (ν) and supercooling (Δt) of Fe-Cr melts at chromium contents of up to 40 at %. The results indicate that there is a minimum in viscosity at 5 at % Cr, a maximum at 12 at % Cr, and a sharp rise in the crystallization tendency of the solid solution in this composition range. The Δt(x) and ν(x) data obtained can be understood in terms of the composition dependence of the bonding energy, which is governed by the geometric and chemical short-range order in the atomic arrangement. Using a probabilistic approach to evaluation of various short-range order configurations and the experimental viscosity and supercooling data at chromium contents of 2.5 and 30 at %, we have calculated Δt(x) and ν(x) up to 40 at % Cr.     

Dynamic Remanent Vortices in Superfluid 3He-B

We investigate the decay of vortices in a rotating cylindrical sample of ³He-B, after rotation has been stopped. With decreasing temperature vortex annihilation slows down as the damping in vortex motion, the mutual friction dissipation α(T), decreases almost exponentially. Remanent vortices then survive for increasingly long periods, while they move towards annihilation in zero applied flow. After a waiting period Δt at zero flow, rotation is reapplied and the remnants evolve to rectilinear vortices. By counting these lines, we measure at temperatures above the transition to turbulence ∼0.6 T _c the number of remnants as a function of α(T) and Δt. At temperatures below the transition to turbulence T≲0.55 T _c, remnants expanding in applied flow become unstable and generate in a turbulent burst the equilibrium number of vortices. Here we measure the onset temperature T _on of turbulence as a function of Δt, applied flow velocity v=v _n−v _s, and length of sample L.     

Influence of quantum confinement on magnetic properties of Fe/Mo/Fe and Fe/Ta/Fe thin-film systems

The magnetic properties of Fe/NML/Fe thin-film structures with Mo and Ta non-magnetic layers (NMLs) grown by magnetron sputtering have been studied using magnetooptical techniques. It is established that the saturation field (H _S) of the samples exhibits oscillations as a function of the NML thickness (t _NML) and the period of oscillations depends on the thickness of iron layers. This behavior of H _S(t _NML) is explained by the oscillatory character of exchange coupling between the ferromagnetic layers.