Thermomechanical properties of iron: viscoplasticity of ferrite and of austenite—ferrite mixtures

The available data on secondary creep in creep tests and on maximum stress in hot deformation tests obtained for δ and α ferrite have been reviewed and expressed in the form of the equation ? = K(σ/E)^P_c sinh(Vσ/RT) exp(—Q_c/RT) with p_c = 2·5, V = 0·0013 m³ mol^?1, and K = 2·6 × 10¹⁷ s^?1, E and Q_c being temperature dependent functions. The approximate range of validity is Z = ? exp (Q_c/RT) ≈ 10¹—10¹¹ s^?1. Together with the corresponding equation for austenite the data can be used to predict the viscoplastic behaviour of austenite—ferrite mixtures.

MST/1538     

Sol–gel auto-combustion synthesis of spinel-type ferrite nanomaterials

Recent developments and trends of sol–gel auto-combustion method for spinel ferrite nanomaterial synthesis are briefly discussed and critically analyzed. The analysis of various parameters of reaction which could be used for better understanding of synthesis process and control of microstructure and property of spinel ferrite nanopowder products was the main objective of this review article. Special attention was paid to variety of particle size and phase purity. For these purposes the correlation between complexant, oxygen balance and combustion process chemical additives, as well as heating mechanism and atmosphere, was established. These results are relevant from standpoints of both application and processing of ferrites.     

Synthesis of ferrite grade γ-Fe2O3

Iron(II) carboxylato-hydrazinates: Ferrous fumarato-hydrazinate (FFH), FeC₄H₂O₄·2N₂H₄; ferrous succinato-hydrazinate (FSH), FeC₄H₄O₄·2N₂H₄; ferrous maleato-hydrazinate (FEH), FeC₄H₂O₄·2N₂H₄; ferrous malato-hydrazinate (FLH), Fein4H₄O₅·2N₂H₄; ferrous malonato-hydrazinate (FMH), FeC₃H₂O₄·1.5N₂H₄·H₂O; and ferrous tartrato-hydrazinate (FTH), FeC₄H₄O₆·N₂H₄·H₂O are being synthesized for the first time. These decompose (autocatalytically) in an ordinary atmosphere to mainly γ-Fe₂O₃, while the unhydrazinated iron(II) carboxylates in air yield α-Fe₂O₃, but the controlled atmosphere of moisture requires for the oxalates to stabilize the metastable γ-Fe₂O₃. The hydrazine released during heating reacts with atmospheric oxygen liberating enormous energy, N₂H₄ + O₂ → N₂ + H₂O; ΔH₂O = −621 kJ/mol, which enables to oxidatively decompose the dehydrazinated complex to γ-Fe₂O₃. The reaction products N₂ + H₂O provide the necessary atmosphere of moisture needed for the stabilization of the metastable oxide. The synthesis, characterization and thermal decomposition (DTA/TG) of the iron(II) carboxylato-hydrazinates are discussed to explain the suitability of γ-Fe₂O₃ in the ferrite synthesis.     

Magnetic and dielectric properties of Co–Zn ferrite

Nanocrystalline powders of Co substituted Zn ferrite with the chemical formula Co_xZn_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1) were synthesized by sol–gel autocombustion method using tartaric acid as fuel agent. The samples were sintered in static air atmosphere for 7 h at 773 K, 7 h at 973 K and 10 h at 1173 K. The organic phase extinction and the spinel phase formation were monitored by means of Fourier transform infrared spectroscopy. The X-ray diffraction patterns analysis confirmed the spinel single phase accomplishment. Crystallite size, average grains size, lattice parameter and cation distribution were estimated. Magnetic behavior of the as-obtained samples by means of M-H hysteresis measurements was studied at room temperature. Permeability and dielectric permittivity at room temperature versus frequency was the subject of a comparative study for the Co_xZn_1−xFe₂O₄ series. In agreement with the proposed cation distribution the sample with Co_0.8Zn_0.2Fe₂O₄ formula exhibits the optimal magnetic and dielectric properties.     

Stabilisation of ferrite in hot rolled δ-TRIP steel

Abstract

A steel has recently been designed to benefit from the deformation induced transformation of retained austenite present in association with bainitic ferrite. It has as its major microstructural component, dendrites of δ-ferrite introduced during solidification. The δ-ferrite replaces the allotriomorphic ferrite present in conventional alloys of this kind. The authors examine here the stability of this δ-ferrite during heating into a temperature range typical of hot rolling conditions. It is found that contrary to expectations from calculated phase diagrams, the steel becomes fully austenitic under these conditions and that a better balance of ferrite promoting solutes is necessary in order to stabilise the dendritic structure. New alloys are designed for this purpose and are found suitable for hot rolling in the two-phase field over the temperature range 900–1200°C.     

Kinetics and dilatometric behaviour of non-isothermal ferrite–austenite transformation

Abstract

A model that describes the ferrite–austenite transformation during continuous heating in Armco iron and three very low carbon, low manganese steels with a fully ferritic initial microstructure is presented. This model allows calculation of the volume fractions of austenite and ferrite during transformation as a function of temperature, and hence knowledge of the austenite formation kinetics under non-isothermal conditions in fully ferritic steels. Moreover, since dilatometric analysis is a technique very often used to study phase transformations in steels, a second model, which describes the dilatometric behaviour of the material and calculates the relative change in length that occurs during the ferrite–austenite transformation, has also been developed. Both kinetics and dilatometric models have been validated by comparison of theoretical and experimental dilatometric heating curves. Predicted and experimental results are in satisfactory agreement.     

On styrene–butadiene–styrene–barium ferrite nanocomposites

Magnetic investigations on a nanocomposite material obtained by spinning solutions of styrene–butadiene–styrene block copolymer containing barium ferrite nanoparticles onto Si wafers are reported. The effect of the spinning frequency on the magnetic features is discussed. It is observed that the magnetization at saturation is decreased as the spinning frequency is increased as the centrifuge force removes the magnetic nanoparticles from the solution. This is supported by the derivative of the hysteresis loops, which show two components, one with a high coercive field and another with a small coercive field. Increasing the spinning frequency increases the weight of the low coercive field component. The anisotropy in the distribution of magnetic nanoparticles, triggered eventually by the self-assembly capabilities of the matrix, is revealed by the difference between the coercive field in parallel and perpendicular configuration. It is noticed that increasing the spinning frequency enhances this difference. The effect of annealing the nanocomposite films is discussed.     

Dielectric relaxation and ac conductivity of polyaniline–zinc ferrite composite

In situ polymerization of aniline is carried out in the presence of zinc ferrite to synthesize polyaniline/ZnFe₂O₄ composites (PANI/ZnFe₂O₄) by chemical oxidation method. The composite has been synthesized with various compositions (10, 20, 30, 40 and 50 wt.%) of zinc ferrite in PANI. From the infrared spectroscopy (FTIR) studies on polyaniline/ZnFe₂O₄ composites, the peak at 1140 cm^?1 is considered to be measure of the degree of electron delocalization. The surface morphology of these composites is studied with scanning electron micrograph (SEM). The ac conductivity and dielectric properties are studied in the frequency range from 10² to 10⁶ Hz. The results obtained for these composites are of scientific and technological interest.     

Magnetic properties of Cu-Cd ferrite investigated by Mössbauer spectroscopy

Samples of the copper-cadmium ferrite series Cd _x Cu_1–x Fe₂O₄ (x=0 to 1) have been investigated by means of Mössbauer spectroscopy at 77 K. Mössbauer spectra forx=0.0 to 0.7 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octahedral ions (B-sites), while forx=0.8 it shows relaxation behaviour and forx0.9 it exhibits a paramagnetic quadrupole doublet. The systematic dependence of the isomer shifts, line widths, quadrupole interactions and nuclear magnetic fields of⁵⁷Fe³⁺ ions in both A and B sites has been determined as a function of cadmium content. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of the A-B and B-B supertransferred hyperfine interactions. Analysis of the average Mössbauer line widths as a function of cadmium concentration suggests that the relaxation spectrum observed atx=0.8 (77 K) is possibly due to domain wall oscillations. It has been found here that QS (B) increases from CuFe₂O₄to CdFe₂O₄ as the cadmium concentration is increased.     

Solution to the Bagaryatskii and Isaichev ferrite–cementite orientation relationship problem

The Bagaryatskii and Isaichev orientation relationships between cementite and ferrite are closely related but not identical. They cannot easily be distinguished using ordinary electron diffraction methods and precise methods indicate that the Bargaryatski orientation does not exist. The issue is important when considering the mechanism by which cementite forms during the tempering of martensite or the formation of lower bainite, where the iron and substitutional solutes are unable to diffuse during the course of precipitation. It is demonstrated here that just one of the orientation relationships is consistent with the mechanism of precipitation at low temperatures, and is associated with much smaller deformations than the other.     

Microwave absorbing properties of double-layer cementitious composites containing Mn–Zn ferrite

Double-layer cementitious composites filled with Mn–Zn ferrite as microwave absorbers were designed based on the impedance matching theory and electromagnetic wave propagation laws. The results showed that the addition of silica fume can improve the impedance matching between the cementitious composites and free space. Comparing with the single-layer structure, the reflectivity of the double-layer cementitious plates can decrease by 6–8 dB and decrease by 15 dB maximum with 30 wt.% ferrite; in addition, the reflectivity of electromagnetic wave is lower than 10 dB in the frequency range of 11.4–18 GHz. These composites can be potentially used as electromagnetic interference (EMI) materials for buildings.     

Synthesis and magnetic properties of hydrothermal magnesium–zinc spinel ferrite powders

Mg–Zn ferrite powders with the nominal composition Mg_0.5Zn_0.5Fe₂O₄ were synthesized via hydrothermal method, and their synthesis, magnetic properties and microstructures were studied. It was found that the pH value affected the amount of impurity Fe₂O₃ and the purity of ferrites greatly. It was also found excess Zn content (5 at.%) in starting materials was not helpful to synthesize pure spinel ferrite, while the prolonged reaction time was harmful for the formation of pure spinel structure. The specimens presented small coercivity lower than 10 Oe, which showed a typical magnetically soft behavior. With the increase of pH value, the saturation magnetization of specimens with excess Zn ions (5 at.%) kept increasing from 23.90 to 41.82 emu/g due to the decreasing amount of impurity Fe₂O₃. The study of microstructures showed that the large particles in powders were the aggregates of small nanoscale crystallites. The analysis of actual Zn and Mg content in synthesized ferrites confirmed that the best experimental conditions for synthesis of pure spinel Mg–Zn ferrite are the hydrothermal temperature is 200 °C, the reaction time is 8 h, the pH value is 12 and the excess amount of Zn(NO)₃ in starting materials is 5 at.%.     

Cleavage fracture behaviour of carbon steels for different ferrite–pearlite contents

Abstract

The effect offerrite–pearlite content on the cleavage fracture behaviour of carbon steels was determined via plain tensile tests at temperatures from 12 to ?196°C. It was found that, under conditions of uniaxial tension within the test temperature range, the cleavage fracture stress for pearlitic eutectoid steel is independent of temperature, whereas that for mild steel changes with temperature and exhibits a depression at low temperatures. An increase in the proportion of pearlite enhances the effect of pearlite colonies as obstacles to cleavage crack propagation. Only when the volume fraction of pearlite is comparable to that of ferrite does the steel begin to display the fracture features of pearlitic steel. Possible mechanisms manifesting different fracture characteristics for steels of various ferrite–pearlite contents are discussed with the aid of fractographic observations.

MST/1075     

Susceptibility,magnetization and Mössbauer studies of the Mg-Zn ferrite system

The variation of susceptibility with temperature of the Zn _x Mg_1–x Fe₂O₄ system has been obtained between 300 K and the Neél temperature at a constant magnetic field of 7.0 Oe for x=0.0 to 0.7. Through the magnetization measurements of this Mg-Zn ferrite system the variation of saturation magnetization with zinc concentration was determined in gaining information about the Yafet-Kittel angles in the system for x0.9 at 298 K. The observations indicate the existence of a Yafet-Kittel type of magnetic ordering in the system. A molecular field analysis of the Y-K spin ordering using a three sublattice model is shown to explain the experimental data satisfactorily. For the sake of verification, Neél temperatures of Mg-Zn ferrites were also determined from Mössbauer studies.     

Anomalies in electrical and dielectric properties of nanocrystalline Ni–Co spinel ferrite

Nanocrystalline Ni-substituted cobalt ferrite sample is prepared by chemical co-precipitation method. X-ray diffraction and scanning electron microscopy techniques are used to obtain structural and morphological characterizations. Nanocrystalline nature is clearly seen in SEM picture. Variation of electrical resistivity as a function of temperature in the range 300–900 K is investigated. ln ρ versus 1/T plot shows four break resulting into five regions in 300–900 K temperature range of measurements. The magnetic transition temperature of the sample is determined from resistivity behavior with temperature. The activation energy in different regions is calculated and discussed. Variation of dielectric constant (?′) with increasing temperature show more than one peak; one at around 773 K and other around 833 K, which is unusual behavior of ferrites. The observed peaks in ?′ variation with temperature show frequency dependence. Electrical and dielectric properties of Ni_0.4Co_0.6Fe₂O₄ sample show unusual behavior in the temperature range 723–833 K. To our knowledge, nobody has discussed anomalous behavior in the temperature range 723–833 K for Ni_0.4Co_0.6Fe₂O₄. The possible mechanism responsible for the unusual electrical and dielectric behavior of the sample is discussed.     

A temperature-dependent Mössbauer study of mechanically activated and non-activated zinc ferrite

The changes of ZnFe₂O₄ caused by mechanical activation as well as the structural evolution of nonactivated and mechanically activated zinc ferrite occurring during heating, have been investigated by in situ Mössbauer spectroscopy. Attention is directed to mechanically induced changes in magnetic properties of zinc ferrite, the variation of nuclear quadrupolar interactions, the thermally induced changes of the Mössbauer shift, and also to the structural response of mechanically activated zinc ferrite to changes in temperature.     

Microstructure and mechanical properties of medium-carbon ferrite–pearlite steel microalloyed with vanadium

Abstract

Microstructural analysis and mechanical testing have been carried out on medium-carbon steels to which additions of vanadium in the range 0·075–0·6 wt-% were made. The steels were either continuously cooled or isothermally heat treated after austenitization. Vanadium carbide precipitation in the proeutectoid ferrite regions of the microstructure and, more unusually, also in the pearlitic ferrite lamellae, were identified by transmission electron microscopy. Moreover, in both ferrite phases the precipitates are aligned in rows, indicative of interphase precipitation at the austenite/ferrite transformation interface. These observations are discussed in terms of the various mechanisms that have been proposed for the interphase precipitation reaction. In the alloys studied the vanadium additions were found to increase the strength of the steels by up to 100%, but to reduce the ductility and notched impact resistance. The most useful combination of increased strength with reasonable ductility and impact toughness was achieved with an addition of 0·15 wt-% V. The vanadium additions contributed to a number of variations in microstructure and therefore in strengthening mechanisms, but the largest effect was the interphase precipitation strengthening of the ferritic phases. The highest strength levels were achieved in fully pearlitic microstructures with the pearlitic ferrite lamellae strengthened by interphase precipitation of the vanadium carbide.

MST/536