Electronic structure and magnetic properties of the α and γ phases of iron, their solutions with carbon, and cementite

The full-potential LMTO method was used to comparatively analyze the band structure, magnetic and cohesive characteristics, energies of formation of α and γ iron, their carbon solid solutions, and cementite. It has been found that the solid solution γ-Fe + C, similarly to γ-Fe, is characterized by an antiinvar transition and that cementite demonstrates invar behavior. The similarity of curves of the density of states, cohesive energies, and magnetic moments for the supersaturated solid solution of carbon in the fcc iron and cementite suggests the preferable formation of cementite in regions of concentration inhomogeneity of carbon distribution in the solid solution of γ iron.     

Thermodynamic Properties of Solid Nickel-Gold Alloys

Free energies, enthalpies, and entropies of mixing of Ni-Au solid solutions containing 5 to 95 atomic pct Ni have been determined by the electromotive force method at 700° to 900°C. The thermodynamic activities exhibit large positive deviations from Raoult’s law, and the entropies of mixing are almost twice those of ideal solutions. The enthalpies of mixing are positive (heat is absorbed) and are attributable to the lattice distortional energy resulting from the size difference between the nickel and gold atoms. This factor appears to be responsible for the miscibility gap at lower temperatures.     

Effect of Small Amount of Manganese on the Interdiffusivities in Fe-Al Alloys

The ability to weld aluminum and steel sheets depends strongly on the formation of intermetallic phases; a process that is, in turn, controlled by the interdiffusion of iron and aluminum across the welded interface. Understanding the interdiffusion behavior, and how it is influenced by tertiary elements such as manganese, will allow for better prediction of the properties of the spot weld. Hence, interdiffusion coefficients and activation energies for interdiffusion were determined in the α solid solution and B2 intermetallic phases of Fe-Al alloys in the presence of 1.5-2 at.% manganese with pseudo-binary diffusion couples investigated at 900-1095 °C. The interdiffusion coefficients in α were found to increase in the presence of Mn at all temperatures compared with those reported in the binary Fe-Al alloys. The activation energies for interdiffusion in α are correspondingly lower than those in the binary Fe-Al alloys. The increase in the main interdiffusion coefficients in the presence of Mn indicates that diffusional interactions between Fe and Al are increased in the presence of Mn. The expected increase in diffusional interactions of Fe and Al are found to be consistent with the thermodynamic interactions between Fe and Al in the binary Fe-Al and ternary Fe-Al-Mn system as estimated from the literature. The presence of Mn is found to decrease the solubility of Al in the α solid solution, which, in turn, is expected to decrease the growth rate of the intermetallic at the interface between steel and aluminum.     

FCC-to-HCP Phase Transformation in CoCrNix Medium-Entropy Alloys

A hybrid first-principles/Monte Carlo simulation is combined with experiments to study the structure and elastic properties of CoCrNi_x (x = 1-0.5) alloys. The experimental X-ray diffraction patterns show that the structures have changed from the single-phase face-centered cubic (FCC) structure at x = 1-0.8 to the coexistence of FCC and the hexagonal close-packed structures at x = 0.7-0.5, which is further confirmed by calculations on mixing energies. The elastic moduli by calculation are basically in agreement with experiments. Room-temperature tension shows that the six alloys have a certain plasticity, the strength and plasticity of the alloys have a linear decrease with the decrease in Ni contents, and the plasticity of the alloys drops from 84 to 23%. Furthermore, first-principles density function theory calculations were employed to reveal the electronic and magnetic structures of alloys. The electron density of states for all alloys is asymmetrical, which illustrates that the alloys are ferromagnetism. It is found that Cr atoms can suppress the ferromagnetism of alloys, since Cr atoms have both positive and negative magnetic moments in all alloys.     

Structural state and magnetic properties of cementite alloyed with manganese

Using X-ray diffraction analysis, M?ssbauer spectroscopy, and magnetic measurements, the structure, parameters of hyperfine interactions, localization of Mn atoms in the lattice, coercive force, and specific saturation magnetization have been investigated in the mechanically alloyed and annealed cementite (alloyed with manganese) of compositions (Fe_{1 ? x} Mn _x )₃C (x = 0?C0.12). It has been shown that strongly deformed cementite resides in the low-coercivity state and, after annealing in the vicinity of 500°C, in the high-coercivity state. Alloying with manganese reduces the coercive force, the specific saturation magnetization, and the Curie temperature of cementite. Inhomogeneities of the distribution of manganese atoms indicate the temperature dependence of the coercive force of mechanically alloyed and annealed cementite samples.     

Ab initio Computer Simulation of Carbon–Carbon Interactions for Various Spacings in BCC and BCT Lattices of Ferrite and Martensite

The ab initio computer simulation of lattice parameters and local structure distortions caused by interstitial carbon atoms in the iron-carbon system has been carried out using WIEN2k software. For the calculations, the full-potential method of linearized augmented plane waves (LAPWs) taking into account the generalized gradient approximation of PBE–GGA was used in a supercell of 54 iron atoms with periodic boundary conditions. The carbon dissolution energy has been found to be 0.85 eV for bcc iron, and 0.79 eV for bct iron. The carbon–carbon interaction energies in the ferromagnetic bct iron have been calculated. It has been found that accounting for tetragonal distortions considerably changes the interaction energy of carbon atoms in comparison with that of the bcc iron. Both the maximum degree of tetragonality of iron and the maximum attraction of carbon atoms have been observed for the case of carbon atoms placed in octahedral pores of the same type. If carbon atoms are in different types of octahedral pores, the tetragonal distortion of the lattice is weak. The obtained results are in good agreement with the Zener–Khachaturyan hypothesis and experimental data of Kurdyumov.     

Crystal structure and magnetic properties of pseudobinary solid solutions Pr(In1 − x Pb x )3

Structure and magnetic properties of Pr(In_{1 ? x} Pb _x )₃ alloys have been studied in some detail. It has been shown that, in this system, in the whole range of compositions (0 ≤ x ≤ 1), there is a continuous series of disordered solid solutions with a structure of the Cu₃Au type. The lattice parameter of these solid solutions varies linearly (according to Vegard’s law). The temperature dependence of the magnetic susceptibility has been studied in the temperature range of 2–300 K. At enhanced temperatures, it is described well by the Curie-Weiss law for all alloys. At low temperatures, an anomalous deviation from this law for the terminal alloys is observed, which is especially strong for PrIn₃. In pseudobinary alloys, this anomaly weakens and the magnetic susceptibility itself increases. In the concentration dependence of the susceptibility measured at 2 K, a maximum is observed. The specific features of the dependence of the magnetic susceptibility of pseudobinary alloys on the temperature and concentration x have been explained by a decrease in the local symmetry of crystal field at the sites of Pr³⁺ ions due to the partial random replacement of their nearest neighbors by atoms of different valences.     

Phase transformations in Ni-Mn-In-based alloys in magnetic field

The effect of a high-strength magnetic field on the phase transformations of ferromagnetic materials based on the Ni-Mn-In system with different degrees of alloying has been revealed experimentally. The effect of the chemical composition of the alloys and magnetic treatment on the martensitic transformations and magnetic transitions has been investigated. It has been established that, upon substituting nickel atoms with manganese atoms, the temperature of the martensitic transformation decreases.     

Effect of chromium on the electronic structure and magnetic properties of cementite

Using ab initio methods, the influence of chromium on the electron structure and magnetic properties of cementite Fe₃C has been investigated depending on the chromium concentration and the possibility of the formation of a chromium carbide Cr₃C has been considered. It has been established that it is the general positions of iron in the lattice of cementite that are most energetically preferable for the substitution by chromium. Chromium leads to a strengthening of interatomic interactions and the change in the sign of the enthalpy of formation in the Fe_{3 ? x} Cr _x C system at x = 1.9. The magnetic moments at the atoms of chromium and iron are oriented antiferromagnetically and the total magnetization of the alloyed system decreases with increasing chromium concentration.     

Ab-initio calculation of enthalpies of formation of intermetallic compounds and enthalpies of mixing of solid solutions

A large number of ab-initio calculations of energies of formation of intermetallic compounds have been performed in the last 15 years. The currently used methods are listed. The paper presents a review of the aluminium based compounds which have been studied. Comparisons of calculated and experimental enthalpies of formation are provided for aluminim-3d and-4d transition metal alloys at equiatomic composition. The modelling of the enthalpies of mixing of solid solutions based on a given lattice is described.     

人造金刚石单晶铁基金属包膜的精细结构

利用扫描Auger微探针和X射线光电子能谱等手段,研究了包覆着高温高压金刚石单晶的铁基金属包膜（Fe—Ni—C合金)的精细结构．结果表明,仅在临近金刚石单晶的包膜内层,C和Fe原子的精细结构有一明显变化;在接触金刚石单晶的包膜表面上,Fe,Ni最外层电子的结合能比包膜其它区域明显增高据此认为,邻近金刚石的包膜内层和接触金刚石的包膜表面在金刚石生长中起重要作用,Fe,Ni原子对C原子的催化很可能是在包膜内层或在金刚石／包膜界面上完成的．     

Solid-state mechanical synthesis of austenitic Fe-Ni-Cr-N alloys

The possibility of producing high-nitrogen nanostructured austenitic Fe-Ni-Cr-N alloys using solid-state mechanical synthesis in a ball mill has been studied. It has been shown that, in the matrices of iron and Fe-xNi (x = 6–20 at %) alloys with chromium nitrides, bcc and fcc Fe-Ni-Cr-N solid solutions are formed. With an increase in the concentration of nickel in the matrix, the amount of mechanically synthesized austenite grows. The subsequent heating of the mechanically synthesized specimens into the austenitic field of the phase diagram of Fe-Ni alloys leads to the α → γ transformation with the retention of the nanostructured Fe-Ni-Cr-N solid solution and precipitation of secondary nitrides CrN, which stabilize the structure.     

Mössbauer investigations of the metastable Fe23B6 phase

Mössbauer spectroscopy was used to study hyperfine interactions in the metastable phase Fe₂₃B₆ that was for the first time obtained in macroscopic amounts in an almost single-phase state. Parameters of ⁵⁷Fe(300 K) spectra, as well as local and average magnetic moments at iron atoms have been calculated. It is indicated that the specific features of Mössbauer spectra and hyperfine-field distributions P(H) in mechanically synthesized Fe₂₃B₆ compounds are determined by the modification of the parameters of local surroundings of Fe atoms in positions 48h and 32f of the unit cell.     

Mössbauer and magnetometric studies of uncommon magnetic properties of ordered (B2 type) Fe-Al alloys

This work presents experimental data on the Mössbauer and magnetic measurements of ordered Fe_65.9Al_34.1 (B2 type) alloy depending on temperature and external magnetic field. The results obtained show that the ground magnetic state of the alloy cannot be referred to any of the known types of magnetic ordering. A model of local magnetic moments is proposed, in which the amplitude and the sign of a local magnetic moment are determined by its local chemical surroundings. The model suggests the existence of local magnetic moments that are antiparallel to the total magnetization of the alloy: the magnetic moment of a Fe atom with five and more Al atoms in the nearest surroundings is antiparallel to the total magnetization. Within the framework of the model proposed, a quantitative agreement of the calculated and experimental values of the average magnetic moment of the Fe atom in the alloy has been obtained. It is shown that in the B2 superstructure there appear magnetic inhomogeneities of a nanometer scale, which consist of magnetic moments that are parallel and antiparallel to the total magnetization. The behavior of the magnetic and Mössbauer parameters depending on the temperature and the applied magnetic field indicates the existence of collective fluctuations of magnetic moments. The quantity of Fe atoms in such fluctuating ensembles, as evaluated from the data on the magnetization, is compared with their quantity in the magnetic inhomogeneities.     

Mössbauer spectroscopy and the structure of interfaces on the atomic scale in metallic nanosystems

A microscopic model of the formation of an alloy on the interface has been constructed, which takes into account the exchange of atoms with the substrate atoms and the “floating up” of the latter into the upper layers in the process of epitaxial growth. The self-consistent calculations of atomic magnetic moments of spatially inhomogeneous structures obtained in this case are used for the interpretation of data of Mössbauer spectroscopy. The proposed scenario of mixing leads to the appearance of a preferred direction in the sample and the asymmetry of interfaces in the direction of epitaxial growth. In the multilayer M ₁/M ₂ (M _1,2 = Fe, Cr, V, Sn, or Ag) systems, this asymmetry makes it possible to understand the difference in the magnetic behavior of M ₁-on M ₂ and M ₂-on-M ₁ interfaces which has been observed experimentally. The correlation between the calculated distributions of magnetic moments and the measured distributions of hyperfine fields at iron atoms confirms the assumption about their proportionality for a broad class of metallic multilayer systems. However, a linear decrease of hyperfine fields at the ⁵⁷Fe nuclei with increasing number of impurity atoms among the nearest and next-nearest neighbors is not confirmed for Fe/Cr systems, although is correct in Fe/V superlattices. In the Fe/Cr multilayer systems, the experimentally measured value of magnetoresistance grows with increasing fraction of the “floated up” atoms of ⁵⁷Fe. Thus, it is the bulk scattering by impurity atoms that gives the basic contribution to the effect of giant magnetoresistance. The problem of the influence of mixing and adsorption of hydrogen in the vanadium layers on the state of the spin-density wave in V/Cr superlattices has been considered.     

Mössbauer study and thermodynamic modeling of Fe–C–N alloy

Mössbauer spectroscopy and Monte Carlo computer simulation have been combined to understand the reason for the solid-solution stability of Fe–0.93 C–0.91 N alloy (mass%). Interstitial concentrations in austenite and ferrite were determined on the basis of X-ray diffraction measurements of the lattice dilatation. The hyperfine structure of Mössbauer spectra was analyzed to identify different atomic configurations in solid solutions and determine their fractions. Thereafter Monte Carlo simulation of the interstitial distribution in ferritic and austenitic solid solutions was performed, and values of the interstitial–interstitial interaction energies were obtained for the first and second coordination spheres in austenite and the first to the fourth coordination spheres in ferrite. Simulation shows that in both austenitic and ferritic phases the interaction of interstitial atoms is characterized by a strong repulsion within the first two coordination spheres. Experimental data and simulated interstitial distributions are consistent and complementary. It is concluded that the absence of interstitial clusters prevents carbide and nitride precipitates and causes the higher thermodynamic stability of Fe–C–N solid solutions as compared with Fe–C and Fe–N ones.     

Structure and magnetic properties of MnSb(Zn) and MnSb(Cu) solid solutions

The MnSb(Zn) and MnSb(Cu) solid solutions with NiAs-type of crystal structure have been obtained. The existence regions of mono-phase solutions have been determined. Zn and Cu can substitute 10% atomic for Mn. On the basis of magnetic measurements it was supposed that the Cu or Zn atoms preferable occupy the MeII sites of NiAs-structure if the amount of nonmagnetic substitution not exceed 10% atomic. The preferable occupation of MeII sites by nonmagnetic atom manifests in rising of the Curie temperature of solid solutions relatively unsubstituted alloy.     

Simulation of the low-temperature stage of annealing of radiation defects in BCC iron using the molecular dynamics method

A procedure for the simulation of the low-temperature stage of annealing of irradiated α iron is suggested. Based on the method of molecular dynamics, the reaction rate constants and the energies of activation for the recombination of Frenkel pairs are calculated. For the simulation of the I _D and I _E stages, the coefficient of diffusion of interstitials is calculated. The calculations are performed for three different potentials of interaction of iron atoms. All three potentials give close results for the majority of the configurations of Frenkel pairs considered. A comparison of the calculated annealing curve with the experimental curve of the stage I is given. Based on this comparison, a correspondence between different configurations of close Frenkel pairs and peaks of the experimental curve has been established. It is shown that one peak can be associated with several configurations of close pairs.