Mössbauer Studies of Electrodeposited and Ion Beam Mixed Alloy Coatings

CEMS, XMS, XRD and electron microprobe analysis were applied to study electrodeposited and ion beam mixed Fe–Cr–Ni as well as electrodeposited and ball-milled Sn–Cr alloys. In Fe–Cr–Ni alloys with composition 40% Cr and 20–30% Ni a metastable ferromagnetic phase has been found beside a metastable paramagnetic and an equilibrium phase in all deposits. The relative occurrence of the ferromagnetic phase exhibits an increase in the plating temperature range: 30–40°C. With plating temperature an increase of the short-range order in the ferromagnetic phase was observed. The highenergy heavy ion irradiation of Fe–Ni–Cr multilayer produces ferromagnetic and paramagnetic metastable phases. The electrodeposition of Sn–Cr alloys results in metastable Sn–Cr phases. In the case of ball-milling preparation the equilibrium Sn₃Cr₂ phase (=2.3 mm/s, =1.2 mm/s) appears in Sn–Cr alloys. The quantity of the Sn₃Cr₂ phase increases with the milling time.     

On the Phase Transformations in Cr–FeB and Fe–CrB Systems at High Temperature

The solid state interactions occurring at high temperature in the Cr–FeB and Fe–CrB systems were studied by transmission Mössbauer and X-ray diffraction techniques on samples prepared by powders carefully mixed, cold-compacted and then treated at 1000°C for times up to 16 h. In the Cr–FeB system, iron atoms liberated by the substitutional diffusion of Cr into FeB lattice preferentially destabilize the iron monoboride with formation of Cr-containing Fe₂B. In the Fe–CrB system, chromium atoms liberated by the substitutional diffusion of Fe into CrB lattice interacts with iron forming an Fe–Cr metal alloy. Moreover, zones of Cr-containing FeB and Fe₂B form at the contact between metal iron and chromium monoboride, and tend to disappear as iron is consumed by the alloying process.     

Microstructure and surface wear resistance in rapidly quenched Fe–Cr–B alloy spray coatings

Fe–Cr–B alloy coatings were fabricated by thermal spraying and investigated in this research as to its wear-resistance and friction properties. The Fe–Cr–B spray-coated layers exhibited much higher wear resistance and significantly low friction coefficient in comparison with those of Fe–Cr base tool steel. The present paper reports that the presence of amorphous surface film formed during the sliding wear will be the main cause of noticeably improved friction properties and wear resistance. It was also observed that the Fe–Cr solid solution phase with supersaturated B and Si was the phase contributing to the crystalline-to-amorphous transition induced by sliding wear. These results imply the possible application of the present alloy coatings to the lubricant-free sliding systems in which the use of organic lubricants is limited or prohibited.     

Processes Proceeding in Fe–0.12C–1Cr–1Mo–1V Steel after Pressure Cycling

The evolution of the phase content and morphology and crystalline defects during temperature and pressure cycling is studied by the methods of transmission electron microscopy and x-ray diffraction analysis. Thermostable Fe–0.12C–1Cr–1Mo–1V steel in the initial ferrite-pearlite state is examined. The evolution of the grain structure and dislocation density is quantitatively described. A low stability of large pearlite grains at temperatures above 600°C is established. The problem of oxidizing of steel of this class is also discussed.     

Pulsed laser deposition of hard magnetic films

Pulsed laser deposition from elemental targets was used to prepare highly textured hard magnetic Nd–Fe–B and Fe–Pt films with coercivities of 2 T and 5.2 T, respectively. In situ methods such as reflection high energy electron diffraction and Auger electron spectroscopy were applied to analyse film composition and structure during growth. Optimisation of the hard magnetic properties is discussed together with the specific advantages of pulsed laser deposition. PACS 81.15.Fg; 75.50.Ww; 75.50.Vv     

Light-induced structural changes in sodiumnitroprusside (Na2(Fe(CN)5NO)·2D2O) at 80 K

Groundstate and electronic excited state (MS_I) of deuterated sodiumnitroprusside (Na₂(Fe(CN)₅NO)·2D₂O) have been investigated by neutron diffraction as well as by optical and Mössbauer techniques. Significant structural changes occur predominantly in the O–N–Fe–C-bond. It has been shown, that the N–O bond-length is not the order parameter, as expected from other studies. We found an increase in the bond lengths Fe–N4 of 0.019(2) Å and N–O of 0.004(4) Å respectively, which is in qualitative agreement with changes determined by Raman spectroscopy and predictions based on diatomic correlations (Badger/Herschbach/Laurie). Additionally we observed a change in the Fe–C1 bond length of 0.012(3) Å in agreement with Raman meaurements.     

Characterization and magnetic properties of Fe–Co ultrafine particles

Characterization and magnetic properties of Fe–Co ultrafine particles were investigated systematically by means of X-ray diffraction, Mössbauer spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, energy disperse spectroscopy analysis, chemical analysis, oxygen determination and magnetization measurement. In comparison with bulk iron–cobalt alloys, the corresponding Fe–Co ultrafine particles have significant difference in the phase structure and magnetic properties, depending on the condition of evaporating and subsequent quenching. The mechanism for the formation of the ultrafine particles as well as the origin of ferromagnetism and paramagnetism (or superparamagnetism) were discussed.     

Temperature dependence of the dislocation starting stress in Ni+17 at.% Cr

The K state can [1–7] have a marked effect on the mechanical parameters of homogeneous solid solutions containing transition elements. The K state is seen as a rise in resistance in low-temperature annealing after quenching from high temperatures and has been ascribed to the formation of short-range order [4, 5, 8–10] or of imperfect long-range order [6, 7]. The mechanical properties are also substantially affected. We have examined the effects on the dislocation starting stress as a function of temperature, as measured by mechanical hysteresis [18–20], in the K state range for Ni+17% Cr.     

Exhibition of High- and Low-spin States of the High-temperature Fcc Phase in Nanoparticles of Fe,Fe-rich and Co-rich Alloys

The results of combined X-ray and Mössbauer studies of structure and local magnetic ordering in massive substances Fe, Fe–Ni, Fe–Mn, Fe–Ni–Mn, Fe–Pt, Fe–Co and aerosol nanoparticles produced by their evaporation in rare Ar atmosphere are discussed. This technique provides a stochiometric composition of alloys in nanoparticles. The smallest (5–8 nm) particles for all alloys containing Fe 60–65% are shown to have a bcc structure whereas with doubling a size the particles acquire a fcc structure. This is explained by the fact that by cooling the particles in the course of preparation they quickly reach a state close to the equilibrium and, according to the constitution diagram, must decompose into two phases. Such decomposition in massive alloys was never observed at temperatures below 300°C because of diffusive difficulties. It is found that as-fresh aerosol particles are covered with an X-ray amorphous oxide shell, which is displayed in the room temperature Mössbauer spectra as a superparamagnetic doublet and is transformed into sextet at lower temperatures. An availability of the oxide shell has no practical influence on the particles structure. The obtained Mössbauer spectra are considered with the model suggested by R.J. Weiss in 1963, on existence of two-spin states in the high-temperature fcc modification of Fe and its alloys. Both states coexist, moreover, in the Mössbauer spectra the ferromagnetic state dominates at high temperature and anti-ferromagnetic one at low temperature. The ferromagnetic state manifests itself as a remnant of the frozen magnetic ordering of the high-temperature fcc modification in the resulting bcc structure, whereas the anti-ferromagnetic state is related to some fcc fraction retained under the particles quenching.     

Preparation of nanocrystalline Fe–Ni powders by mechanical alloying used in soft magnetic composites

This paper focuses on the preparation of nanocrystalline Fe–Ni powders by mechanical alloying method, which can be used in soft magnetic composites. Fe–10 wt% Ni and Fe–20  wt % Ni alloys were prepared using a high-energy ball mill. The magnetic properties of samples were measured by a B–H curve analyzer and microstructures of the samples were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The bcc Fe(Ni) phase formation was identified by XRD and completed after 45 h of milling. It was found that higher milling time resulted in, larger lattice parameter, higher microstrains and smaller crystallite sizes. Also, results showed that with increasing the milling time, coercivity increased and saturation intrinsic flux density firstly increased noticeably and then decreased in higher milling times (>70 h).     

An Investigation of Phase Diagrams of Decomposing Magnetic Alloys

The interaction of phase decomposition and magnetic ordering processes in an A–B alloy of an arbitrary structure is studied theoretically. The decomposition and magnetization temperatures are calculated. The phase diagrams are constructed and compared with experiment on Fe–Cr, Fe–Co, and Pt–Co alloys.     

Structural characterisation of Ni clusters in AlN via X-ray absorption, X-ray diffraction and transmission electron microscopy

Ni ions were implanted in bulk AlN with the goal to form embedded metallic clusters. Combining several characterisation techniques such as X-ray absorption spectroscopy, X-ray diffraction and high resolution transmission electron microscopy, we determined the lattice parameter of the Ni clusters that display a fcc crystalline structure. The average size increases when the ion fluence is increased or after a thermal treatment. Thanks to moiré fringes observed by high resolution transmission electron microscopy and to satellite peaks seen on the diffraction patterns, we concluded that the annealed Ni clusters orientate their (002) planes on the (101) of AlN. Moreover, the satellite positions allowed us to calculate Ni cluster average diameters, that are in agreement with average sizes deduced by X-ray absorption spectroscopy. Received 25 August 1999 and Received in final form 8 February 2000     

Knight shifts of27Al and magnetic susceptibility of intermetallic compound NiAl with Fe additions

Atomic and electronic structures, magnetic properties of intermetallic compound NiAl with Fe additions (up to IO at .% for different types of alloying) were studied using NMR and magnetic susceptibility measurements. The effect of these parameters on the probability of occurence of phase transformation B2-LI_o was explored. Experimental results were compared with those obtained by computer calculations of the electronic structure for atomic arrangement of Fe impurity in both sublattices by means of the cluster variant of TBA-LCAO method. The responsibility of the density of 3d-states at the Fermi level increasing effects (Fe−Ni substitution) and corresponding contributions, associated with orbital interactions (Fe−Al substitution), for observed behaviour of measured parameters were determined. Appearance of B2-LI_o phase transition with increasing of antistructural Fe atoms (ASA) concentration was observed experimentally by means of transmission electron microscopy and X-ray diffraction analysis.     

The Fe–Cr system: atomistic modelling of thermodynamics and kinetics of phase transformations

To understand the behaviour of irradiated defects and kinetic pathways of micro-structural evolution in Fe–Cr alloys, we use a combination of density functional theory with statistical approaches involving cluster expansions and Monte Carlo simulations. A lowest negative mixing enthalpy is found at 6.25% Cr that is consistent with our DFT prediction of an ordered Fe₁₅Cr structure. At 50% Cr, it is found that the predicted enthalpy of formation is 4 times smaller than that calculated by the CPA approach. Thermodynamic and precipitation properties are then discussed in term of segregation between the Fe₁₅Cr and α^′-Cr phases and of vacancy-mediated kMC simulation. To cite this article: D. Nguyen-Manh et al., C. R. Physique 9 (2008).     

The changes in the electronic structure of B2 FeAl alloy with a Fe antisite and absorbed hydrogen

The electronic structure and bonding in a B2 FeAl alloy with and without hydrogen interaction with a Fe antisite were computed using a density functional theoretical method. The hydrogen absorption turns out to be a favorable process. The hydrogen was found close to an octahedral site where one of its Al capped is replaced by a Fe antisite. The Fe–H distance is of 1.45 Å same as the Al–H distance.The density of states (DOS) curves show several peaks below the d metal band which is made up mostly of hydrogen based states (>50% H_1s) while the metal contribution in this region includes mainly s and p orbitals.An electron transfer of nearby 0.21e⁻ comes from the metal to the H. The overlap population values reveal metal–metal bond breaking, the intermetallic bond being the most affected. The H bond mainly with the Al atom and the reported Fe–H overlap population is much lower than that corresponding to FePd alloys and BCC Fe. The changes in the overlap population show the Fe–Al bond is weakened nearly 41.5% after H absorption, while the Fe–Fe bond is only weakened 34.5%. H also develops a stronger bond with the Al atoms. The main bond is developed with Al being twice stronger than Fe–H.     

The effect of cerium oxide argon-annealed coatings on the high temperature oxidation of a FeCrAl alloy

Ceria coatings were applied in order to improve the adherence of alumina scales developed on a model Fe–20Cr–5Al alloy during oxidation at high temperature. These coatings were performed by argon annealing of a ceria sol–gel coating at temperatures ranging between 600 and 1000 °C. The influence of these coatings on the alloy oxidation behaviour was studied at 1100 °C. In situ X-ray diffraction (XRD) was performed to characterize the coating crystallographic nature after annealing and during the oxidation process. The alumina scale morphologies were studied by means of scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The present work shows that the alumina scale morphology observed on cerium sol–gel coated alloy was very convoluted. On the cerium sol–gel coated alloy, argon annealing results in an increase of the oxidation rate in air, at 1100 °C. The 600 °C argon annealing temperature results in a good alumina scale adherence under thermal cycling conditions at 1100 °C.     

Coherent Exchange Cluster Approach for two-dimensional disordered Heisenberg spin systems

The spin wave properties of disordered two-dimensional and quasi two-dimensional disordered Heisenberg spin systems, with a ferromagnetic ground state induced by single-ion anisotropy, are discussed within a Coherent Exchange Cluster Approach (Cluster CEA). The configuration averaged Green's function is described by an effective spin wave Hamiltonian, with complex and energy dependent coherent exchange integrals , where, depends only on the distance of the sitesl andm Considering only nearest neighbour clusters, the more distant exchange is neglected, whereas the nearest neighbour coherent exchange integral is determined by the self-consistency requirement that the most important matrix elements of the scatteringT matrix vanish when the configuration averaging has been performed. The approximations inherent within this approach are discussed, and it is argued that it is practically impossible to improve it essentially.The results of numerical calculations for the square lattice are presented: The parameters have been chosen as to discuss, in a somewhat hypothetical way, the properties of Fe–Ni monolayers on a Cu substrate, especially for dominating Fe concentration, where experiments are still lacking. In contrast to the oversimplified standard Single Bond CEA, the Cluster CEA yields a considerable structure within the density of states. Furthermore, for the almost dilute case, with Ni concentrationc _Ni=0.2 and exchange integralsJ _{Ni, Fe}=2.5·J _{Ni, Ni},J _{Fe, Fe}=0.3·J _{Ni, Ni}, there is even a gap, as expected from exact calculations for isolated impurities. Even within this case, the Green's functions and self-energies are analytic, in contrast to certain generalizations of the well-known CPA method for electronic systems.Concerning the critical concentrations for the appearance of a spin wave instability with negative Fe–Fe exchange, the Cluster CEA yields much better results than the Single Bond CEA.     

Evolution of a Martensite Packet under Multicycle Fatigue Loading

The methods of diffraction electron microscopy of thin metal foils are used to study the defect substructure and the phase content of preliminary quenched Fe–0.60C–1Mn–2Si steel subjected to multicycle fatigue tests. It is demonstrated that steel loading is accompanied by the initial stage of dynamic recrystallization. One of the mechanisms of forming dynamic recrystallization centers is the pair coalescence of packet martensite crystals.     

Sensitivity of ordering kinetics to the doping of Ni3Mn by iron,cobalt, and chromium

The changes in several physical and mechanical properties observed during the ordering of quenched ternary Ni₃Mn-Ni₃Me alloys (Me = Fe, Co, or Cr) were studied. Comparison of the results with results found previously by neutron diffraction shows that the latter method does not always give reliable information about the state of the ordering of ternary alloys, so it must be supplemented by a study of structure-sensitive physical and mechanical properties.Translated from Izvestiya VUZ. Fizika, No. 10, pp. 116–124, October, 1969.     

Electrical and magnetic properties of mercury selenide doped with nickel and chromium

The electrical properties of mercury selenide single-crystals doped with nickel and chromium are studied over the temperature range 80–400 K in magnetic fields of 0–8 kOe. It is established that in the mercury selenide lattice the impurity Ni is electrically and magnetically inactive. With growth in Cr concentration the paramagnetism of mercury selenide specimens increases. Results of a study of magnetic susceptibility are compared to theoretical calculations for a point model and a point model with consideration of covalence effects.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 44–47, November, 1990.