J Cieślak

Sigma-Phase in Fe-Cr and Fe-V Alloy Systems and its Physical Properties

This review addresses the physical properties of the σ-phase in Fe-Cr and Fe-V alloy systems as revealed both with experimental—mostly with the Mössbauer spectroscopy—and theoretical methods. In particular, the following questions relevant to the issue have been addressed: identification of σ and determination of its structural properties, kinetics of α-to-σ and σ-to-α phase transformations, Debye temperature and Fe-partial phonon density of states, Curie temperature and magnetization, hyperfine fields, isomer shifts and electric field gradients.

Titanium-induced changes in the electronic structure of iron

Abstract Mossbauer effect was used to study changes in the electronic structure of iron caused by substitutional Ti atoms. It was found that one Ti atom increases the charge-density at nuclei of Fe by 0.007 s-like electrons, if present in the nearest-neighbour shell, and by 0.002 s-like electrons, if situated in the next-nearest-neighbour shell. The effect of more distant Ti atoms is the opposite, i.e. they decrease the charge-density, and, consequently, the average effective charge-density at nuclei of Fe atoms hardly changes with Ti content. The estimated change of the charge-density due to one Ti atom per unit cell, η=0.035, which is by a factor of four to five less than the corresponding figure expected from the Miedema–van der Woude model. On the other hand, the average Fe-site spin-density decreases linearly with Ti content. This fact accompanied by the constant value of the average charge-density can be accounted for by postulating that electrons with spin-up spins flip into the spin-down state.

Discovery and characterization of magnetism in sigma-phase intermetallic Fe-Re compounds

Systematic experimental (vibrating sample magnetometry) and theoretical (electronic structure calculations using charge and spin self-consistent Korringa-Kohn-Rostoker Green function method) studies were performed on a series of intermetallic sigma-phase Fe(100-x)Re(x) (x = 43-53) compounds. Clear evidence was found that all investigated samples exhibit magnetism with an ordering temperature ranging between 65 K for x = 43 and 23 K for x = 53. The magnetism was revealed to be itinerant and identified as a spin-glass (SG) possibly having a re-entrant character. The SG was found to be heterogeneous viz. two regimes could be distinguished as far as irreversibility in temperature dependence of magnetization is concerned: (1) of a weak irreversibility and (2) of a strong one. According to the theoretical calculations the main contribution to the magnetism comes from Fe atoms occupying all five sub lattices. Re atoms have rather small moments. However, the calculated average magnetic moments are highly (ferromagnetic ordering model) or moderately (antiparallel ordering model) overestimated relative to the experimental data.

Debye temperature of disordered bcc-Fe?Cr alloys

The Debye temperature, Θ(D), of Fe(100-x)Cr(x) disordered alloys with 0 ≤ x ≤ 99.9 was determined from the temperature dependence of the centre shift of (57)Fe Mössbauer spectra recorded in the temperature range of 60-300 K. Its compositional dependence shows an interesting non-monotonous behaviour. For 0<x ≤ ∼ 45, as well as for ∼ 75 ≤ x ≤ ∼ 95, the Debye temperature is enhanced relative to its value of a metallic iron, and at x ≈ 3 there is a local maximum having a relative height of ∼ 12% compared to a pure iron. For ∼ 45 ≤ x ≤ ∼ 75 and for x ≥ ∼ 95 the Debye temperature is smaller than the one for the metallic iron, with a local minimum at x ≈ 55 at which the relative decrease of Θ(D) amounts to ∼ 12%. The first maximum coincides quite well with that found for the spin-waves stiffness coefficient, D(o), while the pretty steep decrease observed for x ≥ ∼ 95, which is indicative of a decoupling of the probe Fe atoms from the underlying chromium matrix, is likely related to the spin-density waves which constitute the magnetic structure of chromium in that interval of composition and show also anomalous dynamic behaviour. The harmonic force constant calculated from the effective Debye temperature of the least Fe-concentrated alloy (x ≥ 99.9) amounts to only 23% of the one characteristic of a pure chromium as determined from the heat capacity experiment.

Magnetic properties of a nanocrystalline σ-FeCr alloy

Magnetic properties of a nanocrystalline σ -Fe55.4Cr44.6 alloy were investigated by means of M¨ ossbauer spectroscopy and vibrating sample magnetometry both as a function of temperature (4 K T 300 K) and, for the latter, also as af unction of external magnetic field (Ba 15 T). The methods used enabled us to determine the mean Curie temperature, � TC �≈ 57 K, and the average magnetic moment per Fe atom, � µ �= 0.34 µB ,a s well a st o fi nd that the sample behaves lik ea n ensemble of interacting superparamagnetic particles. Comparison of these results with our recent corresponding investigations on microcrystalline σ -Fe100−x Crx alloys with 45 x 50 shows that the ratio between the average hyperfine field, � B� ,a nd� µ� fo rt he nanocrystalline sample fits well to that of the microcrystalline ones of similar composition. The non

Investigation of a Cr42.2Fe57.8 alloy prepared by mechanical alloying

AC r 42.2Fe57.8 nano-crystalline alloy prepared by mechanical alloying for 100 h in argon atmosphere wa si nvestigated with Mossbauer spectroscopy (MS), differential scanning calorimetry (DSC), SQUID magnetometry and x-ray diffraction (XRD) techniques. Evidence was found that the final product was composed of two crystalline phases, namely (a) Cr45Fe55 with an abundance of ∼50% and (b) Cr86.5Fe13.5 with an abundance of ∼15%, and an amorphous phase having two crystallization temperatures: one at TC1 ≈ 870 K and another one at TC2 ≈ 920 K. The results prove that the milling process has led both to a partial amorphization and to phase decomposition into Fe-rich and Cr-rich phases.

The Debye temperature of quasi-equi-atomic α-Fe–Cr alloys

A series of quasi-equi-atomic bcc Fe100−xCrx alloys (40.5≤x≤52.6) was investigated with Mossbauer spectroscopy. The Debye temperature, ΘD, was determined from the spectra measured in the temperature range of 30–300 K. It was revealed that the values of ΘD found here increase linearly with x, but both the values and their rate of increase are slightly smaller than those recently found for σ-Fe–Cr alloys of similar composition. The small difference in ΘD-value between the two phases do not reflect the huge crystallographic and mechanical differences between them.

Distribution of Cr atoms in the surface zone of Fe-rich Fe–Cr alloys quenched into various media: Mössbauer spectroscopic study

Effect of a quenching medium (water, liquid nitrogen and block of brass) on a short-range ordering in Fe(100-x)Cr(x) (x less than 19) alloys was studied with the Moessbauer spectroscopy. The distribution of Cr atoms was expressed in terms of the Cowley-Warren short-range order (SRO) parameters: alpha1 for the first neighbor-shell, alpha2 for the second neighbor-shell and alpha12 for both neighbor-shells. It was revealed that none of the quenching media resulted in a random distribution of atoms, yet the degree of randomness was the highest for the samples quenched onto the block of brass. The quenching into water and liquid nitrogen caused a partial oxidation of samples surface accompanied by a chromium depletion of the bulk. Quantitative analysis of various phases in the studied samples both in their bulk as well as in pre surface zones was carried out.

Study of a sigma-phase formation in an equiatomic Fe-V alloy

Kinetics of the ?- to ?-phase transformation in an equiatomic Fe-V alloy has been studied by M?ssbauer Spectroscopy and X-ray diffraction. The B2 superstructure seems to form prior to the precipitation of the ?-phase. The results obtained in this study give evidence that in the investigated interval, the transformation kinetics strongly depends on temperature. The kinetics of the ?- to ?-phase transformation, as determined from the temperature dependence of the average hyperfine field derived from the M?ssbauer spectra, has been quantitatively described in terms of the Johnson-Mehl-Avrami equation, from which the kinetics parameters n and k have been determined.

Unusual dynamics of Fe atoms in a chromium matrix

(57)Fe site Mössbauer spectroscopy (MS) was used to investigate the dynamics of (57)Fe atoms embedded in a chromium lattice as impurities. From the Mössbauer spectra recorded in the temperature range of 80-350 K, a temperature dependence of the Lamb-Mössbauer factor, f, was determined. The latter revealed an unusual dynamics of (57)Fe atoms, namely a harmonic mode below T≈145 K with a characteristic effective Debye temperature Θ(eff) = 190.2 K and a strongly anharmonic one above T≈145 K. The latter mode exists in two clearly defined temperature intervals with significantly different Θ(eff) values, namely (i) ∼155 K for ∼145 K ≤ T ≤ ∼240 K and the record-high anharmonic coefficient ε = -25.8 × 10(-4) K(-1), and (ii) ∼151 K for T ≥ ∼240 K with ε = -14 × 10(-4) K(-1). Based on Visschers theory, the record-low value of the relative binding force constant for Fe atoms was determined as 0.0997 for the harmonic regime. It is suggested that the unusual dynamics observed in this study might be related to the underlying spin-, charge- and strain-density waves of chromium.