S.M. Dubiel

Mössbauer-effect study of the phase separation in the Fe-Cr system

In situ and conventional room temperature 57Fe-site Mossbauer spectroscopy (MS) were used to study the phase separation process in the Fe-Cr alloy system. It was shown that MS is well able to distinguish between (i) nucleation and growth and (ii) spinodal decomposition, the two mechanisms held responsible for the separation. The Fe-rich branch of the spinodal line was located at 80.8≤x≤84 at.% Fe at T = 415 °C. It was also shown that the saturation of the average hyperfine field cannot be taken as an indicator of the termination of the separation process. The kinetics of the process dramatically depends on the aging temperature, being faster by a factor of 1.4 at T = 440 °C than at T = 415 °C.

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.

Electronic structure of a σ -FeCr compound

The electronic structure of a σ-FeCr compound in a paramagnetic state was calculated for the first time in terms of isomer shifts and quadrupole splittings. The former were calculated using the charge self-consistent Korringa-Kohn-Rostoker (KKR) Greens function technique, while the latter were estimated from an extended point charge model. The calculated quantities combined with recently measured site occupancies were successfully used to analyze a Mössbauer spectrum recorded at room temperature using only five fitting parameters namely background, total intensity, linewidth, IS0 (necessary to adjust the refined spectrum to the used Mössbauer source) and the QS proportionality factor. Theoretically determined changes of the isomer shift for the σ-FeCr sample were found to be in line with the corresponding ones measured on a α-FeCr sample.

Influence of Si on spin and charge density changes in bcc-iron

Abstract The influence of Si on the 57 Fe and 119 Sn site hyperfine (hf) fields and isomer shifts has been studied for a series of Fe-Si alloys containing up to ca. 13 at% Si and ca. 0.9 at% 119 Sn. The observed changes of the hf fields and the isomer shifts have been interpreted as reflecting spin and charge density changes, respectively. The following correlations could be established: hf field H (0,0) vs. isomer shift IS(0,0) of undisturbed atomic neighbour configurations (0,0), average hf field, H vs. average isomer shift, IS ; average hf field, H vs. average number of Si atoms within the first two neighbour shells, N . Based on the f correlations the following hf coupling constants have been determined: a) the hf coupling constant for s-like itinerant electrons are 690 kOe/s-el for Fe and 2100 kOe/s-el for Sn, b) the average hf coupling constants are 660 kOe/s-el and 2100 kOe/s-el for Fe and Sn, respectively. From the correlation between H and N the changes in the spin or charge densities caused be one Si atom per unit cell, η, have been deduced as follows: η(Fe)=0.17 and η(Sn)=0.03. Comparison is made with previously reported equivalent results for the Fe-Al system.

Effect of titanium on the kinetics of the σ-phase formation in a small grain Fe–Cr alloy

Abstract Using Mossbauer Spectroscopy the kinetics of the σ-phase formation was studied in situ on Ti-doped quasi-equiatomic Fe–Cr alloys having an average grain size equal to 31(2) μm. The kinetics was studied both with a traditional approach i.e. the amount of the σ-phase precipitated, A σ , was determined from the spectral area, as well as with a novel approach i.e. in terms of the average isomer shift. In both cases the data could be well fitted in terms of the Johnson–Avrami–Mehl equation which yielded the kinetics parameters: the time constant, k , and the form factor, n . Assuming the k -values follow the Arrhenius law, the Ti-induced change in the activation energy, E , was determined. In particular, it was revealed that addition of 1.5 at% Ti or less accelerates the transformation, with the highest rate for 0.3 at% Ti, and the difference in E equals to 18.8 kJ/mol. In alloys with Ti-content higher than 1.5 at%, the formation of the σ-phase was retarded. The value of E =207±20 kJ/mol was found for the Fe 53.8 Cr 46.2 alloy.

Electronic structure of the σ phase of paramagnetic Fe-V alloys

The electronic structure of σ-phase Fe100−xVx compounds with 33.3 ≤ x ≤ 60.0 was calculated from the charge self-consistent Korringa-Kohn-Rostoker method. For the first time, charge densities ρA(0) and electric field gradients were determined at Fe nuclei, that occupy five nonequivalent lattice sites. The highest ρA(0) values were found on sites A and D, and the lowest one on site B, the difference ranging between 0.162 and 0.174 s-like electrons per Fe atom for x = 33.3 and x = 60, respectively. The calculated quantities combined with experimentally determined site occupancies were successfully applied to analyze Fe Mössbauer spectra recorded on a series of 8 samples in a paramagnetic state.

Extended scaling in the magnetic critical phenomenology of the σ-phase Fe0.53–Cr0.47 and Fe0.52–V0.48 alloys

The magnetization of the sigma-phase Fe(0.53)Cr(0.47) and Fe(0.52)V(0.48) alloys was studied as a function of temperature and field. The experiments show that both materials behave magnetically as re-entrant spin glass systems. Field versus temperature diagrams were obtained where the locations of the paramagnetic phase, the intermediate ferromagnetic-like phase and the spin glass fundamental state were displayed. These diagrams are in qualitative agreement with the predictions of the mean field theory for the interplay between the ferromagnetic and spin glass orderings. The critical phenomenology near the para-ferromagnetic transition could be investigated. It was found that the paramagnetic susceptibility is quite well described by the extended scaling scheme, where the reduced temperature is written as τ = (T - T(c))/T. The value obtained for the susceptibility critical exponent γ is intermediate between the prediction of the 3D Heisenberg universality class and the large values observed in spin glasses, as previously found in other re-entrant systems. The data do not confirm the validity of the extended scaling in the ferromagnetic-like phase. Using either the conventional or extended scaling protocols, the exponents β and δ were found to have values close to those reported for spin glass transitions. Despite the relevance of disorder and the anomalous values determined for β, γ and δ, the Widom scaling relation holds as an equality.

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.

Magnetic properties of σ-FeCr alloys as calculated with the charge- and spin-self-consistent KKR(CPA) method

Magnetic properties of a