Evidence for a re-entrant character of magnetism of sigma-phase Fe-Mo alloys: non-linear susceptibilities

Non-linear ac magnetic susceptibility terms viz. quadratic, chi2, and cubic, chi3, were measured versus temperature and frequency for a series of the sigma-phase Fe(100-x)Mo(x) (47<x<53) compounds. Clear evidence was found that the ground magnetic state of the samples is mixed i.e. constituted by two phases: a spin glass (SG) and a ferromagnet (FM), hence the magnetism of the investigated samples can be regarded as re-entrant (RSG). Based on the present data, previously reported magnetic phase diagram has been upgraded [J. Przewoznik, S. M. Dubiel,J. Alloy. Comp., 630 (2015) 222].Comment: 10 pages, 4 figures, 22 references, 1 tabl

Change of Cr atoms distribution in Fe85Cr15 alloy caused by 250 keV He+ ion irradiation to different doses

Redistribution of Cr atoms in a Fe85Cr15 alloy caused by its irradiation with 250 keV He ions to different doses viz. 5, 10 and 30 dpa was investigated by means of conversion electrons Mossbauer spectroscopy. The redistribution was expressed in terms of the Warren-Cowley short-range order parameters alpha1, alpha2 and alpha12 pertaining to the first, second and both neighbor shells, respectively. Clear evidence was found, both for non-irradiated and irradiated samples that the actual distribution of Cr atoms is characteristic of the shell, and for a given shell it depends on the irradiation dose. In particular, alpha1 is positive, hence indicates an under population of Cr atoms in 1NN with respect to the random case, alpha2 is negative, giving evidence thereby that 2NN is overpopulated by Cr atoms, and alpha12 is weakly positive. Under the applied irradiation the number of Cr atoms in both neighbor shells decreased signifying thereby a clustering of Cr atoms. The underlying decrease of Cr concentration within the two-shells volume around the probe Fe atoms was estimated at 1.5 at% ranging between 2.1 for the lowest and 0.8 at% for the highest dose.Comment: 16 pages, 8 figures, 2 table

Phase-decomposition-related short-range ordering in a Fe-Cr alloy

A redistribution of Cr atoms related to a phase decomposition (PD) caused by an isothermal annealing at 415 C in a 15.15 at%Cr Fe-Cr alloy was studied in an ex-situ way by the conversion electrons M\"ossbauer spectroscopy. Analysis of the spectra in terms of a two-shell model enabled determination of probabilities of 17 different atomic configurations and average numbers of Cr atoms within the first (1NN) and the second (2NN) neighbor shells versus annealing time, separately. The annealing-time evolution of these numbers, expressed in terms of the Cowley-Warren short-range order (SRO) parameters, was shown to follow the Johnson-Mehl-Avrami-Kolgomorov equation. The SRO-parameter averaged over the 1NN-2NN shells was revealed to be linearly correlated with the average hyperfine field. Signatures in favor of the nucleation and growth mechanisms responsible for PD are discussed, too.Comment: 12 pages, 8 figures, 3 tables, 29 reference

Kinetics of phase separation, border of miscibility gap in Fe-Cr and limit of Cr solubility in iron at 832 K

Kinetics of phase decomposition accompanied by precipitation of sigma-phase in a Fe73.7Cr26.3 alloy isothermally annealed at 832 K was studied by means of M\"ossbauer spectroscopy. Two stage decomposition process has been revealed by three different quantities viz. the average hyperfine field, , the short-range parameter, alpha1, and the probability of atomic configuration with no Cr atoms within the first two coordination shells around the probe Fe atoms, P(0,0). The first stage, that has terminated after ca.300 h of annealing, has been associated with the decomposition into Fe-rich phase in which the concentration of Cr, determined as 20.9 at.%, can be interpreted as the border of the metastable miscibility gap at 832 K. The second stage can be regarded as a continuation of the phase decomposition process combined with a precipitation of sigma. The three relevant parameters for this stage have also saturation-like behavior vs. annealing time and the saturation can be interpreted as termination of the two processes. The concentration of Cr in the Fe-rich phase has been determined as 19.8 at.% and this value can be regarded as the limit of Cr solubility in iron at 832 K. Both stages of the kinetics were found to be in line with the Johnson-Mehl-Avrami-Kolgomorov equation yielding values of the rate constant and the Avrami exponent. The activation energy of the second-stage process was determined to be by ca.12 kJ/mol higher.Comment: 20 pages, 11 figures, 2 tables, 20 reference

On the miscibility gap at 800 K in the Fe-Cr alloy system

Issues pertinent to the miscibility gap at 800K in a Fe73.7Cr26.3 alloy viz. the kinetics of the phase decomposition and borders of the latter were investigated by means of the M\"ossbauer-effect spectroscopy. The kinetics was revealed to well follow the Johnson-Mehl-Avrami-Kolgomarov equation testifying to a nucleation and growth mechanism underlying the decomposition. A solubility limit of Cr in the Fe matrix was determined to be 17.3(6) at.% and that of Fe in the Cr matrix was found as equal to 18.5(9) at.%. The results obtained have been compared with various theoretical predictions.Comment: 10 pages, 1 table, 6 figurse, 13 referene

Fe-rich border of miscibility gap and activation energy of phase decomposition in a Fe-Cr alloy

Concentration of Cr in the Fe-rich alpha-phase, x, resulted from a phase decomposition caused by an isothermal annealing at T = 415 and 450 C of a non-irradiated (NR) Fe-Cr14 EFDA sample and that of a He-ions irradiated (IR) one annealed at 415 C was determined with M\"ossbauer spectroscopy. The x-value in the latter was by ~3 at% higher than the one in the NR-counterpart. The activation energy for the phase decomposition in the NR-sample was 122 kJ/mol. In the IR-sample its value was by 12 kJ/mol lower. Avrami exponents for the NR-samples were close to 0.5, and that for the IR-sample had a value of about 1.Comment: 5 pages 3 figures 1 Tabl

Effect of thermal history on the short-range order in Fe-Cr alloys

Effect of a thermal history of Fe(100-x)Cr(x) (x < 20) samples on a Cr atoms distribution within the first (1NN) and the second (2NN) neighbor-shells was studied with the M\"ossbauer spectroscopy. The distribution was expressed in terms of the Cowley-Warren short-range order (SRO) parameters: for 1NN, for 2NN and for 1NN-2NN. It was shown to be characteristic of the thermal treatment and of the neigbor shell. For quenched samples, is positive for all x-values, while shows inversion at x about 8 from positive to weakly negative. Similar character has , but the degree of ordering in 1NN-2NN is lower than that in 2NN. Isochronally annealed samples exhibit similar behvior for x > ~8, but significantly different for x and occurs at x about 3, yet in the opposite direction. The follows the trend predicted by Erhart et. al. [PRB 77, 134206 (2008)]. A clear-cut inversion induced by an isothermal annealing at 415 C was found for the Fe85Cr15 sample.Comment: 3 figures, 1 table, 18 reference

The Debye temperature of sigma-phase Fe-Mo compounds as determined with M\"ossbauer spectroscopy

The Debye temperature, T_D, of sigma-phase Fe_(100-x)Mo_x compounds with 47 < x < 56.7 was determined from the temperature dependence of the centre shift of M\"ossbauer spectra recorded in the temperature range of 80 - 300 K. Its compositional dependence shows a weak increase with x whose rate, in a linear approximation, is equal to 3.1 K/at%. The results are compared with the corresponding ones found previously for the sigma-phase in Fe-Cr and Fe-V compounds.Comment: 4 pages, 3 figure

Magnetism of sigma-phase Fe-Mo alloys: ac magnetic susceptibility study

A series of four sigma-Fe(100-x)Mo(x) samples was investigated with ac magnetic susceptibility measurements. An evidence was found that the ground magnetic state of the samples is constituted by a spin glass (SG) with the spin glass temperature ranging between ca. 34K for x=47 and ca. 16K for x=53. The SG state is heterogeneous and it can be divided into a weak-irreversibility and a strong-irreversibility domains. Its figures of merit are typical of metallic (canonical) spin glasses.Comment: 8 pages, 4 figures, 1 table, 18 reference

Magnetism of sigma-phase Fe-Mo alloys: its revealing and characterization

A low-temperature magnetism was revealed in a series of sigma-Fe(100-x)Mo(x) alloys (x=45-53). Its characterization has been done using vibrating sample magnetometry, M\"ossbauer spectroscopy, and ac magnetic susceptibility. The magnetic ordering temperature was determined to lie in the range of 46 K for x=45 and 22K for x=53, and the ground magnetic state was found to be typical of a spin-glass.Comment: 7 pages, 5 figures, 20 reference