659 research outputs found
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
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
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: 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
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