Structure and Mössbauer spectra for the Fe–Cr system: From bulk alloy to nanoparticles

Abstract

The structure and hyperfine fields of Fe1−xCrx (x=0.0236–0.803) nanoparticles (average size of 27±2 nm) are studied at room temperature by combined x-ray diffraction and Mössbauer spectroscopy techniques. They are produced by fast evaporation of bulk alloys at 3 Torr Ar pressure. The bulk alloys of any composition are shown to exhibit a bcc structure, whereas the nanoparticles demonstrate a mixture of bcc and tetragonal σ phases in the Cr range from 24.4 to 83.03 at. %. At the Cr content of 2.36 at. % the lattice constant for nanoparticles is larger than that of the bulk alloy, though the values of hyperfine fields on Fe nuclei do not differ. The Mössbauer spectrum of nanoparticles contains an oxide doublet in addition to the sextet specific to that of the bulk alloy. In both cases the width of the sextet lines is rather narrow. However, even at ∼8 at. % Cr the lines of the sextet are broadened so much that it can be decomposed by two-three components. This is explained by freezing the high-temperature ferromagnetic fcc phase regions in the bcc lattice. As the Cr content increases, the Mössbauer spectra become more complex, transforming finally into a paramagnetic singlet. A complete ferromagnetic→paramagnetic transition is observed for the bulk alloy at 68 at. % Cr and for nanoparticles at 35 at. % Cr. The results are discussed under the assumption that at high temperatures the alloys are not homogeneous and exhibit fluctuations of the composition. With decrease of temperature these fluctuations result in decomposition of the alloy into two phases for nanoparticles whereas they are frozen at the cluster level in the bulk alloys holding a macroscopic homogeneity.

Aerosol nanoparticles in the Fe1−xCrx system: Room-temperature stabilization of the σ phase and σ→α-phase transformation

“…In the past, metastable -FeCr phase has been detected at room temperature in sputtered thin films [13][14][15] and in alloys with additions of a third element ͑e.g., Ti, Al, and Sn͒ [16][17][18][19] or after subjecting ␣-FeCr to long thermal treatments. [20][21][22] However, as previously shown by Petrov and co-workers, 23,24 nanoparticles of hightemperature phases of metals and alloys can be stabilized at room temperature by gas evaporation. Figure 3 presents the dependencies of the -phase content and the ␣-FeCr lattice parameter on the sample composition, as obtained from Rietveld refinements of the diffraction data.…”

Non-Stoichiometry and Calphad Modeling of Frank-Kasper Phases

“…It has proved to be useful in order to study the magnetic properties of the σ phase [9][10][11] or even to show phase transitions [12,13].…”

Mössbauer Spectroscopy Study of Laboratory Produced ODS Steels

“…The Möss-bauer spectra of sample KOT 17 and KOT 18 do not show sextets of narrow lines which can be seen in all previously mentioned samples, identified as non-pure bcc Fe phase. The spectra can be decomposed into two components [9], with mean hyperfine field values B hf 1 ≈ 27 T, B hf 2 ≈ 22 T, respectively. These two sextets of hyperfine field distribution can be assigned to phases of bcc iron with gradually increasing Cr content in the samples.…”

“…The first sextet (34 T) has to be assigned to Fe-atoms in the iron matrix that are mostly surrounded by Fe atoms in their first coordination shells (named as Fe), while the two other magnetically split subspectra (B hf 1 ≈ 31 T, B hf 2 ≈ 29 T) have to be associated with iron atoms surrounded in their first or second coordination shells by alloying elements (named as Fe-Cr). Similar fitting model was used by Petrov et al [9]. Minor paramagnetic singlet in the middle of the spectrum can be described as chromium rich bcc α phase [10].…”

“…According to [9], the chosen fitting procedure depends on the amount of Cr in Fe-Cr alloys. In the case of 9 to 13.6 wt% Cr containing samples the applied fitting model consists of four components.…”