Chunfei Wu

Chunfei Wu

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Materials characterisation - Solid – Composition analysis - X-ray diffraction analysis (XRD) - XRD application in research

Phases and Interfaces in the Cr–Fe–Si Ternary System: X-ray Diffraction and Electron Microscopy Study

https://doi.org/10.3390/inorganics11020073

“The ternary Cr-Fe-Si system was investigated with X-ray diffraction, energy dispersive X-ray spectrometry, scanning and transmission electron microscopy, and electron diffraction. Samples melted at 1723 K were examined right after cooling or after annealing at 1073 K for 3 days to determine phases, grain sizes, and interphase interfaces. During annealing, a polymorphic transformation of the tetragonal α-FeSi2 to the orthorhombic β-FeSi2 phase occurs, while CrSi2 retains its hexagonal structure at high-temperature treatment. Thin layers of ε-FeSi with a cubic structure were observed and identified within the CrSi2 grains. Crystallographic orientation relationships are determined at the interphase interfaces. The contributions of lattice mismatch and thermal expansion coefficient misfit to deformation are discussed.”

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“High-purity iron (99.5 wt.%, powder, Sigma-Aldrich, St. Louis, MI, USA), chromium (99.5 wt.%, powder, Sigma-Aldrich), and silicon (99.0 wt.%, powder, Sigma-Aldrich) were used as starting materials to prepare stoichiometric FeCrSi4. The powder mixture was melted in an arc melting furnace at T = 1723 K, and then then crystallized under vacuum (about 0.1 Pa). A part of the samples was finally annealed in air at T = 1073 K for 72 h.

X-ray powder diffraction (XRD) patterns were obtained in reflection mode using a Rigaku MiniFlex 600 diffractometer (Rigaku Corporation, Japan) with the CuKα radiation (45 kV, 40 mA, Ni–Kβ filter) in the angle range 2θ = 15°–100° with a scanning step of 0.02° and a rate of 1°/min. The quantitative ratio of the identified phases was estimated by the Rietveld method using PANalytical X’Pert High Score Plus software (Malvern Panalytical, UK). In the initial stages the overall zero error, phase scale factors, background, profile parameters, and lattice parameters of identified phases were refined. The peak shapes were approximated with pseudo-Voight profile function. Since the CrSi2 and FeSi2 phases exhibited the different pronounced orientations, the preferred texture parameters were also refined in the final stages.

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2.1. Powder X-ray Diffraction Study

In accordance with Cr-Fe-Si ternary phase diagrams [13,14], the position of the sample composition of FeCrSi4 is at the point 17 at% Cr: 17 at % Fe: 66 Si at.%. Thus, we expected the following phases to be formed: FeSi2, CrSi2, and (Cr, Fe)Si for the temperature interval and starting stoichiometric composition used in the work. Close to this point, pure Si can also be formed.
The possible phases, their lattice parameters and space groups of the Cr and Fe silicides and Si used in this work for identification are listed in Table 1 according to corresponding ICSD database (FIZ Karlsruhe, Germany) and references. We will use general notations for phases of iron silicides: α-FeSi2 (tetragonal structure), β-FeSi2 (orthorhombic structure) and ε-FeSi (cubic structure).
Table 1. Chemical and phase composition of binary Fe and Cr silicides.
Fe–Si Cr–Si
Phase Space Group; Lattice Parameters (nm) # ICSD Phase Space Group; Lattice Parameters (nm) # ICSD
Fe3Si Fm3¯3¯m; a = 0.5665 [30]; 412838 Cr3Si Pm3¯3¯n; a = 0.45599 [31]; 32509
Fm3¯3¯m; a = 0.5662 [32]. 633537 Pm3¯3¯n; a = 0.4539 [33]. 199130
Fe5Si3 P63/mcm; a = 0.67552, c = 0.47174 [34]; 42585 Cr5Si3 I4/mcm; a = 0.917, c = 0.4636 [35]; 15683
P63/mcm; a = 0.6755, c = 0.4715 [36]; 633525 I4/mcm; a = 0.915, c = 0.464 [37]. 626782
P63/mcm; a = 0.6756, c = 0.4718 [38]. 633540
ε-FeSi P213; a= 0.445507 [39]; 5250 CrSi P213; a = 0.4629 [33]. 626772
P213; a= 0.4489 [40]; 76945
β-FeSi2 Cmca, a= 0.9863, b = 0.7791, c = 0.7833 [41]; 9119 CrSi2 P6222; a = 0.4428, c = 0.6364 [42]; 626776
Cmca, a= 0.9876, b = 0.7798, c = 0.7836 [43]; 163384 P6222; a = 0.4428, c = 0.6364 [44]; 626787
Cmca, a= 0.988, b = 0.7798, c = 0.7839 [45]. 603890 P6422; a = 0.4283, c = 0.6368 [46]; 96026
α-FeSi2 P4/mmm, a = 0.2725, c = 0.5202 [47]; 5258 Si Fd3¯3¯m, a = 0.54307 [48]; 29287
P4/mmm, a = 0.269, c = 0.5133 [49]. 633544 Fd3¯3¯m, a = 0.543086 [50]. 76268
The Rietveld refined powder XRD patterns obtained from the samples are shown in Figure 1. The reflections on the XRD patterns from the sample before annealing can be indexed with three phases tetragonal α-FeSi2, hexagonal CrSi2, and cubic FeSi (Figure 1a). While the XRD pattern obtained from the annealed material shows the presence of four phases orthorhombic β-FeSi2, hexagonal CrSi2, cubic ε-FeSi, and cubic Si (Figure 1b). No cubic CrSi was revealed in any of the samples, nor was any traces of α-FeSi2 found in the annealed sample. Thus, α-FeSi2 phase with space group P4/mmm completely transforms into orthorhombic Cmca β-FeSi2 phase and silicon appears in the composition of the sample after 3 days of annealing at 1073 K.
Inorganics 11 00073 g001 550
Figure 1. Phase analysis using Rietveld refinement for the Fe−Cr−Si samples before (a) and after annealing (b). The Bragg peaks of α−FeSi2 tetr and β−FeSi2 orth, ε−FeSi and Si cub were labeled with 3 indices, and the planes in hexagonal CrSi2 are shown with the Miller-Bravais indices (hkil), labels of visible Si (111, 022 and 113) peaks are shown under the XRD pattern.
The quantitative composition of phases in the samples together with the Rwp (weighted least-squares error) and GOF (goodness of fit) in accordance with Rietveld refinement are reported in Table 2. It shows that CrSi2 hex has retained its space group and quantity, while iron disilicide has undergone a tetragonal to orthorhombic phase transition due to the tetragonal FeSi2 phase and partly of cubic ε−FeSi phase.
Table 2. Phase composition of the samples according to the Rietveld refinement.
As Grown (Non-Annealed) Annealed
Rwp = 13.4%, GOF = 6.4 Rwp = 11.5%, GOF = 5.2
phase weight % Phase weight %
CrSi2 50.5 (4) CrSi2 50.7 (3)
α−FeSi2 41.9 (4) β−FeSi2 46.8 (3)
ε−FeSi 7.5 (2) ε−FeSi 1.3 (1)
Si Not found Si 1.2 (1)
The presence of minor other phases found by TEM will be described later in the article. (See for instance Section 2.3 and the corresponding figures).

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