Research Article | Open Access
Experimental Investigation of Isothermal Section of the B-Cr-Fe Phase Diagram at 1353 K
The isothermal section of the B-Cr-Fe ternary system was studied experimentally at 1353 K. X-ray diffraction and scanning electron microscopy equipped with EDX analyzer were used for determination of phase equilibria and composition of the coexisting phases in the B-Cr-Fe model alloys after long-term annealing (1500–2205 h). Two iron borides FeB and Fe2B, six chromium borides Cr2B, Cr5B3, CrB, Cr3B4, CrB2, and CrB4 and also iron and chromium solid solutions (α(Fe,Cr), α(Cr,Fe), γ(Fe,Cr)) and β-rhombohedral B were observed in the alloys. High solubilities of the third element in binary borides and no ternary phase were found. Based on the experimental results, isothermal section of the B-Cr-Fe system at 1353 K was determined.
The knowledge about phase equilibria of the B-Cr-Fe system is important in several fields of materials research. The system is subsystem of various complex alloys, for example, modified ferritic steels for energy industry [1–3], steels for production abrasive, and corrosion-resistant components . Information about the system is also important for study of boride coatings on various alloys and metals, including iron [5, 6].
Borides of transition metals, including chromium borides, have unique combination of thermal, mechanical, magnetic, and electrophysical properties and are being widely used in atomic power and chemical industry [7, 8].
The B-Cr-Fe system has been experimentally studied by only a few authors. Gorbunov and Bodurjan  investigated system at 1373 K after annealing for 320 h and Chepiga and Kuzma  at 973 and 1173 K after annealing for 400 and 750 h, respectively. In the works [9, 10], some questionable phases were found. CrB6 boride, found by Gorbunov and Bodurjan  and also by Chepiga and Kuzma , is not boride, but it is oxyboride as it was later clarified . Besides the phase, Gorbunov and Bodurjan  found Cr4B and Cr2B5 phases. The phase that was predicted as Cr2B5 was not observed by any other investigators neither in ternary B-Cr-Fe  nor in binary B-Cr system . Regarding the Cr4B phase, it was shown that the phase is orthorhombic Cr2B phase . The phase constitution of the system at 1373 K was investigated also by Gianoglio et al. , but, however, only in region up to 50 at% boron.
The present work is focused on experimental study of phase equilibria of the B-Cr-Fe system at 1353 K in the entire composition range after long time annealing within 1500–2205 h that guarantee to achieve equilibrium state.
2. Experimental Procedure
High purity powders of Fe (99.98%), B (99.9%), and Cr (99.99%) were used as input material for the production of twenty B-Cr-Fe alloys. Powders were mixed, pressed into cylindrical pellets at a pressure of 700 MPa, and subsequently melted. The melting was carried out in an argon arc furnace on water cooled copper plate in argon atmosphere of 99.999% purity. The solidified alloys (15 g) were remelted several times in order to achieve good homogeneity. The final chemical compositions of the produced alloys were determined by atomic absorption spectroscopy; see Table 1. The alloys were then sealed into evacuated silica glass tubes with titanium turnings and annealed at 1353 K within 1500–2205 h. After the annealing, the samples were quenched into cold water.
The annealed material was studied by X-ray diffraction (Philips X’Pert Pro) and scanning electron microscopy (JEOL JSM-7000F) equipped with “Thermal FEG” and INCA EDX analyzer. The analyzer enables quantitative analysis for elements above and including atomic number 5 (boron). Backscattered electron image mode of scanning electron microscopy was used to study the alloy microstructure.
3. Results and Discussion
All phases identified by experimental method in alloys are given in Table 1. Two iron borides FeB and Fe2B, six chromium borides Cr2B, Cr5B3, CrB, Cr3B4, CrB2, and CrB4, and also iron and chromium solid solutions (α(Fe,Cr), α(Cr,Fe), γ(Fe,Cr)) and β-rhombohedral B were found in alloys. The borides in the system form at high temperatures. Most of them form from the liquid during production process of the alloys. Following long-term isothermal annealing leads to achievement of equilibrium chemical compositions of the phases. Crystallographic phase data of observed borides are given in Table 2. The CrB4 boride was found in 17Fe-65B-18Cr and 33Fe-62B-5Cr alloys. This phase, known from binary B-Cr system , has not been observed in ternary B-Cr-Fe system till now [9, 10]. The Cr2B5 boride that was found by Gorbunov and Bodurjan at 1373 K after annealing for 320 h was not found. No ternary phase was found in the B-Cr-Fe system. It is in accordance with the mentioned literature [9, 10, 14].
Four three-phase equilibria FeB+CrB4+B (Figure 1(d)), FeB+CrB4+CrB2, FeB+CrB2+Cr3B4, and FeB+Cr3B4+CrB (Figure 3(a)) were found in the part of phase diagram with boron content up 50 at.% (Figure 4). There are some differences between phase equilibria presented in this paper and equilibria at 1373 K from work , as shown in Table 3. The differences in this part of the phase diagram are connected with the CrB6 and Cr2B5 phases . However, the CrB6 phase, as was mentioned in introduction, is not right determined. The existence of Cr2B5 boride was not confirmed by present experimental results.
A part of the isothermal section of the phase diagram with boron range 33–50 at.% contains three three-phase equilibria CrB+Cr5B3+Cr2B (Figure 3(c)), FeB+Cr2B+Fe2B, and FeB+CrB+Cr2B (Figure 4). The same three-phase equilibria were found also at 1373 K in the works [9, 14].
Two three-phase equilibria Fe2B+Cr2B+γ(Fe,Cr) (Figure 3(f)) and Cr2B+γ(Fe,Cr)+α(Fe,Cr) are in the iron rich corner of the phase diagram at the temperature 1353 K (Figure 4). About 9 at.% of chromium was found in the γ(Fe,Cr)-phase in 45Fe-30B-25Cr alloy with phase equilibria Fe2B+Cr2B+γ(Fe,Cr). Both Gorbunov and Bodurjan  and Gianoglio et al.  found Fe2B+Cr2B+α(Fe,Cr) and Fe2B+γ(Fe,Cr)+α(Fe,Cr) equilibria at 1373 K (Table 3), however, with different amount of chromium in the α(Fe,Cr)-phase in Fe2B+Cr2B+α(Fe,Cr) equilibrium, about 35 at% of Cr according to  and 18 at% according to . The differences between observed phase fields of the part of isothermal section in the present work at 1353 K and in works [9, 14] at 1373 K indicate that the equilibrium phase fields are changed at temperature from temperature range 1353–1373 K.
All found borides dissolve third element. It is illustrated by EDX-spectra on Figure 2. Cr2B phase has the highest solubility of the third element among all borides in the system. The boride dissolves up to 39 at.% of Fe. The solubility is high also in CrB, FeB, and Fe2B phases. It was found that Fe2B contains up to 15 at.% of chromium and FeB dissolves up to 16 at.% of Cr. The CrB phase dissolves up to 15 at.% of Fe. Besides them, high solubility of iron is observed also in Cr3B4 boride, Figure 2(c), that contains up to 15 at.% of Fe. Various amounts of dissolved iron were measured also in other chromium borides. In particular, Cr5B3 contains up to 6 at.% of Fe, CrB2 dissolves up to 2 at.% of Fe, Figure 2(b), and CrB4 contains up to 7 at.% of Fe, Figure 2(a).
All the mentioned experimental results, that is, equilibrium phase compositions and composition of phases, were used to determine the isothermal section of B-Cr-Fe phase diagram at 1353 K that is shown in Figure 4.
In the work, phase equilibria of the B-Cr-Fe model alloys at 1353 K were studied experimentally. The achieved results can be summarized as follows:(i)Two iron borides FeB and Fe2B, six chromium borides Cr2B, Cr5B3, CrB, Cr3B4, CrB2, and CrB4, and also iron and chromium solid solutions (α(Fe,Cr), α(Cr,Fe), γ(Fe,Cr)) and β-rhombohedral B are equilibrium phases in the system at 1353 K.(ii)8 three-phase equilibria were found in the phase diagram at the temperature.(iii)Solubilities of the third element in all binary borides were determined. High solubilities were found in Cr2B, CrB, Cr3B4, FeB, and Fe2B phases.(iv)No ternary phase was found.
Based on the experimental results, isothermal section of the B-Cr-Fe system at 1353 K was established.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The present work was supported by Slovak Grant Agency (VEGA) under Grant no. 2/0153/15. Thanks are also due to Dr. P. Repovský for help at production of alloys.
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