Liquidus Projection of the B-Fe-U Diagram: The Fe-Rich Corner

Abstract

The liquidus projection of the Fe-rich corner of the B-Fe-U phase diagram is proposed based on powder X-ray diffraction measurements, differential thermal analysis, and scanning electron microscopy observations complemented by energy dispersive X-ray spectroscopy. The present work demonstrates the existence of 10 ternary reactions together with 18 monovariant lines and establishes their location and approximate temperatures. The ternary compounds existing in this region of the B-Fe-U ternary phase diagram, UFe₃B₂, UFe₄B, and U₂Fe₂₁B₆, were confirmed to form by ternary peritectic reactions, yet UFe₄B and U₂Fe₂₁B₆ have minute primary crystallization fields, indicating a more challenging preparation of single crystals for these compounds when compared with UFe₃B₂.

Appendix

1.1 Solidification Paths

Figure 10 shows the solidification path for 38B:52Fe:10U, 33B:50Fe:17U (UFe₃B₂ stoichiometry), 17B:73Fe:10U, 17B:66Fe:17U (UFe₄B stoichiometry), and 9B:87Fe:4U alloys. In agreement with the liquidus surface proposed, the solidification path followed by the 38B:52Fe:10U alloy presented the following solidification sequence: L → UFeB₄ → UFe₃B₂ → Fe₂B → γ-Fe + UFe₄B → γ-Fe + UFe₄B + UFe₂ (Figure 4(e)). The primary solidification of UFeB₄ indicates that the alloy is situated in the crystallization field of this compound. According to the transition temperatures (Figure 7(a)), formation of UFe₃B₂ (below l₁₅) occurred at 1503 K (1230 °C), formation of Fe₂B (along and below l₁₉) occurred at 1453 K (1180 °C), and solidification of γ-Fe + UFe₄B mixture (along l₂₂) occurred at 1303 K (1030 °C). The fact that l₁₅ converges to U₅ and l₁₉ diverges from U₅ implies that this ternary reaction occurs at 1453 K (1180 °C) < T < 1503 K (1230 °C). Moreover, since l₁₅ operated at a temperature near 1503 K (1230 °C) (UFe₃B₂ formation) and is diverging from P₃, it implies that P₃ occurs at T > 1503 K (1230 °C) (Figure 10).

Fig. 10

Solidification path for alloys 38B:52Fe:10U, 33B:50Fe:17U (UFe₃B₂ stoichiometry), 17B:73Fe:10U, 17B:66Fe:17U (UFe₄B stoichiometry), and 9B:87Fe:4U alloys. Dashed lines represent the liquid composition at the monovariant lines

The microstructure of the 33B:50Fe:17U (UFe₃B₂ stoichiometry) alloy presents the following solidification sequence: L → UFeB₄ → UFe₃B₂ → UFe₄B + UFe₂ → γ-Fe + UFe₄B + UFe₂ (Figure 4(f)). Solidification started with UFeB₄, indicating that the alloy is situated inside the primary crystallization field of this compound. According to the transition temperatures (Figure 7(b)), the solidification of UFeB₄ occurred at 1538 K (1265 °C) (above l₁₅); the solidification of UFe₃B₂ (below l₁₅) occurred at 1481 K (1208 °C). The solidification ended in the γ-Fe + UFe₄B + UFe₂ ternary eutectic, which occurs at 1263 K (990 °C). The fact that l₁₅ diverges from P₃ and converges to U₅ implies that P₃ occurs at T > 1538 K (1265 °C) and U₅ occurs at T < 1538 K (1265 °C). Since l₁ is diverging from P₁[7] and converging to U₃, the latter reaction must occur at T < 1933 K (1660 °C). Additionally, since l₁₇ is diverging from the binary phase diagram[14,15] where it operates at 1662 K (1389 °C) and is converging to U₄, the latter reaction must occur at T < 1662 K (1389 °C).

In agreement with the liquidus surface proposed, the solidification path followed by the 17B:73Fe:10U alloy presented the following solidification sequence: L → Fe₂B → UFe₃B₂ → UFe₂ + UFe₄B → γ-Fe + UFe₄B → γ-Fe + UFe₂ + UFe₄B (Figure 5(a)). Therefore, according to the DTA results, solidification started at 1468 K (1195 °C) with the formation of Fe₂B and this indicates that the alloy is situated inside the primary crystallization field of this compound. Solidification of UFe₃B₂ (along or below l₁₉) occurred at 1443 K (1170 °C), solidification of the UFe₂ + UFe₄B eutectic mixture (along l₂₂) occurred at 1323 K (1050 °C), and solidification of γ-Fe + UFe₄B (along l₂₇) occurred at 1303 K (1030 °C). The fact that U₂Fe₂₁B₆ phase is not present in the microstructure suggests that nucleation of this phase was hindered by kinetic reasons. The fact that l₂₇ diverges from U₈ enables the inference that this ternary reaction occurs at T > 1303 K (1030 °C). Since l₁₉ converges to P₄, this ternary reaction occurs at T < 1443 K (1170 °C).

The microstructure of the 17B:66Fe:17U (UFe₄B stoichiometry) alloy evidences the following solidification sequence: L → UFe₃B₂ → UFe₄B → UFe₂ → UFe₄B + UFe₂ → γ-Fe + UFe₂ + UFe₄B (Figure 5(b)). Therefore, solidification initiated at 1373 K (1100 °C) with the formation of UFe₃B₂, indicating that the alloy is situated inside the primary crystallization field of this compound. The solidification of UFe₄B (below l₂₀) together with the solidification of the UFe₂ + UFe₄B eutectic mixture (along l₂₂) occurred at 1293 K (1020 °C). The solidification ended in E₃ with the γ-Fe + UFe₂ + UFe₄B ternary eutectic mixture at 1253 K (980 °C). Since l₂₀ is converging to, and l₂₂ is diverging from, U₆, this ternary reaction occurs in the 1293 K (1020 °C) < T < 1373 K (1100 °C) temperature range. Consequently the lower temperature P₅ occurs at T < 1373 K (1100 °C).

The microstructure of the 9B:87Fe:4U alloy presents the following solidification sequence: L → γ-Fe → U₂Fe₂₁B₆ → γ-Fe + UFe₄B → γ-Fe + UFe₂ + UFe₄B (Figure 5(e)). The primary solidification of γ-Fe occurred at 1438 K (1165 °C), the solidification of γ-Fe and U₂Fe₂₁B₆ (along l₂₆) occurred at 1323 K (1050 °C), and solidification of γ-Fe + UFe₄B eutectic mixture (along l₂₇) occurred at 1303 K (1030 °C). The solidification ended at 1258 K (985 °C) in E₃ with the γ-Fe + UFe₂ + UFe₄B ternary eutectic mixture. The fact that l₂₆ diverges from U₇ and converges to U₈ implies that U₇ occurs at T > 1323 K (1050 °C) and that U₈ occurs at T < 1323 K (1050 °C). On the other hand, l₂₇ also diverges from U₈, which enables the inference that this ternary reaction occurs at T > 1303 K (1030 °C). The configuration of U₇ implies the convergence of l₂₅, which diverges from the binary phase diagram at T = 1447 K (1174 °C)[14,15] without crossing the respective Alkemade line. This points to a monotonic temperature decrease in l₂₅ and as a result, U₇ occurs at T < 1447 K (1174 °C).