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

Abstract

The liquidus projection at the boron-rich corner of the B-Fe-U phase diagram is proposed based on powder X-ray diffraction measurements, heating curves, and scanning electron microscopy observations, complemented with both energy dispersive X-ray spectroscopy and electron probe microanalysis. Evidence for six ternary reactions is presented, the corresponding 12 monovariant lines are drawn, and the nature and location of the ternary reactions are given. The ternary compounds existing in this region of the B-Fe-U ternary phase diagram, UFeB₄ and UFe₂B₆, were confirmed to be formed by ternary peritectic reactions, yet UFeB₄ has a considerably larger primary crystallization field, which points to an easier preparation of single crystals of this compound, when compared with UFe₂B₆.

Appendix

1.1 Solidification Paths

Figure 7 shows the solidification path of three alloys: 66B:17Fe:17U (UFeB₄ stoichiometry), 67B:22Fe:11U (UFe₂B₆ stoichiometry) and 77B:8Fe:15U. In agreement with the liquidus surface proposed, the solidification path followed by the 66B:17Fe:17U (UFeB₄ stoichiometry) alloy can be described as: L → UB₄ → UFeB₄ → FeB (Figure 2(d)). The primary solidification of UB₄ implies that this alloy is situated inside the UB₄ crystallization field, which is delimited by the l₁ and l₂ monovariant lines. According to the detected transition temperatures (Figure 4(a)), solidification of UB₄ (above l₂) occurred at ~1973 K (~1700 °C), and formation of UFeB₄ (below l₂) occurred at ~1973 K (~1660 °C). This suggests that the two transitions are proximal. The fact that l₂ is diverging from P₁ enables us to infer that this ternary peritectic occurs at T > 1973 K (1660 °C).

Fig. 7

Solidification path for alloys 77B:8Fe:15U, 67B:22Fe:11U (UFe₂B₆ stoichiometry), 66B:17Fe:17U (UFeB₄ stoichiometry) alloys. Dashed lines represent the liquid composition at the monovariant lines

The solidification path of the 67B:22Fe:11U alloy (UFe₂B₆ stoichiometry) was: L → UB₄ → UFeB₄ → UFe₂B₆ → UFe₂B₆ + UB₁₂ + FeB → UB₁₂ + FeB + B (Figures 2(b) and (c)). The primary solidification of UB₄ indicates that the alloy is situated inside the crystallization field of this compound. According to the transition temperatures (Figure 4(c)), solidification of UB₄ (above l₂) occurred at ~1973 K (~1700 °C); formation of UFeB₄ (below l₂) occurred at ~1833 K (~1560 °C); formation of UFe₂B₆ (below l₆) occurred at ~1743 K (~1470 °C). This implies that E₁ occurs at T < 1743 K (1470 °C). Solidification ended in the ternary E₂ eutectic at ~1683 K (~1410 °C) because of the proximity of the E₁ and E₂ reactions and possible local composition variations. Since l₂ converges to P₂ this ternary reaction occurs at T ~ 1833 K (~1560 °C). The fact that the solidification path of the alloy 67B:22Fe:11U crosses both l₂ and l₆ is consistent with their peritectic nature and indicates that P₂ is situated to the left of this alloy.

According to liquidus surface proposed, the solidification path followed by the 77B:8Fe:15U alloy was L → UB₄ → UB₄ + UB₁₂ → UB₁₂ + UFe₂B₆ → UFe₂B₆ + FeB (Figure 2(a)). According to the results presented in Figure 4(b), UB₄ solidified at T > 2273 K (2000 °C); UB₄ + UB₁₂ (l₄) formed at ~2003 K (~1730 °C); UFe₂B₆ + UB₁₂ (U₁ and l₇) solidified at ~1833 K (~1560 °C); and UFe₂B₆ + UB₁₂ + FeB (E₁) formed at ~1743 K (~1470 °C). Because l₆ diverges from P₂ and converge to U₂, it implies that the later ternary peritectic occurs at T < 1833 K (1560 °C).