Liquidus Projection of the B-Fe-U Diagram: The Boron-Rich Corner
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COMPOUNDS with atypical magnetic structures have been previously identified in Fe-U-X systems with X elements from the III or the IV group, e.g., Al[1] or Sn, respectively.[2] Unusual magnetic properties can also be expected in compounds from the B-Fe-U system, where B can act as an interstitial element with high electronegativity and decrease the hybridization between the 5f and 3d orbitals. However, data on this system are scarce, and further investigation is required. Results on the B-Fe-U ternary diagram were previously reported by Valyovka et al.,[3,4] who identified the UFeB4 and UFe3B2 borides. Recent systematic studies on the isothermal section at 1223 K (950 C) revealed the existence of three other ternary compounds: (i) UFe4B, with a hexagonal structure closely related to the CeCo4B-type (a = 0.4932(1) and c = 0.7037(2) nm[5]); (ii) U2Fe21B6, with a cubic Cr23C6-type structure (a = 1.0766(4) nm[5]); and (iii) UFe2B6 with a CeCr2B6-type structure (a = 0.3137(6), b = 0.6181(11), and c = 0.8225(17) nm[6]). However, attempts to prepare these ternary compounds as singlephase materials have not been successful, impairing their MARTA DIAS, Post-Doc, is with Instituto Tecnolo´gico e Nuclear, Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa, Estrada Nacional 10, 2686-953 Sacave´m, Portugal, and also with ICEMS, Departamento de Bioengenharia, Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal. Contact e-mail: marta.dias @itn.pt PATRI´CIA ALMEIDA CARVALHO, Researcher, is with ICEMS, Departamento de Bioengenharia, Instituto Superior Te´cnico. MARCEL BOHN, Researcher, is with Departement DRO/ Geosciences Marines, Ifremer Centre de Brest, B.P. 70, 29280 Plouzane, France. OLIVIER TOUGAIT and HENRI NOE¨L, Researchers, are with Institut des Sciences Chimiques de Rennes, Chimie du Solide et Mate´riaux, UMR CNRS 6226, Universite´ de Rennes 1, Avenue de Ge´ne´ral Leclerc, 35042 Rennes, France. ANTO´NIO PEREIRA GONC¸ALVES, Researcher, is with Instituto Tecnolo´gico e Nuclear, Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa. Manuscript submitted February 24, 2011. Article published online September 1, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
complete physical characterization. A detailed knowledge of the B-Fe-U ternary phase diagram is essential for a comprehensive understanding of the alloys solidification behavior, which in turn is required for an optimized selection of the single-crystal growth methods suitable for the synthesis of each boride. In the case of compounds with high melting temperature such as borides, and especially when heavy atoms such as U are involved, general equilibrium conditions are difficult to attain during solidification (even at moderate rates or in subsequent annealing treatments). However, in these conditions the solid-liquid interface can be assumed to behave locally as if it were in a state of equilibrium, as long as the reaction rates at the interface are fast compared with the rate of interface advance.[7–9] In other words, as long as
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