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TRODUCTION

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.[2] Unusual magnetic properties can also be expected for compounds of the B-Fe-U system, where, due to its high electronegativity, the interstitial B atoms can decrease the hybridization between the 5f and 3d orbitals of the U atoms.[3] However, due the large range of solidification temperatures and the numerous peritectic reactions present in the B-Fe-U system,[4] most attempts to prepare ternary compounds as single-phase materials have not been successful, which impaired their complete physical characterization. A detailed knowledge of the B-Fe-U ternary phase diagram, and particularly of the liquidus projection, is therefore essential for a comprehensive understanding of its ternary borides solidification behavior. Early results on the B-Fe-U ternary diagram were reported by Valyovka and Kuzma,[5] who have identified the UFeB4 and UFe3B2 ternary compounds and proposed (incomplete) compatibility triangles for the

MARTA DIAS, Postdoctoral Position, is with the Associac¸a˜o Euratom/IST, Instituto de Plasmas e Fusa˜o Nuclear, Instituto Superior Te´cnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal. Contact e-mail: [email protected] PATRI´CIA ALMEIDA CARVALHO, Researcher, is with the ICEMS, Departamento de Engenharia Mecaˆnica, Instituto Superior Te´cnico, Universidade de Lisboa. UMESH VINAICA MARDOLCAR, Researcher, is with the Departamento de Fı´ sica, Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal, and also with the Centro de Cieˆncia Moleculares e Materiais, Faculdade de Cieˆncias da Universidade de Lisboa, Campo Grande, 1749-016 Lisbon, Portugal. OLIVIER TOUGAIT and HENRI NOE¨L, Researchers, are with the Institut des Sciences Chimiques de Rennes, UMR CNRS 6226, Universite´ de Rennes 1, Avenue de Ge´ne´ral Leclerc, 35042 Rennes, France. ANTO´NIO PEREIRA GONC¸ALVES, Researcher, is with the Campus Tecnolo´gico e Nuclear, IST/ITN, Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa, Estrada Nacional 10 ao km 139,7, 2695-066 Bobadela LRS, Portugal. Manuscript submitted May 29, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A

isothermal section at 1073 K (800 C).[5,6] Recently, the study of the isothermal section at 1223 K (950 C)[7] and systematic investigations on the B- and Fe-rich corners of the diagram[8,9] showed the existence of three new compounds, UFe4B,[7] U2Fe21B6,[7] and UFe2B6,[10] in addition to those initially reported. The B-rich corner presents 12 monovariant lines and 6 invariant reactions,[8] while the Fe-rich corner[9] exhibits additional 18 monovariant lines and 10 invariant reactions. However, the U-rich corner remained unexplored. In the case of compounds with high melting temperature such as borides, and especially when heavy atoms like U are involved, general equilibrium conditions are difficult to attain in solidified microstructures (even at moderate cooling rates or with