Cascade of Peritectic Reactions in the B-Fe-U System

M. Dias 0 P.A. Carvalho 0 A.P. Dias 0 M. Bohn 0 N. Franco 0 O. Tougait 0 H. Noe l 0 A.P. Gonc alves 0 0 M. Dias, Departamento de Qumica, Instituto Tecnologico e Nuclear/ CFMC-UL , P-2686-953 Sacavem, Portugal and Departamento de Engenharia de Materiais, Instituto Superior Tecnico , Av Rovisco Pais, 1049-001 Lisboa, Portugal ; A.P. Goncalves, Departamento de Qumica, Instituto Tecnologico e Nuclear/CFMC-UL , P-2686-953 Sacavem, Portugal ; P.A. Carvalho, Departamento de Engenharia de Materiais, Instituto Superior Tecnico , Av Rovisco Pais, 1049-001 Lisboa, Portugal ; A.P. Dias, Departamento de Engenharia Qumica e Biologica, Instituto Superior Tecnico , Av. Rovisco Pais, 1049-001 Lisboa, Portugal ; M. Bohn, Departement DRO/Geosciences Marines, Ifremer Centre de Brest , B.P. 70-29280 Plouzane, France ; N. Franco, Departamento de Fsica, Instituto Tecnologico e Nuclear/CFN-UL , P-2686-953 Sacavem, Portugal ; O. Tougait and H. Noel, Laboratoire de Chimie du Solide et Inorganique Moleculaire , UMR CNRS 6511, Universite de Rennes 1 , Avenue de General Leclerc, 35042 Rennes, France . Contact The solidification paths for UFeB4, UFe3B2 and UFe4B, ternary compounds, situated along the U:(Fe,B) = 1:5 line in the B-Fe-U phase diagram, are proposed based on x-ray powder diffraction measurements, differential thermal analysis, heating curves and scanning electron microscopy observations complemented with energy and wavelength dispersive x-ray spectroscopies. The compounds melt incongruently and are formed by peritectic reactions. The present work demonstrates the existence of a cascade of peritectic reactions along the U:(Fe,B) = 1:5 composition line, establishes peritectic temperatures and proposes an isopleth diagram along this line. 1. Introduction Borides play an increasingly important role in present day engineering due to their high melting temperature as well as chemical and thermal stability. Moreover, ternary intermetallic borides of AMxBy type (with M a d-transition metal and A an actinide or rare earth) have attracted considerable interest due to a diversity of unusual physical characteristics,[1] which extend from permanent magnetism with unusually large magnetic coercive fields, like in SmCo4B[2] and SmNi4B,[3] to unconventional magnetic ordering, as seen for UNi4B.[4] Several compounds with atypical properties have been previously identified in U-Fe-X ternary systems (namely for X = Al[5] or Sn[6]), and interesting compounds can be also expected in the B-Fe-U system. However, data on this system is scarce and requires further investigation. Results on the B-Fe-U ternary diagram were previously reported by Valyovka and Kuzma,[7,8] who identified the UFeB4 and UFe3B2 compounds. Recent systematic studies on the isothermal section at 950 C revealed the existence of three other ternary compounds: (i) UFe4B, with a hexagonal structure closely related to the CeCo4B-type structure (a = 0.4932(1) nm and c = 0.7037(2) nm[9]); (ii) U2Fe21B6, with a cubic Cr23C6-type structure (a = 1.0766(4) nm[9]) and (iii) UFe2B6 with a CeCr2B6-type structure (a = 0.31401 nm, b = 0.61842 and c = 0.82218 nm[10]). The present study aims to analyze the solidification path and identify the formation reactions of ternary compounds with an atomic U:(Fe,B) ratio of 1:5, i.e., UFeB4, UFe3B2 and UFe4B. This knowledge is required to establish adequate processes for pure compounds synthesis, necessary in turn to their subsequent physical properties characterization. Powder (XRD) and high temperature (HTXRD) x-ray diffraction, scanning electron microscopy (SEM), complemented with energy and wavelength dispersive x-ray spectroscopies (respectively, EDS and WDS), differential thermal analysis (DTA) and heating curves obtained from an induction furnace (IF) have been used in this study. 2. Experimental Over 60 alloys with general xU:yFe:zB compositions were prepared by melting together the elements (purity > 99.9 at.%) in an arc furnace equipped with a cold crucible under an argon atmosphere. The surface of uranium pieces was deoxidized in diluted nitric acid prior to melting. In order to ensure homogeneity, the samples were melted at least three times before quenching to room temperature. No losses higher than 1 wt.% were observed. The high cooling rate of the solidification process enabled the solidification path of the alloys to be followed under non-equilibrium conditions. Subsequent heat treatments at 950 C allowed inferring the transitions leading to equilibrium. X-ray powder diffractograms of the as-cast samples were collected at room temperature with monochromatic Cu Ka radiation using an Inel CPS 120 diffractometer, equipped with a position-sensitive detector covering 120 in 2h with a resolution of 0.03 , and a Philips XPert diffractometer with a 2h-step size of 0.02 from 10 to 70 . The Powder Cell software package[11] was used to simulate diffractograms for comparison with experimental data. The microstructures were observed in secondary and backscattered electron modes (respectively, SE and BSE) on polished and etched surfaces using a JEOL JSM-7001F field emission gun scanning electron microscope equipped for EDS. This spectroscopy technique was primarily used for efficient x-ray map collection, whereas (quantitative) analysis was carried out with a Cameca SX100 electron microprobe micro-analyzer (EPMA) equipped with five wavelength dispersive spectrometers. In the present study, a multilayer Mo-B4C crystal with a large interplanar distance (2d = 210.36 nm) was used to detect boron, a lithium fluoride (LIF) crystal (2d = 4.03 nm) was used to detect uranium and a pentaerythrirol (PET) crystal (2d = 8.75 nm) was used to detect iron. The elements were analyzed simultaneously using BKa, UMb and FeKa transitions, for an acceleration voltage of 15 kV, a beam current of 20 nA, and using CeB6, UC and a-Fe as standards. Typical beam sizes were 100 nm and the interaction volume was 1 lm3. The X-phi correction software package was used to calculate the relative element proportions.[12] Quantitative analyses were performed in 13 representative alloys with nominal compositions close to or on the U:(Fe,B) = 1:5 line. Each phase was analyzed in more than 6 randomly selected points. DTA measurements were carried out for 6 alloys up to 1600 C, using a Setaram DTA Labsys and employing open alumina crucibles and a permanent argon flow. Sample masses of 60-120 mg were used on the experiments. The optimized heating and cooling rates for clear peak evidence vs acquisition efficiency were 5 and 10 C/min. The difference in temperature measured for the same transformations at different cooling rates indicated that the undercooling/overheating (DT) values were below 5 C. DTA curves were normalized for mass and the transition temperatures were determined from the derivative curves. Additionally, heating curves up to 2000 C were obtained at 10 W/min in an induction furnace (IF) coupled with an optical pyrometer. Due to t (...truncated)