ELSEVIER Journal of Alloys and Compounds 221 (1995) 157-160 Journal of /~LOYS AND C@MPOJb~S The phase diagram of the U-Sn system A. Palenzona, P. Manfrinetti lstituto di Chimica Fisica, University of Genova, Corso Europa, Palazzo delle Scienze, 1-16132 Genova, Italy Received 16 September 1994 Abstract The phase diagram of the U-Sn system has been investigated using differential thermal analysis, metallographic analysis, X-ray diffraction and electron microscopy. The U-rich side of the system (0-15 at.%Sn) has not been studied owing to the high reactivity and contamination of the samples by the container material (Mo). Five intermediate phases have been found, four of which not previously known: UsSn4 (TisGa4-type), USn (Thin-type), USn2 (ZrGa2-type), U3Sn7 (CeaSnT-type) and USn3 (AuCu3-type). The main feature of the U-Sn phase diagram is that the relevant temperatures are confined in a narrow range of about 50* (1340-1390 °C) giving rise to some difficulties in determining the melting behaviour of the compounds and in drawing the shape of the diagram. Another singular behaviour has been observed for the mean atomic volumes along the whole system. Keywords: Uranium alloys;Tin alloys;Crystal structure; Phase diagrams 1. Introduction Further to the study of tin alloys with Sc, Y, rare earths (R), Th and U, after the determination of the Sc-Sn [1] and Th-Sn [2] phase diagrams and the investigation of the tin-rich side of the heavy R systems [3] we have moved our attention to the U-Sn system. A recent assessment of this phase diagram [4] revealed the existence in the current literature of a lot of information acquired using different experimental tech- niques, namely differential thermal analysis, X-ray dif- fraction, metallographic analysis, electron microprobe, microhardness and vapour pressure measurements. From the above data the existence of about eight intermediate compounds has been proposed but only USn 3 (AuCu3-type) was completely characterized. This last compound has been extensively studied [5], showing singular electronic properties. Therefore, in view of a systematic examination of the physical prop- erties of all the U-Sn compounds, a complete deter- mination of the phase diagram to identify the present phases was undertaken and we give here the results obtained. 2. Experimental details Elemental, depleted uranium (purity 99.8 wt.%) and tin (purity 99.99 wt.%) were commercial products from Koch-Light Laboratories, UK. Samples of mass about 1.0 g each were prepared by melting weighed amounts of the two elements, in the form of fine turnings pressed into small pellets, in a semilevitation high frequency induction furnace. The samples were then transferred to the differential thermal analysis equipment and were cycled at heating and cooling rate of 10 or 20° min -1. The temperature measurements were accurate to + 5°. Difficulties were encountered in the range 0-15 at.%Sn owing to the high reactivity of the corresponding alloy and to the contamination of the samples by the container material (Mo), therefore this side of the diagram has not been investigated. The central part of the system also (35-75 at.%Sn), where the melting temperatures rise to 1400 °C, showed some contamination problems by the con- tainer so only one thermal cycle was performed and the data used in drawing the diagram were mostly recorded on heating or, sometimes, averaged between heating and cooling. For compositions richer in uranium than USn3, the alloys are pyrophoric and must be handled carefully under a protective atmosphere. X-ray investigation were carried out on powders using a Debye or a Guinier-Stoe camera with silicon as internal standard: a =5.4308/~; the powder intensities were compared with those calculated using the LAZY-PULVERIX program [6]. Metallographic examination was carried out under protective Na-dried paraffin oil using standard teeh- 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)01463-9