Abstract
Element clustering and structural features of liquid lead-bismuth eutectic (LBE) alloy have been investigated up to 720 °C by means of high temperature X-ray diffraction (HT-XRD), X-ray Photoemission Spectroscopy (XPS) and Scanning Photoemission Microscopy (SPEM) at the Elettra synchrotron in Trieste. The short-range order in liquid metal after melting corresponds to the cuboctahedral atomic arrangement and progressively evolves towards the icosahedral one as temperature increases. Such process, that involve a negative expansion of the alloy, is mainly connected to the reduction of atom distance in Pb–Pb pairs which takes place from 350 °C to 520 °C. On an atomic scale, it is observed a change of the relative number of Bi–Bi, Pb–Pb, and Pb–Bi pairs. The Pb–Bi pairs are detected only at a temperature above ~350 °C, and its fraction progressively increases, giving rise to a more homogeneous distribution of the elements. SPEM results showed evidence that the process of chemical homogenization on an atomic scale is preceded and accompanied by homogenization on micro-scale. Clusters rich of Bi and Pb, which are observed after melting, progressively dissolve as temperature increases: Only a few residuals remain at 350 °C, and no more clusters are detected a 520 °C.
Highlights
Many physical properties of liquid metals depend on their structure, its knowledge is of fundamental importance
The samples used for X-ray Photoemission Spectroscopy (XPS) and Scanning Photoemission Microscopy (SPEM) measurements were prepared through rapid quenching of the liquid from different temperatures
The short-range order in liquid metal corresponds to a cuboctahedral arrangement of atoms that progressively evolves towards an icosahedral one as temperature increases. This structural transformation involves a negative expansion of the interatomic distances
Summary
Many physical properties of liquid metals depend on their structure, its knowledge is of fundamental importance. The following issues are of particular interest for scientific knowledge and metallurgical applications: (i) The structural correlations between solid and liquid during the solidification, (ii) the phase de-mixing in metal systems with a miscibility gap in the liquid state, (iii) the liquid–liquid phase transitions in pure metals and alloys. Present work investigates the behavior of the liquid Pb–Bi eutectic (LBE) alloy, a good candidate as coolant and neutron spallation source in MYRRHA reactor [37] For this application more information on atomic-scale interactions, structure and thermal physical properties, as a function of the temperature must be collected, because one of the main limitations of this reactor is the compatibility of structural materials with liquid LBE at high temperature, where corrosion and embrittlement take place. XPS and scanning photoemission microscopy (SPEM) at the Elettra synchrotron (Trieste, Italy) with high lateral resolution [41,42] were used to investigate possible clustering of alloying elements
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