Thermodynamic and thermophysical properties of Mg-Pd alloys: case study of Mg6Pd alloy

A peculiar feature of the Mg-Pd phase diagram is the presence of several "Complex Metal Alloys (CMAs)" which are phases with giant unit cells and inherent structural disorder; their structure can be described with Mackay clusters as fundamental structural unit [1]. These polyhedra consist in an arrangement of 52 atoms disposed in three shells with local icosahedral order: the first is an icosahedron, the second an icosidodecaedron and the third another icosahedron. The assessment of the Mg-Pd phase diagram was published in 1988 [2] with many uncertainties and subsequently, considering the existence of the CMAs, was revised by [1]. At present a definitely assessed version of the Mg-Pd phase diagram is not available, and great differences can be found for instance in the formation of the Mg6Pd phase which is indicated to be a peritectic one by [2] and having a congruent melting by [1]. As for the standard enthalpies of formation (?fH°) there is a great lack in literature for the Mg-Pd phases; nowadays predictive methods are very developed, but it is not trivial to apply common models to giant cells like those of the CMAs. Based on the phase diagram evidence of a several intermetallic compounds, the mixing behaviour of liquid Mg-Pd alloys has been analysed through the study of thermodynamic, surface and structural properties in the framework of compound formation model (CFM) and quasi chemical approximation (QCA) for regular solution.