Bulk and Surface Properties of Liquid Cr-Nb-Re Alloys

In order to replace Ni-based superalloys in view of high technological applications, new perspectives are open for advanced Nb-based alloys. The addition of refractory metals such as rhenium and tungsten make these alloy systems particularly suitable for coatings. Indeed, Re based diffusion barrier on Nb-based alloys can significantly improve the oxidation resistance and mechanical properties of the coated layer. To this purpose, in the present work the Cr-Nb-Re system has been studied. Direct measurements of bulk and surface properties of both solid and, in particular, liquid alloys at elevated temperatures are often technically difficult or even impossible, and therefore theoretical models can be used to estimate missing property values. Preliminary inspection of the literature related to the Cr-Nb-Re system and its binary subsystems reveals mainly theoretical results obtained in the framework of different theories, but the corresponding experimental data are still sparse. The energetics of mixing in liquid Cr-Nb, Cr-Re and Nb-Re systems has been analysed through the study of the concentration dependence of various thermodynamic, surface (surface tension and surface composition) and structural properties (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the framework of statistical mechanical theory in conjunction with the Quasi-Lattice Theory (QLT). The results obtained for these binary systems have been extended to study the thermodynamics and surface properties of ternary Cr-Nb-Re liquid alloys.