Rhenium Alloys: Modelling, Intermetallics and Phase Diagrams – ARMIDE

This project consists in a systematic and fundamental study of transition metals-rhenium systems. The central place of rhenium in the periodic table provides this element a peculiar role in the constitution of transition metal element systems and in the formation of intermetallic compounds of these elements.

The rhenium-based systems are interesting for the development of applications connected to its use, in particular as an alloying element in Ni-based superalloys. They are characterized by the presence of Frank-Kasper (or topologically close packed) intermetallic compounds which present a high degree of non-stoichiometry, like the sigma and chi phases. The precipitation of these brittle phases represents an obstacle to the development of applications.

The Calphad thermodynamic modelling method allows the prevision of the equilibrium state in multi-element systems after modelling of the constituting simple systems. It allows to predict the precipitation of these intermetallic compounds. Their description benefits from recent contributions of ab initio calculations results such as the enthalpy of formation of compounds, from the use of the measured site fractions and from the proposal of new models.

Systems have been selected and will be studied in the frame of this project:
- Mn-Re and Fe-Re, because they are important systems that are not well known experimentally
- Cr-Mo-Nb-Re-Ta-W because extrapolations can easily be obtained from the only binary systems
- Mo-Re-Hf and Mo-Re-Zr, since they allow a fundamental study of the ternary chi phase
- Mo-Ni-Re that will be studied in details since it is a deciding system for Ni-based super-alloys
These systems will be studied by different techniques:
- experimental measurements of phase diagrams: alloys will be synthesized and characterized in the equilibrium state and/or by thermal analysis
- determination of crystalline structures: the atomic distribution on the different sites in non-stoichiometric phases will be obtained by the Rietveld method
- measurement of the thermodynamic properties: measurement of the enthalpies of formation will be performed by calorimetry
- ab initio calculations: the enthalpies of formation of ordered compounds at 0 K will be obtained, as well as the electronic structures allowing to understand bonding, phase stability and interactions between elements

The obtained data will be used in the frame of Calphad modelling. Models adapted to each phase will be developed and compared.

The aims of the project are the following:
- to obtain a complete description of transition metals-rhenium systems, a better knowledge of intermetallic compounds and a better understanding of the peculiar role of rhenium in the transition metal systems
- to understand the phenomena governing the site distribution of the elements in ternary sigma and chi phases
- to study completely a ternary system (Mo-Ni-Re) fundamental for applications of rhenium as alloying element in Ni-based superalloys
- develop a thermodynamic database on transition metal-rhenium systems