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Abstract

Despite the great advances in DFT calculations it is still too time consuming to apply these methods to complex materials as Ni-base superalloys. However, coupling DFT calculations and thermodynamic modelling using the CALPHAD approach can overcome these drawbacks and represent a powerful tool to investigate real systems.
As a first example of the possible applications of this approach, an improved modelling of sigma phase in several systems will be presented. The precipitation of TCP (Topologically Close- Packed) phases as sigma phase in Ni-base superalloys and in the interface between the protective coating layer and the superalloy, can significantly degrade material properties, especially creep and oxidation resistance. In order to prevent the precipitation of sigma phase, computer simulations are helpful and knowledge of phase diagrams and thermodynamic properties is essential. To this purpose, DFT calculations have been carried out and site fractions in sigma phase were calculated and compared with experimental data. Using the CALPHAD approach and DFT results, the phase diagrams have then been assessed and an improved thermodynamic description has been obtained.
As a second case of coupling DFT calculations and the CALPHAD approach, some systems of interest as hydrogen storage materials will be considered. For example, the Al-Mg-H system has been thermodynamically assessed including the magnesium alanate Mg(AlH₄)₂ which has a theoretical hydrogen content of 9.3 wt%. Key ab initio calculations have been carried out when thermodynamic data were scarce. Another interesting system the La-Ni-H will be presented and discussed, where the LaNi5 compound is well know hydrogen storage properties. Calculated and experimental thermodynamic properties have then been compared and a satisfactory agreement has been achieved.