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Modelling topologically close-packed phases in superalloys: Valence-dependent bond-order potentials based on ab-initio calculations
Refractory elements are used in Ni-based superalloys to increase creep resistance (Mo, Re, W) and to retard the coarsening of the γ’ phase (Re). At high concentrations of refractory elements precipitation of topologically close-packed (tcp) phases is detrimental to the creep properties of the alloys. A more detailed understanding of the formation kinetics and thermodynamic stability of tcp phases will therefore be beneficial for the design of the next generation superalloys. Atomistic modeling of tcp stability with interatomic potentials requires to go beyond the second-moment approximation to the electronic density of states by including up to at least the sixth moment. We have developed an analytic bond-order potential (BOP) that systematically takes into account higher moment contributions to the density of states and depends explicitly on the valence of the transition-metal elements. For the parameterization of the new BOP, we performed extensive density functional theory (DFT) calculations of elemental and binary compound phases of Ni, the technologically important alloying element Cr, and the refractory metals Mo, Re, and W. We will discuss the structural trends of the DFT calculations, compare to the predictions of the analytic BOP and report on our progress on the parameterization of the analytic BOP for the Re-W system. This work is part of the EPSRC-funded collaborative multi-scale project ’Alloys by Design’.