Structural Stability of Topologically Close-packed Phases: Understanding Experimental Trends in Terms of the Electronic Structure

Abstract

Topologically close-packed (TCP) phases in single crystal Ni-based superalloys have a detrimental effect on the mechanical properties. In order to gain a microscopic understanding of the factors that control TCP phase stability, we carry out atomistic calculations based on the electronic structure. In particular, we use a hierarchy of methods that treat the electronic structure at different levels of coarse-graining, i.e. at different levels of computational cost and accuracy.The applied levels of approximation range from density functional theory (DFT) to tight-binding (TB) to bond-order potentials (BOPs). This hierarchy of electronic structure methods allows us to interpret the findings of a recently derived structure map of experimentally observed TCP stability. The TB and BOP calculations are compared to extensive high-throughput DFT calculations for the TCP phases A15, C14, C15, C36, µ, σ, and χ of transition-metal elements.These findings are extended to binary systems based on DFT heat-of-formations for TCP phases in the systems V/Nb-Ta, Nb/Mo-Ru, V/Cr/Nb/Mo-Re, V/Cr/Nb/MoCo. By pairwise comparisons of selected systems, we illustrate the interplay of the difference in average valenceelectron concentration N and the composition-dependent relative volume difference ∆V /V . Such an approach could be useful to predict the change of expected TCP phase stability due to changes of the composition for a given multi-component alloy.

Microsegregation and precipitates of an as-cast Co-based superalloy—microstructural characterization and phase stability modelling

“…The influence of N on the relative structural stability of TCP phases is well known from the calculations with density functional theory (DFT) [18,19] and approximate electronic structure methods [20][21][22][23]. The interplay of N and DV=V was discussed in DFT calculations for various binary systems [22][23][24][25].…”

BOPfox program for tight-binding and analytic bond-order potential calculations

“…Here we describe our implementation of analytic BOPs in the software package BOPfox 10 . BOPfox has already been used in several publications 8,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] and is being continuously extended and optimised. We point out similarities of TB/BOP calculations and computations carried out using other electronic structure methods, and discuss the peculiarities of analytic BOPs in detail.…”