Superalloys are high-temperature application alloys combining high mechanical strength and high resistance to surface degradation. These alloys are widely used in gas turbines, power plants, and chemical process industries and for other specialised applications that require heat and corrosion resistance [1]. Applications at extreme temperatures close to the melting points require a stable two-phase microstructure, which is resistant to coarsening and eventual property degradation. The safe and reliable operation of the turbine is based on microstructural stability against ageing, which also implies low creep deformation at high temperatures and maintaining high strength at low temperatures. The coherent or semi-coherent interface of the 𝛾 and 𝛾′ phases enable these requirements. The formation of typical cuboidal 𝛾′ is attributed to the elastic misfit strain (i.e., elastic free energy), which is required to form a coherent interface of cubic lattices (fcc, L12) with different lattice constants [2]. Although, precipitate morphology and interface instabilities are affected by the anisotropy of the interface energy, most mesoscale simulations use isotropic interface models [3].

Using Molecular dynamics (MD) simulation, the complete interface energy anisotropy of coherent interfaces is determined for all possible orientations beyond the usual high symmetry direction. Hence, we assess the reliability and show the limitations of the established interpolation scheme for the interface energy, which are based on ab initio simulations of a set of high symmetry interface orientations. At the same time, we have analysed the structure of low symmetry interface directions and the emergence of a mesoscale coherent interface structure beyond the conventional broken-bond model [4]. From these insights, we have developed the structural model of coherent interfaces, which can describe the population of defects at the coherent interfaces based on simple crystallography of coherent interfaces. This structure model is the basis of an interface energy model, which allows rationalising local interface energy minima and maxima. We will discuss the similarities and differences with surface energy models of vicinal free surfaces of fcc metals [5] and the necessary ingredients for a free energy model for fcc-L12 coherent interfaces. We outline the strategy for incorporating the interface anisotropy into phase-field models for Ni-based and Co-based superalloys to study ageing utilising chemically accurate ab initio materials data of a minimal set of simulations as an input.

References 

[1] R. C. Reed; Cambridge University Press, 2006, 1-28

[2] G. Brunetti et al.; Micron, 2012, 43, 396-406

[3] A. Roy et al.; Philosophical Magazine, 2017, 97, 2705–2735

[4] B. Sonderegger et al.; Materials Science Forum, 2010, 638, 2730-2735

[5] D. Wolf et al.; Surface Science, 1992, 277, 301-322