New insights into the nucleation and growth of topologically close-packed phases in superalloys

Abstract

Topologically close-packed phases (TCPs) are key limitations of the development of new generations of superalloys needed for high-efficiency, low-CO₂-emitting advanced gas turbines. Their nucleation and growth remain puzzles in the two-phase microstructure (γ/γ′) of superalloys. Here, we find that the TCP precipitation is not a simple thermal activation process instead being significantly affected by plastic activity in superalloys. Atomic-resolved analysis via scanning transmission electron microscopy and atomic probe tomography reveals a diffusion highway across the γ/γ′ microstructure with the shear of superlattice stacking faults, leading to discontinuous nucleation and scatter-gather-merge growth of σ phase. Such pathway is energetically favorable according to first-principles calculations compared to ordinary case without fault, to induce large growth momentum of TCPs in a “straight-across” pattern. Our findings can be generalized to other TCPs, such as μ and P, which are usually reaction products of the σ phase, generating new insights into the nucleation and growth of TCPs in superalloys.