Insights into the effects of Re and Ta on TCP phase evolution in nickel-based SX superalloys: Experimental and simulation analysis

Abstract

This study explores the effects of Re and Ta interactions on TCP (topologically close-packed) phase precipitation in experimental alloys under long-term thermal exposure. TCP phase is thoroughly examined and analyzed in terms of its location, morphology evolution, structural identification, and phase transition, employing thermodynamic calculations and first-principles analysis. The main findings indicate that the standard heat treatment used in the experiment does not fully eliminate microstructure segregation. Particularly, W and Re exhibit significant segregation in the dendrite region, resulting in preferential TCP phase formation in this region. Re induces destabilization of the γ-matrix and greatly enhances TCP phase precipitation, whereas Ta enhances the supersaturation of the γ-matrix. The interaction between Ta and Re further enhances the precipitation of the TCP phase. Thermodynamic calculations and first-principles analysis indicate that Re significantly improves the driving force of TCP phase precipitation from the γ-matrix, whereas the effect of Ta is found to be slight. Under thermal exposure at 900 °C, the σ-phase is firstly observed in the 8Ta2Re alloy, exhibiting a long needle-like shape, followed by subsequent evolutions resulting in the transition to the P-phase. The phase transition from the σ-phase to the P-phase is governed by the diffusion of elements, with Ni playing a crucial role in continuous diffusion.