Coarsening kinetics of γ′ precipitates in a Re-containing Ni-based single crystal superalloy during long-term aging

The morphological evolution and coarsening kinetics of γ′ precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900, 950 and 1000 °C. After heat treatment, well-defined cuboidal γ′ precipitates with low misfit was obtained within the experimental alloy. Then coarsening rate constants and particle size distribution (PSD) of γ′ phases were calculated and specified based on the measured precipitate sizes for varying periods of aging times from 100 to 2000 h. After aging for 2000 h, γ′ precipitates maintained cubical shape at 900 °C, while exhibited sphere at 950 and 1000 °C. Coarsening models based on diffusion-controlled process with a functional relationship of r³ vs. t (classic Lifshitz-Slyozov-Wagner coarsening model) and interface-controlled model with a function of r² vs. t (trans-interface diffusion-controlled coarsening model) were investigated to fit between the experimental results and theoretical analysis. It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model, while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through the γ/γ′ interface. The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models. Finally, coarsening mechanism could be divided into four regimes: (i) coarsening by diffusion-controlled; (ii) coarsening by diffusion and interface co-controlled; (iii) coarsening by interface-controlled; (iv) coarsening by interface-controlled accompanied with γ′ coalescence.