Coarsening transitional kinetics of γ′ precipitates in a nickel-based single crystal superalloy during thermal exposure

Abstract

The microstructural stability and coarsening kinetics of γ′ precipitates in a nickel-based single crystal superalloy (Ni-SX) were thoroughly investigated under various thermal exposure treatments. The evolutions of characteristic parameters including γ′ size, morphology, area fraction, and γ channel width were documented. Coarsening rate constants and particle size distributions (PSDs) were calculated and recorded based on measured precipitate sizes at temperatures ranging from 800 ℃ to 1100 ℃ and durations ranging from 100 h to 1010 h. The experimentally derived rate constants were compared with the calculated theoretical values from two mainstream models: matrix-controlled diffusion (LSW model) and interface-controlled diffusion (TIDC model). Notably, transitional coarsening kinetics of γ′ precipitates from the LSW model to the TIDC model was observed at high temperatures over time. Through a comprehensive consideration of key coarsening kinetics parameters such as elemental diffusivity, trans-interface diffusion coefficient, coarsening rate constant, γ/γ′ interfacial area, and solute mass transfer, the transformation of the coarsening kinetics from matrix-controlled diffusion to interface-controlled diffusion was quantitatively discussed. The emergence of the TIDC model as the dominant mechanism was attributed to relatively smaller solute mass transfer through the interface. Moreover, the degradation of γ/γ′ interfacial energy and interfacial area, coupled with the increase in solute diffusion coefficient in the matrix, facilitated the trans-interface diffusion to act as a limiting factor throughout the coarsening process.