Abstract
Second phase particles (SPP) play an essential role in controlling grain size and properties of polycrystalline nickel base superalloys. The understanding of the behavior of these precipitates is of prime importance in predicting microstructure evolutions. The dissolution kinetics of the primary γ′ precipitates during subsolvus solution treatments were investigated for three nickel base superalloys (René 65, AD730 and N19). A temperature-time codependency equation was established to describe the evolution of primary γ′ precipitates of each material using experimental data, the Thermo-Calc software and the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. The dissolution kinetics of precipitates was also simulated using the level-set (LS) method and the former phenomenological model. The precipitates are represented using an additional LS function and a numerical treatment around grain boundaries in the vicinity of the precipitates is applied to reproduce their pinning pressure correctly. Thus, considering the actual precipitate dissolution, these simulations aim to predict grain size evolution in the transient and stable states. Furthermore, it is illustrated how a population of Prior Particle Boundaries (PPB) particles can be considered in the numerical framework in order to reproduce the grain size evolution in the powder metallurgy N19 superalloy. The proposed full-field strategy is validated and the obtained results are in good agreement with experimental data regarding the precipitates and grain size.
Highlights
In polycrystalline nickel base superalloys, the γ0 phase is found in several types of precipitates that can basically be distinguished based on their size and specific role in the microstructure
A pre-simulation was made in order to generate an initial numerical microstructure similar to the one observed experimentally; a short heat treatment of 400 s ≈ 6 min at 1070 ◦ C was simulated starting from an initial smaller mean grain size than the one measured in order to get the boundaries stopping at the Second phase particles (SPP)
A new approach using a mean-field model to describe the dissolution of second phase particles was introduced in a finite element framework using the Level-Set model to simulate grain growth for different γ-γ0 nickel base superalloys as primary γ0 precipitates dissolve
Summary
In polycrystalline nickel base superalloys, the γ0 phase is found in several types of precipitates that can basically be distinguished based on their size and specific role in the microstructure. The dissolution of primary γ0 precipitates in nickel base superalloys has mainly been studied experimentally [1,8,9,10,11,12] but not so much in full-field numerical works [13,14,15] at the polycrystalline scale Such simulations may help clarify, predict, and control microstructure evolution when precipitate dissolution occurs during heat treatments (HT), at subsolvus and near-solvus temperatures for each considered material. Both the phenomenological model for precipitate evolution and GB motion parameters are used in full-field simulations conducted within an LS framework. The last section summarizes the conclusions and opens perspectives for further works
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