Coarsening and dissolution of γ′ precipitates during solution treatment of AD730™ Ni-based superalloy: Mechanisms and kinetics models
Résumé
The kinetics of γ′ size and volume fraction changes during solution treatment of the advanced Ni-based superalloy, AD730 are determined and the underlying mechanisms are investigated. High resolution Differential Thermal Analysis (DTA) and thermodynamic modeling were used to design and perform solution heat treatment experiments. Semi-analytical models are developed to describe the dissolution and coarsening processes. The results from the proposed models, supported by electron microscopy observations, indicate that coarsening occurs before complete dissolution takes place. Agglomeration is shown to be the governing coarsening mechanism for this alloy after calculation of the coefficients for both the Ostwald ripening and agglomeration mechanisms. Electron microscopy observations revealed that the early stages of agglomeration occur by neck formation between two neighboring particles. The splitting of γ′ particles was identified as one of the main dissolution mechanisms. Based on the obtained results, a dissolution kinetics model is proposed to quantify the volume fraction of dissolved γ′ particles and estimate the activation energy of this process for AD730. A coarsening model based on the time-temperature dependence of the γ′ coarsening rate coefficient is also proposed taking the concentration of elements, γ′ volume fraction and the temperature into consideration. Based on this model, a method is developed to predict γ′ size evolution during aging heat treatment process and an optimum heat treatment to reach the desired γ′ distribution is proposed. The validity and accuracy of the proposed models were verified by carrying out different heat treatment experiments.