Dissolution and precipitation kinetics of γ' in nickel base superalloy Udimet 520

Abstract

The influence of various solution andaging heat treatments on the dissolution and precipitation kinetics of γ' in wrought nickel base Udimet 520 superalloy was investigated, with the aim of determining optimum preforge and post-forge heat treatment schedules. Solution heat treatments at 1070, 1100, 1110, and 1120°C for 1, 2, and 4 h followed by water quenching were applied. Examination by SEM showed that in specimens solution treated at 1070°C the necklace type microstructure remained and partialagglomeration of γ' precipitates was observed even after 4 h holding. By contrast, after solutionising at 1100°C for 4 h, the γ' volume fraction decreased from 28 to 5% and when solution treated at 1140°C, abnormal grain growth was observed. The optimum solution treatment leading to an almost complete dissolution of γ' particles was determined to be 1120°C/4 h. Double aging treatments were carried out at 925°C/4 h/ AC (air cooled) followed by aging at 700 or 800°C for times ranging from 1 to 100 h. The results indicated that the precipitates have almost spherical shapes at the beginning and as the aging time is increased a partial change to a cuboidal shape is observed. This behaviour was more pronounced when aging at 800°C was employed. Microhardness measurements under both aging conditions showed that at 700°C hardness values increase about 60 HV when γ' particles grow 70 nm. On the other hand, in the samples aged at 800°C, hardness values increase with the particle size until a certain value and then drop. The precipitate growth process was followed by image analysis and the activation energy for γ' growth was evaluated to be 104 kJ mol ^-1. The results obtained are discussed in the framework of classic nucleation theory and compared with other workers' results.