A new type-γ′/γ′′ coprecipitation behavior and its evolution mechanism in wrought Ni-based ATI 718Plus superalloy

Abstract

In the precipitation-hardened Ni-based superalloy, typified by ATI 718Plus, the nano-scale γ′ and γ′′ phase in duplet or triple coprecipitate morphology can provide superior high-temperature strength. Thus, it is of great sense to study the evolution of γ′/γ′′ coprecipitate during long term service at elevated temperature. In this study, the new-type γ′/γ′′ coprecipitates with a sandwich or compact configuration were found firstly in wrought ATI 718Plus superalloy during long term thermal exposure at 705 °C. These co-structure of the γ′/γ′′ precipitates evidently inhibit the coarsening of γ′ phase. The increase of thermal exposure time evidently leads to the increase of the volume fraction of γ′/γ′′ coprecipitate and transformation of sandwich-type γ′/γ′′ coprecipitate to compact-type γ′/γ′′ coprecipitate, which is characterized as γ′′ phase precipitate at several faces of the γ′ phase. The main evolution mechanism of γ′/γ′′ coprecipitates is element segregation, especially the composition variations of Al + Ti and Nb and their ratio of Al+Ti/Nb. In addition, the interfacial energy between γ′′ phase and γ matrix also plays a key role on the γ′/γ′′ coprecipitates evolution. The calculated results show that the longer thermal exposure time leads to the higher interfacial energy, which is beneficial for nucleation and precipitation of γ′′ phase on the faces of γ′ phase.