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

In the precipitation-hardened Ni-based superalloy,typified by ATI 718 Plus,the nano-scaleγandγphase in duplet or triple coprecipitate morphology can provide superior high-temperature strength.Thus,it is of great sense to study the evolution ofγ’/γ’’coprecipitate during long term service at elevated temperature.In this study,the new-typeγ’/γ’’coprecipitates with a sandwich or compact configuration were found firstly in wrought ATI 718 Plus superalloy during long term thermal exposure at 705℃.These co-structure of theγ’/γ’’precipitates evidently inhibit the coarsening ofγ’phase.The increase of thermal exposure time evidently leads to the increase of the volume fraction ofγ’/γ’’coprecipitate and transformation of sandwich-typeγ’/γ’’coprecipitate to compact-typeγ’/γ’’coprecipitate,which is characterized asγphase precipitate at several faces of theγphase.The main evolution mechanism ofγ’/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 betweenγ’’phase andγmatrix also plays a key role on theγ’/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 ofγ’’phase on the faces ofγ’phase.