Development of bimodal size distribution of γ’ precipitates in Allvac 718Plus Ni-based superalloy

Alloy Allvac 718Plus is a relatively new superalloy developed to improve the properties of the widely-used superalloy Inconel 718. The major strengthening phase of the alloy is γ’. The size, morphology, and distribution of the γ’ phase dominantly decides the strength of the alloy. The current work aimed to develop a bimodal size distribution of γ’ precipitates. Initially, the samples were solution treated at 1000ºC for 1 hr followed by water quenching (WQ). Then, various single-step aging treatments were conducted to develop unimodal γ’ precipitate size distributions. Then, based on the results obtained from the single-step aging treatments, two-step aging treatments were developed to produce the desired bimodal distribution. All the aging temperatures lied between 720-920℃. Different cooling methods were incorporated, such as WQ, air cooling, and furnace cooling. The two-step aging treatment of 900 ºC for 2 hr followed by WQ, then re-heating to 720℃ for 10 hr followed by WQ resulted in a uniform bimodal distribution with 11 and 55 nm average sized γ’ precipitates. A detailed microstructural analysis was performed using SEM and TEM to identify and measure the γ’ precipitates. A separate work will focus on understanding particle-dislocation interactions for the bimodal microstructure using tensile deformations.