Multi-length-scale study on the heat treatment response to supersaturated nickel-based superalloys: Precipitation reactions and incipient recrystallisation

Abstract

A supersaturated γ phase microstructure is produced in Ni-based superalloys using laser powder bed fusion (L-PBF) – the cooling rate arising from the process is shown to suppress the solid-state precipitation of the γ′ phase. The response of the material to a heat treatment therefore requires new understanding at the fundamental level, since the first population of γ′ precipitate forms upon heating, in contrast to cooling from homogenisation above the γ′ solvus. Here, we have interrogated two new nickel-based superalloys designed for the L-PBF technology, both in situ and ex situ, at multiple length scales using advanced characterisation methods. First, we conducted in situ synchrotron X-ray diffraction during various heat treatments to trace the evolution of the γ′ volume fraction with temperature. The first structural changes were detected at an unexpectedly low temperature of ∼445 °C. Second, the temperature for γ′ nucleation and its sensitivity to heating rate was studied using an electrical resistivity method. Then, the γ′ composition upon heating, isothermal holding and cooling is analysed using atom probe tomography (APT), the result is rationalised by further scanning-transmission electron microscopy and nanoscale secondary ion mass spectroscopy. Finally, static recrystallisation during isothermal exposure was investigated, which occurs within minutes. This work sheds light on a new strategy of tailoring microstructure for additively manufactured superalloys by manipulation of the γ′ precipitate distribution upon heating.