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

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
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.