Investigation of microstructural changes in INCONEL 706 at high temperatures by In-Situ small-angle neutron scattering

Abstract

The Ni-Fe based superalloy INCONEL 706 (IN706) has a complex microstructure and many A₃B type phases, e.g., Ni₃Al γ′, Ni₃Nb γ″, Ni₃Nb δ, or Ni₃Ti η precipitates can form. The various precipitates can also have different morphologies, i.e., plate, needle, cube, or disc shape, and some of them exist as co-precipitates—γ′/γ″. In the present study, in-situ measurements by small-angle neutron scattering (SANS) were performed to monitor the microstructural evolution at elevated temperatures. The SANS measurements were complemented by microstructural observations at room temperature by scanning electron microscopy and transmission electron microscopy (SEM and TEM). It is demonstrated that η plates and γ″ needles form directly at high temperatures. Cooling from high temperature produces fine dispersions of γ′/γ″ precipitates, which coarsen on reheating to lower stabilization temperatures (893 and 993 K). The final morphology of the γ′/γ″ co-precipitates, i.e., compact/noncompact type very much depends on the stabilization temperature and the cooling rates from prior higher-temperature stabilization steps.