Precipitate evolution in the early stages of ageing in Inconel 718 investigated using small-angle x-ray scattering

Microstructural evolution during the early stages of ageing (less than one hour) in a Ni-Cr-Fe based superalloy Inconel 718 (IN718) has been investigated using Small-Angle X-ray Scattering (SAXS). The effects of precipitate kinetics on the precipitate size distribution are compared indirectly with SAXS measurements by using Vickers microhardness data. The microhardness increased after 4 min of ageing at a temperature of 760 °C, although the recorded SAXS data did not reveal the precipitate size distribution. This indicates that the precipitates had not evolved enough to be detected, but still a small number of precipitates increased the yield strength. After ageing the alloy for the shortest period for which data were available, 8 min, clear evidence of precipitates could be found from the SAXS data, showing that the γ″- precipitates are about 6 nm in width and 3 nm in height.