Grain size effect on precipitation behavior of nanostructured Inconel 718

To provide insight into the effect of grain size on the precipitation behavior of γ strengthening super-
alloy Inconel 718, a gradient nanostructure with a large grain size span (from 9 nm to tens of microns)
along the depth direction was achieved by mean of surface mechanical grinding treatment, followed by
annealing upon 70 0–10 0 0 °C for 1 h. The results reveal significant differences in the type and size of
precipitates in samples with different grain sizes. No γ precipitate was detected inside the grains as the
grain size was refined down to 40 nm (NG-40) and 9 nm (NG-9). For δ phase, a significantly accelerated
precipitation along grain boundary was observed in NG-40 upon 700 °C annealing. Interestingly, with
the grain size drops to 9 nm, the precipitation of δ was suppressed, with some nanosized MC carbides
appearing upon annealing. The grain size effect of precipitation behavior endows NG-9 an ultra-high RT-
hardness (5.2 GPa) after 1000 °C thermal exposure and an ultra-high hot-hardness (3.2 GPa) at 800 °C.