Abstract: To provide insight into the effect of grain size on the precipitation behavior of γ″ strengthening superalloy 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 700-1000 °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.

Key words: Inconel 718, Precipitation, Grain size effect, Nanograins