Thermal stability of surface nanostructure produced by laser shock peening in a Ni-based superalloy

A nanocrystalline layer with the grain size of about 20– 200 nm in a Ni-based superalloy was fabricated by the means of laser shock peening (LSP). The microstructure characterization of nanocrystalline layer was systematically investigated by transmission electron microscopy (TEM) and nanoindentation. And the thermal stability of surface nanocrystalline layer was studied by in-situ TEM annealing with different temperatures and duration times (500 °C/1 h, 700 °C/1 h and 900 °C/3 h). In addition, we designed a kind of nanoindentation test with the sample being post annealed at 900 °C for 10 h to verify the thermal stability of nanocrystalline layer. The results indicated that nanostructure has superior thermal stability after annealing at 700 °C for 1 h, which is higher than the dynamic crystallization temperature, 0.36 T_m (melting temperature T_m is about 1300 °C). Grain growth occurred at 900 °C, but some nano-grains with the size of around 50 nm still existed, and the nanohardness test further strengthened the evidence of good thermal stability. All these experimental results indicated that such Ni-based superalloy with nanocrystalline layer exhibited relatively good thermal stability after annealing at 900 °C. Lastly, the mechanism of thermal stability of surface nanostructure produced by LSP in a Ni-based superalloy was discussed in detail.