Effect of shot peening on residual stress stability and high-temperature mechanical properties of powder metallurgy Ni-based superalloy

This study investigated the effects of shot peening on the microstructure and high temperature creep property of a René 95 nickel-based superalloy produced by powder metallurgy. The alloy was processed by hot isostatic pressing followed by solution treatment at elevated temperatures and subsequent two stage aging. Following shot peening treatment a high compressive residual stress of approximately 1200MPa was induced near the surface and local plastic deformation together with grain refinement was observed. Heat treatment in the range of 400 °C to 800 °C revealed that, although no significant changes were observed in the average grain size and local misorientation based on EBSD analysis, compressive residual stresses were rapidly relaxed at temperatures above 700 °C. This stress relaxation may be attributed to the dissolution of the fine tertiary γ′ precipitates near 700 °C, which can alter dislocation behavior and compromise microstructural stability. Tensile tests over a range of temperatures indicated that both yield strength and ultimate tensile strength were maintained up to 650 °C, but decreased significantly above 700 °C. In parallel, high-temperature creep tests conducted at 650 under a stress of 1034MPa revealed that the shot-peened specimens exhibited an approximately 82% longer creep life than the base material. These results suggest that the compressive residual stresses induced by shot peening can partly offset deleterious high temperature effects such as phase transformations, thereby enhancing the mechanical properties of René 95 in high temperature applications.