Effect of surface nanostructure on tensile behavior of superalloy IN718

• A nanograins of ≈12 nm up to a depth of ≈90 μm was produced in IN718 by USSP.
• There was no phase change in this alloy even after 90 min USSP.
• The microhardness of surface was enhanced by ∼20%.
• Yield and tensile strength were increased by 3.5% and 2.15% respectively.

Peak aged nickel based superalloy Inconel 718 was subjected to ultrasonic shot peening for different durations of 45, 60 and 90 min. The transverse and longitudinal sections of the ultrasonic shot peened (USSP) specimens were examined by scanning electron microscope (SEM) and transmission electron microscope (TEM) and the shot peened microstructures were characterized. A nanostructure of nearly 12 nm grain size was observed to develop in surface region of the USSP specimen. X-ray diffraction (XRD) of the USSP samples did not reveal any phase change. Microhardness of the shot peened region was increased by 20% in the as shot peened condition and it was reduced to 8% in the stress relieved condition. Yield and tensile strengths were found to increase only marginally by 3.5% and 2.15%, respectively and the ductility was reduced by 3.4% following ultrasonic shot peening. The results are discussed in terms of grain refinement in the shot peened region.