Mechanism for stress relaxation behavior of the residual stress improving treatments with water jet peening and buffing

In this study, the relaxation behavior of the compressive residual stress in nickel-based alloy 600 caused by water jet peening (WJP) and buffing stress improving treatments has been investigated, through the hardness reduction kinetic analysis, electron back scattered diffraction (EBSD) crystal orientation analysis and positron annihilation spectroscopy analysis. Hardness reduction behavior in WJP and buffing processed samples during thermal aging has been found to follow the Johnson-Mehl-Avrami equation and the rate constant for hardness reduction to be expressed by the Arrhenius equation. According to the result of EBSD crystal orientation analysis, it has been confirmed that the strain is introduced in the mainly surface region, which is corresponding to the hardness measurement results. Positron lifetime measurement using positron annihilation lifetime spectroscopy has revealed that the positron lifetime of the WJP and buffing processed samples is larger than that of solution heat-treated sample, indicating the lattice defects have been introduced with the stress improvement process. In addition, it becomes apparent that a large amount of vacancies introduced have disappeared after thermal aging, however some parts of dislocations have still remained. On the basis of the results, the mechanism for the stress relaxation behavior of the residual stress improving treatment effects with water jet peening and buffing has been discussed.