Effect of ultrasonic nanocrystal surface modification on elevated temperature residual stress, microstructure, and fatigue behavior of ATI 718Plus alloy

Abstract Ultrasonic Nanocrystal Surface Modification (UNSM) is a mechanical surface treatment that induces large compressive residual stresses and near-surface microstructural changes in the material by using repetitive impacts at ultrasonic frequencies with a WC tip. In this study, we investigate the use of UNSM to improve the fatigue life of ATI 718 Plus (718Plus) at high temperature of 650 °C. UNSM induced severe surface plastic deformation which led to nano-sized crystallites, twins, and high dislocation density in the near surface regions, coupled with an increase in surface hardness and high magnitude of compressive residual stresses. This change in the near-surface microstructure remained stable at elevated temperatures. In addition, UNSM retained ∼−680 MPa near-surface residual stress (56% of its initial residual stress) even after 140 h exposure to 650 °C. The retained near-surface residual stresses and the stable microstructure from the UNSM increased the yield strength by ∼11% (∼105 MPa) and endurance limit by ∼8% (∼70 MPa) in corresponding tests at 650 °C. This improvement in fatigue life was attributed to near-surface microstructure changes, material hardening and high compressive residual stress induced by UNSM. The estimated crack growth rates were ∼60% lower for UNSM-treated 718Plus as compared with untreated material. The thermal-mechanical residual stress relaxation indicates the effectiveness of UNSM in improving the fatigue life of 718Plus at 650 °C.