Effect of gradient microstructure on elevated temperature mechanical properties of Ni-based superalloy ATI 718Plus

Abstract

The effect of the ultrasonic surface rolling process (USRP) and post-treatment on the microstructure and elevated-temperature tensile properties of ATI 718Plus were investigated. The gradient distribution of the η-Ni3(Al0.5Nb0.5) phase and recrystallized grains was generated on the surface of the USRP-treated samples after heat treatment. The amount of η phase in the gradient layer decreased with increasing depth, whereas grain size showed the opposite trend. These η phase particles, distributed in the grain boundaries (GBs) of the gradient microstructure with refined grains, stabilized the trans-scale grain hierarchy at a high temperature of 704 °C. This heterogeneous microstructure promotes the accumulation of high-density geometrically necessary dislocations (GNDs) near the GBs and η phase during plastic deformation, resulting in additional hardening. The central coarse grains in the heterogeneous structure were subjected to the most plastic deformation and conserved ductility during the tensile tests. Compared with the standard sample, the yield strength (YS) and ultimate tensile strength (UTS) of the ultrasonically rolled samples increased by approximately 12% during tensile tests at 704 °C, without distinct plastic deterioration occurring. This study helps to improve the understanding of the functional mechanism of the equilibrium secondary phase in superalloys and the influence of the distribution state of the equilibrium phase on the mechanical properties.