Temperature-dependent deformation mechanisms and strengthening behavior of a novel Ni-based superalloy with bimodal γ′ precipitate distribution

Abstract

High-temperature tensile performance is a critical indicator of the mechanical reliability of Ni-based superalloys. In this study, a newly designed Ni-based superalloy was systematically investigated from room temperature to 900 °C to elucidate the coupling between microstructural characteristics, mechanical response, and deformation mechanisms. Fine-grained (10.7 μm) and coarse-grained (97 μm) microstructures were obtained through multi-stage heat treatments. The FG alloy exhibits superior yield and tensile strength below 750 °C, reaching 1340 MPa and 1740 MPa at room temperature and retaining 1208 MPa and 1267 MPa at 750 °C, which is primarily attributed to its higher grain boundary density and enhanced grain-boundary-mediated strengthening. However, the FG alloy shows a pronounced ductility drop at 815 °C, with the elongation decreasing to 9 %. In contrast, the CG alloy demonstrates enhanced strength retention at elevated temperatures, maintaining an ultimate tensile strength and yield strength of 801 MPa and 770 MPa at 900 °C, corresponding to a tensile strength retention of 54 % (compared with 38 % for the FG alloy), together with a stable elongation of 8.5 %. This superior high-temperature stability is associated with a lower grain boundary density and a higher fraction of Σ3 special boundaries (42.6–55.9 %). Microstructural analysis reveals a bimodal γ′ precipitate distribution in both microstructures, with fine secondary γ′ precipitates (FG: 82 nm, 48 %; CG: 92 nm, 57 %) dominating strengthening through short-range dislocation–precipitate interactions, and coarse primary γ′ precipitates (FG: 1.54 μm, 7.6 %; CG: 3.19 μm, 0.5 %) facilitating deformation accommodation by pinning grain boundary migration, thereby enabling a synergistic balance between strength and ductility over a wide temperature range. Transmission electron microscopy and critical resolved shear stress analysis demonstrate a temperature-dependent transition in deformation mechanisms from γ′ shearing to Orowan looping in the range of 815–900 °C. In addition, Co/Ti solute segregation at twin interfaces reduces interfacial energy and suppresses defect migration, further stabilizing the γ/γ′ microstructure and enhancing high-temperature strength. These findings provide mechanistic insight for the microstructural design of advanced Ni-based superalloys.