Microstructural evolution and mechanical properties of IN718 under coupled high-temperature and stress conditions

The stability of high-aspect-ratio IN718 superalloy under prolonged high-temperature service is critical for gas turbine applications, yet a systematic understanding of its microstructural evolution and mechanisms remains insufficient. This study investigates the evolution of microstructure and properties in such forgings subjected to thermomechanical exposure at 235°C/308 MPa and 359°C/499 MPa simulating extended service over 10,000 h. Microstructural analysis under elevated thermomechanical coupling revealed reduced fine grains and an 11.34% increase in Σ3 twin boundaries, optimising grain boundary structure. γ’ rose to 7.74%, while γ″ dropped by 62.63% with a stable δ-phase, indicating Nb release that elevated elastic strain energy. Thermal recovery reduced dislocation density by 53.57%, which synergised with stress-induced bowing to eventually produce microplastic deformation. This mechanism yielded 4.96% higher yield strength and 6.13% better elongation, demonstrating superior strength-ductility synergy. These findings offer novel insights for service stability and life prediction of high-strength fasteners under thermomechanical exposure.