Coupled Effects of Pre-Strain and Heat Treatment on the Mechanical Response and Fatigue Performance of Inconel 718 superalloy

Pre-strain is a critical manufacturing variable that must be considered in the design and performance assessment of metallic components. This study examines the macro-/micro-mechanical responses and fatigue behavior of Inconel 718 superalloy at room temperature after controlled pre-straining and standard aging heat treatment. Uniaxial tensile tests show a substantial strength increase accompanied by reduced elongation as pre-strain rises, with further strengthening after heat treatment. Low-cycle fatigue (LCF) life, evaluated under a peak stress of 0.95σ s for each condition, decreases markedly with increasing pre-strain and is further shortened by subsequent heat treatment. EBSD, SEM, and TEM reveal increased lattice misorientation after pre-straining, persistent subgrain boundaries, and the formation of strengthening precipitates after heat treatment. Fractography confirms a three-stage fatigue process (crack initiation, propagation, and final fracture) that is insensitive to pre-strain level or heat treatment route. Mechanistically, fatigue degradation in pre-strained specimens is dominated by planar slip and severe dislocation entanglement, whereas in pre-strained and heat-treated specimens, the cyclic softening and life reduction arise from the combined effects of γ″ shearing and residual dislocation tangles. The results, obtained for pre-strain levels of 7-35%, provide an important practical implication for material design under extreme loads: achieving a balance between strength and damage tolerance is crucial, rather than solely pursuing maximum strength.