Microstructure-property correlation and low-cycle fatigue life prediction in Inconel 718 superalloy: A comprehensive study on solution heat treatment effects

This study examines the effects of three solution heat treatment processes—Direct Aging (DA), Standard Treatment (ST), and High-Strength Treatment (HST)—on the microstructure, mechanical properties, and low-cycle fatigue (LCF) behavior of Inconel 718 superalloy at room temperature. The results indicate that heat treatment significantly governs grain size and precipitate characteristics, thereby dictating material performance. Microstructural analysis showed that the DA process produced the finest grains (82.27 μm) and the highest volume fraction of γ″ strengthening precipitates. The ST sample exhibited mixed grain sizes (92.52 μm) with granular and short-rod δ phases at grain boundaries, whereas HST led to complete δ-phase dissolution and grain coarsening (98.33 μm), resulting in uniform equiaxed grains. Mechanically, the DA sample achieved the highest hardness (521 HV) and superior tensile properties at both room temperature and high-temperature (650 °C), though with reduced ductility. In contrast, HST resulted in the lowest strength due to precipitate dissolution and grain growth. LCF tests revealed that DA samples consistently delivered the longest fatigue life across strain amplitudes of 0.4–0.8%, attributed to their fine-grained structure which promotes homogeneous deformation and delays crack initiation. A Manson-Coffin model was established with a prediction accuracy within a factor of 1.5. Fractography confirmed that coarse-grained HST microstructures accelerate fatigue damage. These findings underscore the critical role of microstructure in fatigue performance and provide valuable guidance for heat treatment optimization of Inconel 718 in aerospace components.