The Effects of Long-Term High-Temperature Aging on the Microstructural Evolution and Impact Fracture Behavior of Inconel 625 Superalloy

Inconel 625 is widely used in high-temperature structural components because of its excellent strength, toughness, and corrosion resistance. However, long-term exposure to elevated temperatures can induce precipitation of carbides, γ″ phase, and δ phase, leading to microstructural degradation and reduced mechanical reliability. Although precipitation evolution and tensile properties of aged Inconel 625 have been widely studied, the relationship between long-term precipitate evolution and impact fracture behavior remains insufficiently clarified. In this study, solution-treated Inconel 625 alloy was aged at 700 °C and 750 °C for up to 5000 h, with additional stress-assisted aging at 750 °C under 30 MPa and 51 MPa. Impact toughness, microhardness, fracture morphology, and precipitate evolution were systematically investigated. The results show that long-term aging significantly reduces impact toughness at both room and elevated temperatures, with a more pronounced reduction at room temperature. The room-temperature impact energy decreases from 314 J to approximately 10 J and stabilizes after 2000 h. Quantitative analysis shows that γ″ precipitate coarsening follows the Lifshitz–Slyozov–Wagner relationship, indicating diffusion-controlled growth. Stress-assisted aging under the present low stress levels has only a limited influence on precipitate evolution and impact toughness. The toughness degradation is mainly attributed to chain-like grain-boundary carbides and needle-like or plate-like δ phase, which embrittle grain boundaries, segment the austenitic matrix, and limit impact energy absorption.