Creep rupture mechanisms and life prediction of IN617 for VHTR applications

The creep rupture mechanisms of IN617 were systematically investigated by establishing the relationship between microstructural characteristics and creep performance, including rupture time, percentage elongation, microhardness, and minimum creep rate. The Wilshire equation was employed to extrapolate long-term creep life by normalizing the stress, σ, using the ultimate tensile strength, σTS. The results indicated that at temperatures ≤950 °C, when σ/σYS values were >0.2 (σYS = yield strength), the creep rupture mechanism involved the sliding and migration of recrystallized grain boundaries, leading to matrix softening and necking fractures. When applied stresses ranged from 0.12σTS to 0.2σYS, creep voids along the grain boundaries became a pronounced feature, implying the rupture mechanism of the weakening of the grain boundaries without sufficient secondary-phase pinning. At a low stress level (σ/σTS < 0.12), creep ruptures were governed by a composite mechanism of microstructural degradation and internal oxidization, which was reflected in the brittle fractures caused by the connection of cracks along carbide networks and internal oxidized voids. A temperature–stress–rupture mechanism map was constructed by combining a comparison and summary from the literature. The Wilshire extrapolation curve described the creep rupture times well with temperatures and stresses, and the turning point of the curve suggested that microstructural degradation and internal oxidization increased the minimum creep rates and accelerated creep rupture.