Microstructure evolution characterization of Ni-based GH4720Li superalloy during strain-controlled fatigue at 550 °C

High-temperature fatigue property of Ni-based GH4720Li superalloy at 550 °C has been investigated at maximum strain from 0.8% to 1.1%. Microstructural characterization and oxidation behavior of superalloy during high-temperature fatigue have been analyzed by transmission electron microscopy and scanning transmission electron microscopy. The results show that a stable response following a slight cyclic hardening during initial cycles was revealed at the maximum strain from 0.8% to 1.0%. The stable response decreased with an increase in maximum strain. Continuous cyclic hardening was observed at the maximum strain of 1.1%. There is difference in dislocation substructures between primary γ′ precipitates and γ grains. Dislocation cell and mechanical twin were formed in the interior of primary γ′ precipitates and γ grains. The primary γ′ precipitate interface would migrate toward the interior of primary γ′ precipitates along twin boundaries, leading to instability of primary γ′ precipitates. The secondary γ′ depleted zone was distinctly generated near the surface due to the decomposition of secondary γ′ precipitates. The crack initiation and propagation during high-temperature fatigue were found inside the secondary γ′ depleted zone. The primary γ′ precipitates could effectively hinder the crack propagation. Al-rich oxide films (Al2O3) were initially produced at crack tips, because the rate of diffusion of Al was relatively higher than that of other elements at crack tips.