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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.

The corrosion resistance of Zr-0.7Sn-0.35Nb-0.3Fe-xGe(x=0.05,0.1,0.2,mass fraction,%) alloys was investigated in lithiated water with 0.01 mol/L LiOH at 360℃/18.6 MPa by autoclave tests.The microstructures of the alloys and oxide films on the corroded specimens were observed by TEM and SEM.The results show that the corrosion resistance of the Zr-0.7Sn-0.35Nb- 0.3Fe alloys in lithiated water at high temperature under high pressure is markedly improved by Ge addition.The alloy with 0.1%Ge shows the best corrosion resistance.In Zr-0.7Sn-0.35Nb-0.3Fe-xGe alloys,there exists fine Zr(Fe,Cr,Nb)_2 and Zr(Fe,Cr,Nb.Ge)_2 second phase particles(SPPs) with a close-packed hexagonal crystal structure(hep) and coarse Zr_3Ge SPPs with a tetragonal crystal structure(TET).The oxide films formed on the Zr-0.7Sn-0.35Nb-0.3Fe-0.1Ge alloys corroded for 220 d are compact and thin.The micro-pores and micro-cracks are hardly detected and many ZrO_2 columnar grains exist in the oxide films formed on the Zr-0.7Sn-0.35Nb-0.3Fe-0.1Ge alloys. This indicates that the suitable amount of Ge could not only delay the process that the vacancies diffuse to form micro-pores and micro-pores develop to form micro-cracks,but also could retard the evolution from ZrO_2 columnar grains to ZrO_2 equiaxed grains.