Oxidation behavior of Ni-based superalloy GH4738 under tensile stress

Revealing the oxidation behavior of superalloys is crucial for optimizing material properties and extending service life.This study investigated the oxidation behavior of superalloy GH4738 under stress states at 850℃.High-throughput specimens were fabricated to withstand different stresses at the same time.Isothermal oxidation s amples were analyzed using the mass gain method to obtain oxidation kinetic curves.The results show that the external stress below 200 MPa could improve the oxidation resistance of the GH4738.With tensile stress increasing,the oxide layer becomes thinner,denser and more complete,while internal oxidation decreases.The tensile stress alters the structure of the external oxide layer from a two-layer to a threelayer configuration.The Cr_(2)O_(3) oxide layer inhibits the outward diffusion of Ti,leading to Ti enrichment at the oxide-matrix interface and altering the oxidation mechanism of GH4738.

Inclusions in wrought superalloys:a review

Inclusions in wrought superalloys significantly affect the stability of the alloy properties and limit their use and devel-opment in aeroengines and other applications.The types and sources of inclusions in wrought superalloys were reviewed with analysis of the conditions of inclusion formation from the viewpoints of thermodynamics and kinetics.The ther-modynamic data for inclusion formation in nickel-based and cobalt-based alloys were summarized and improved.The damage of inclusions to the fatigue and tensile properties and workability of these alloys and the mechanisms of crack initiation and propagation caused by inclusions were also discussed,and the effects of inclusions with different charac-teristics on crack propagation were reviewed.In addition,the control methods and mechanisms of inclusions in the triple smelting process(vacuum induction melting+protective electroslag remelting+vacuum arc remelting)were covered,providing a reference for improving the control technology of inclusions in wrought superalloys.Finally,the difficulties and development trends for inclusion control in wrought superalloys were discussed.

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

High-temperature fatigue property of Ni-based GH4720Li superalloy at 550℃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 decomposi-tion 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(Al_(2)O_(3))were initially produced at crack tips,because the rate of diffusion of Al was relatively higher than that of other elements at crack tips.

Interaction between γ'precipitate distribution and microstructure homogeneity during hot deformation in a Ni-based superalloy

The inhomogeneous microstructure of the Ni-based superalloys used for turbine disks was an intolerable defect for the mechanical performance.The effects of the distribution ofγ'precipitate,forging temperature,and strain level on the microstructure evolution of GH4730 alloy were investigated by EBSD during hot deformation.The results showed that the heterogeneous factor peaked at the transition temperature from the single-phase to the double-phase region.The coupling effect of heat and stress led to the heterogeneous precipitation and distribution ofγ'phase during the transition region,which was the main reason for the formation of inhomogeneous microstructures.The coherentγ'phases of approximately 0.3μm were diffusely distributed inside the large grains,increasing the grain strength,making recrystallization refinement difficult,and thus forming large unrecrystallized grains.The incoherentγ'phases with a size of approximately 1.1μm located at the grain boundaries and pinned the grain boundaries,and thus the accumulated strain at the grain boundaries caused the occurrence of discontinuous dynamic recrystallization and promoted continuous refinement of the grains.The microstructure evolution of new Ni-based superalloys during hot forging was focused,and the formation mechanism of inhomogeneous microstructure and control measures was explained.A theoretical basis for improving the microstructure homogeneity of the new cast and wrought superalloys was provided.