Research Status and Prospect of Oxidation Behavior of Nickel-Based Single Crystal Superalloys
Nickel-based single crystal superalloys have become increasingly important in the manufacturing of aircraft engines and gas turbine blades due to their superior mechanical properties,including excellent high-temperature strength and superior fatigue resistance.However,these alloys suffer from the problem of high-temperature oxidation,which can significantly shorten their service life.In order to improve the oxidation resistance and prolong the service life,it is necessary to have a comprehensive understanding of their oxidation behavior.The oxidation kinetics of nickel-based single crystal superalloys can be divided into three stages:the first stage,where the oxidation follows the parabolic law controlled by diffusion;the second stage,where the oxidation rate decreases significantly due to the formation of a continuous dense alumina layer;and the third stage,where the thickness of the oxide film reaches a critical value and detachment occurs,which accelerates the oxidation rate.The oxidation kinetics of different alloys may differ due to the influence of factors such as element content and oxidation temperature.The oxidation kinetics curve exhibits a parabolic law,subparabolic law,and cubic law,and different oxidation stages show different oxidation laws due to changes in the oxide layer(whether a dense alumina layer is formed).The oxide film of nickel-based single crystal superalloys can be divided into three layers:the outer layer of nickel oxide,the middle layer of spinel,and the inner layer of dense alumina.After a period of oxidation,γ'phase in the matrix can be transformed into γ'phase at the inner side of alumina film due to the depletion of Al element,forming γ'-free zone.After long-term oxidation,the thickening of the oxide film leads to the peeling of the oxide layer,leaving a large area of y'-free zone.During the initial oxidation process,the dislocation generated by lattice mismatch can aggregate in the two phases to form a dislocation network,providing a channel for oxygen atoms to diffuse inward.Moreover,the oxidation behavior of nickel-based single crystal superalloys is affected by many factors.First,the element content of the alloy,such as Al,Cr,Si,Hf,and rare earth elements can improve its antioxidation performance of the alloy,while S,Ti,Ru,and other elements have the opposite effect.In addition,the segregation during the solidification process of the alloy can cause inconsistent oxidation behavior between dendrites and intradendritic regions.Specifically,the initial oxidation product between dendrites is mainly aluminum oxide,while the intradendritic region is mainly nickel oxide.In the long-term oxidation process,the alumina layer in the intradendritic region undergoes a process of changing from continuous dense to fragmented and dispersed,while those in the interdendritic region always remains continuous until detachment.Different oxidation conditions also affect the oxidation behavior:as the temperature increases,the oxidation rate of the alloy accelerates,and the thickness of the oxide layer also increases.When loaded applied,the dislocations in the y matrix increase and aggregate at the interface of y and γ'phases,accelerating the outward diffusion rate of Al elements and causing the growth of the oxide layer to accelerate.The oxidation behavior also show anisotropy,specifically,the oxidation resistance of(100)surface is worse than that of(110)surface.This difference may be attributed to the distinct growth structures of oxide on various oriented surfaces,leading to different numbers of short-circuit diffusion paths on the surface,or may be caused by varying diffusion path lengths at the two-phase interfaces in different orientations.The differences in antioxidant properties can also be explained from the perspective of atomic adsorption.First-principles calculations show that different oriented surfaces have different adsorption capacities for oxygen atoms,which lead to the anisotropy of oxidation.The higher the surface roughness is,the more dislocations and defects are near the surface.These defects provide outward diffusion channels for Al ions,rapidly generate a dense alumina layer and improve the antioxidation performance of the alloy.In the future,research on the oxidation of single crystal superalloys can be carried out from the following directions:(1)Utilize in-situ oxidation technology with longer coverage time,more accurate stress-temperature control,and finer observation to establish a more precise oxidation model.(2)Use more high-precision instruments to characterize trace rare earth elements and explore its modification mechanism deeply to achieve precise and efficient doping of rare earth elements.(3)Carry out the research on the multi-factor synergy,coupling two or more conditions to study the oxidation behavior of alloys closest to actual service environment,thus establishing an oxidation model more in line with real situations.(4)Conduct research on the influence of oxidation behavior on alloy creep/fatigue fracture behavior.In-depth study of the relationship between oxidation and mechanical performance such as creep and fatigue are helpful to deepen the understanding of material deformation and failure behavior and improve the service life of components.
nickel-based single crystal superalloyhigh-temperature oxidationelement diffusionanisotropy
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