Coarsening Behavior Of γ’ Phase And Microstructure-dependent Oxidation Behavior Of Ni-based Single Crystal Superalloy

Ni-based superalloys are widely used in aerospace,mechanical manufacturing,energy chemical industry,nuclear engineering and other fields due to their excellent mechanical properties at room temperature/high temperature,good high temperature structure stability and oxidation resistance and corrosion resistance.The mechanical properties of Ni-based superalloys are acknowledged to depend strongly upon theγ′+γtwo phase microstructure.Theγ′precipitates with an L1₂-type ordered structure embedded into theγmatrix act as the main strengthening phase.γ′precipitate is the structural stable phase,and the coarsening ofγ′precipitates in Ni-based alloy occurs inevitably over the course of long-time exposure at high temperature.Moreover,theγ′volume fraction,morphology and size distribution play a vital role in determining the mechanical properties of superalloys.In this paper,Ni-7.8wt.%Al binary single crystal superalloy was adopted as the model alloy,the coarsening behavior ofγ′precipitates in the model alloy during isothermal aging at different temperatures（700,800 and 900°C）was investigated.The aging temperature has a significant effect on the coarsening behavior ofγ′precipitates.At 700°C,the size ofγ′precipitates in the alloy has no significant change.When aged at 800°C and 900°C,the distribution ofγ′precipitates is long-range disorder and short-range order.With the increase of aging temperature and time,the morphology ofγ′precipitates will gradually develop from cuboidal to strip-like,and the average size ofγ′precipitates and the width ofγchannel will increase.The coarsening ofγ′precipitates can be divided into two stages:early stage coarsening and later stage coarsening.The LSEM model fits better for the early stage coarsening ofγ′precipitates,while the TIDC model fits better for the later stage coarsening ofγ′precipitates.Based on the above studies,alloys with differentγ′+γtwo phase microstructure（γchannel widths andγ′phase dimensions are different）were obtained by different heat treatment regimes,and the microstructure-dependent oxidation behavior of the alloys was investigated.In the initial stage of oxidation at 650°C,the oxidation behavior depends mainly on the growth pattern of oxides in theγchannels.Ni and O in the wideγchannel will preferentially diffuse,forming outwardly convex Ni O regions.The subsequent formation of Ni-Al spinels below the Ni O region provides a large number of short-circuit channels that further accelerate the inward diffusion of O and the outward diffusion of Ni.However,the slenderγchannels favor the formation of a continuous protective Al₂O₃ layer on the alloy surface,leading to a relatively uniform Ni O layer and a regular Ni-Al spinel structure.In the late stage of oxidation,the influence of Ni-Al spinel structure within theγchannel on oxidation is attenuated,and the oxidation behavior is primarily dependent on the structure of the oxide in the IOZ.The small size of theγ′phase limits the growth of Al₂O₃ within it.While in the largeγ′phase,a continuous network of endoxides is formed between the dense,elongated Al₂O₃,greatly facilitating the diffusion of the elements along the oxide/metal interface.Therefore,the alloy with wideγchannels and largeγ′phases exhibits poor oxidation performance.