Effect of NiRe Layer on Oxidation Resistance of NiAl Coatings
Owing to their excellent high-temperature mechanical properties,nickel-based superalloys have been widely used in aviation,particularly in the hot end-components of aeroengines.Al and Cr contribute significantly to the oxidation resistance and thermal corrosion resistance of superalloys;however,they adversely affect the microstructure stability and plasticity of superalloys.Owing to the continuous development of superalloys,the proportions of these two elements are decreasing,thus resulting in a decline in the corrosion resistance of superalloys.To improve the oxidation resistance and thermal corrosion resistance of superalloys,researchers have adopted high-temperature protective coatings in superalloys,which endow excellent comprehensive mechanical properties to the latter.β-NiAl has a high melting point and its Al content is higher than the critical concentration for the selective oxidation of aluminum;thus,it is the main forming phase of simple aluminide coatings.The addition of modified elements intoβ-NiAl coatings is being actively investigated,whereas studies regarding the Re modification of β-NiAl coatings are few.The method of electroplating NiRe,arc ion plating Al and vacuum heat treatment is adopted to perform Re modification on β-NiAl.To investigate the effect of the NiRe layer on the oxidation resistance of aluminide coatings,the isothermal oxidation properties of Al-diffusion and 1Re-NiAl coatings at 1 100 ℃ are evaluated.The cross-sectional,surface-morphology,and microstructural evolution of the coatings are analyzed via scanning electron microscope(SEM)and X-ray diffraction(XRD).Both coatings are primarily composed of theβ-NiAl phase,whereas Zones 1 and 2 of the Al-diffusion coating and Zone 2 of the 1Re-NiAl coating are composed of β-NiAl and white α-W precipitated phases.Similarly,Zone 1 of the outer layer of the 1Re-NiAl coating is composed of β-NiAl and white γ-Re precipitated phases,thus indicting that the formation of α-W can be avoided by incorporating a NiRe layer.The isothermal oxidation kinetic results show that the total mass gain of the 1Re-NiAl coating is 0.62 mg/cm2 after isothermal oxidation for 300 h,which is lower than that of the Al-diffusion coating(2.77 mg/cm2).A significant amount of β-NiAl and slight oxide-scale spallation are indicated for the 1Re-NiAl coating.However,the diffraction peak of the β-NiAl phase cannot be detected and oxide-scale spallation on the surface is particularly severe in the Al-diffusion coating after isothermal oxidation for 150 h.The Al-diffusion coating shows severe internal oxidation and oxide-scale spallation(composed primarily of NiAl2O4 spinel),whereas the surface oxide scale of the 1Re-NiAl coating comprises single Al2O3 and indicates slight spallation after isothermal oxidation for 300 h.In the annealing process of the 1Re-NiAl coating,Al first diffuses into the NiRe layer,thus resulting in a gradual increase in Al content in the NiRe layer and a transformation into the β-NiAl phase.Simultaneously,the Re in the original NiRe layer is precipitated in situ to form a Re-rich precipitated phase in the outer layer of Zone 1 owing to the extremely low solid solubility of Re in β-NiAl.Thus,the incorporation of the electroplated NiRe layer can restrain the formation of α-W precipitates in the outer layer of the 1Re-NiAl coating.Moreover,incorporating Re improves the stability of the β-NiAl phase during oxidation and promotes Al diffusion from the inside to the surface of the coating.Thus,the oxidation resistance is improved significantly by incorporating an electroplated NiRe layer.In this study,Re is successfully introduced via electroplating and arc ion plating,and the effect of the NiRe layer on the oxidation resistance of the coating is investigated,which can facilitate future studies regarding Re and other element-modified β-NiAl coatings.Additionally,the reported method has broad application prospects.
composite electroplated NiRearc ion platingβ-NiAl coatingisothermal oxidation
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