Study On Behavior And Mechanism Of MCrAlY Coatings In High Temperature Environment

Nickel-based superalloys have good mechanical properties,stable physical and chemical properties.Alloys are generally doped with chromium,iron,molybdenum,cobalt,aluminum,titanium and other elements.It has high mechanical strength and resistance to oxidation and hot corrosion at high temperature of 650-1000?.It is the most widely used and highest temperature strength alloy in high temperature alloys.Nickel-based superalloys are mainly used in the manufacture of high-temperature components for aerospace industry flight blades,high-power jet engines,nuclear reactors,thermal power,photovoltaics and other energy conversion equipment.However,traditional nickel-based superalloys are difficult to maintain basic mechanical strength in high temperature,oxygen-rich,and chlorine-rich environments.Therefore,it is of great significance to develop coatings with excellent oxidation resistance and excellent corrosion resistance.In this thesis,NiAlSiY coating,NiCrAlY coating and NiCrAlSiY coating were prepared on the surface of GH4169 alloy by arc ion plating technology.The corrosion behavior of NiAlSiY coating on GH4169 alloy under constant temperature oxidation and NaCl at 500?was studied;The isothermal oxidation behavior of NiCrAlY coating and NiCrAlSi Y coating on GH4169 alloy at 750?was also studied.The experimental results show that:?1?The oxidation resistance and corrosion resistance of Ni-21Al-3Si-1Y coating are stronger than those of Ni-17Al-3Si-1Y coating and Ni-13Al-3Si-1Y coating.The protection mechanism of MCrAlY coating is to form a stable and dense oxide layer on the coating after high temperature oxidation,thereby blocking the further diffusion of O₂ and protecting the coating and substrate.Compared with other oxides,Al₂O₃ has the characteristics of not easy to fall off,good performance and moderate formation rate,and is an ideal oxide.The Ni-21Al-3Si-1Y coating has the densest Al₂O₃ layer and can most effectively block the diffusion of O₂,so the Ni-21Al-3Si-1Y coating has the best oxidation resistance.?2?The NiAlSiY coating can be oxidized for 2 h to form an oxide layer of NiO and the inner layer is Al₂O₃.After 30 h of oxidation,the Ni-13Al-3Si-1Y coating has an outer layer of SiO₂,a middle layer of NiO and an inner layer of Al₂O₃.The Ni-17Al-3Si-1Y coating has an oxide layer of NiO on the outer layer and Al₂O₃ on the inner layer;the Ni-21Al-3Si-1Y coating has only one dense Al₂O₃ oxide layer.The higher the Al content,the more difficult it is to form SiO₂.When oxidation occurs,Al and Ni are preferentially oxidized,and the diffusion rate of Si is slower than that of Al and Ni,and is finally oxidized.Selective oxidation results in delamination of the oxide layer.The reason why different oxidation layer delamination occurs after 30 h of oxidation is mainly due to the difference between the decomposition rate and the generation rate of the oxide layer.The lower the Al content,the fewer atoms can form Al₂O₃,and the slower the generation rate,O₂ will It reacts with Ni and Si,and finally shows a strange distribution of SiO₂ in the outer oxide layer.After oxidation,the distribution of Si and Y in NiAlSiY coating is consistent,and they are distributed at the grain boundary;NiCrAlY coating and NiCrAlSiY coating are different in distribution law of Si and Y after oxidation.Provides a fast path for atomic diffusion.?3?The deposited NiAlSiY coating,NiCrAlY coating and NiCrAlSiY coating mainly consist of?-NiAl phase,and after oxidation,Al₂O₃ phase and?'-Ni₃Al phase appear.The Al phase also appears in NiAlSiY coatings.This is because a separate Al target is used in the preparation process,and the elemental Al disappears after oxidation.The transformation of?-NiAl into?'-Ni₃Al phase provides an Al source for Al₂O₃ in the oxide layer.The remaining phases are difficult to find in XRD because of less content or amorphous state.?4?During the hot corrosion process,pores will appear inside the NiAlSiY coating.These pores are caused by the combined action of O₂ and Cl₂,and eventually the gaseous AlCl₃ will escape along the pores of the coating and the oxide layer.When Al is consumed too much,it will appear the phenomenon of coating disintegration and collapse.?5?The NiCrAlY coating and NiCrAlSiY coating oxidized to form Al₂O₃ oxide layer at100 h of oxidation.At the same time,O₂ diffused inward along the grain boundary,oxidized Cr-rich areas,and Cr₂O₃ also appeared inside the coating.However,no oxidation phenomenon was found in the Ni/Al-rich region.The grain size of the oxidized coating becomes smaller.The closer to the oxide layer,the finer the grains,the larger the grain boundary per unit area,and the more favorable the diffusion of atoms.