| Ni-based superalloys are widely used in key hot-end parts of aero-engines due to their excellent high-temperature strength and good plasticity.With the increase of the thrust-to-weight ratio of aero engines,the temperature at the front end of turbine inlet has increased significantly,making the service temperature of other key hot-end components behind turbine increase greatly.Among them,the service temperature of a large and complex thin-walled bearing part of engine in our country has increased from below 650? to above 700?.At the same time,it needs to withstand long-term mechanical stress at high temperature and thermal stress caused by temperature unevenness,so high-temperature oxidation usually occurs in the alloy surface.Engine fuel contains a large number of impurities such as S,Cl,Na,K,etc.,these impurities can react with O or NaCl in the air(high content of NaCl over the coast or over the ocean)during combustion,and a mixed salt film such as Na2SO4 and NaCl is deposited on the surface of components,which is easy to cause hot corrosion of superalloy components.Therefore,it is very necessary to carry out research on high-temperature oxidation resistance and hot corrosion resistance of the key materials such as K4750 alloy and K4169 alloy,so as to provide a guarantee for the reliable application of the alloys at higher temperature.This thesis focused on studying the high-temperature oxidation behavior and hot corrosion behavior of two alloys under same conditions,analyzing the oxidation kinetics and corrosion kinetics of two alloys,and clarifing the high-temperature oxidation mechanism and hot corrosion mechanism.It provided a theoretical basis for the application of alloys under high temperature and harsh conditions.The high-temperature oxidation behavior of K4750 alloy was studied.During the constant temperature oxidation at 700?-1000? in air,the oxidation weight gain increased with the increase of temperature,and the oxidation kinetics curve basically followed to the parabolic law.The alloy was at complete anti-oxidation grade below 900?.The surface oxidation products mainly included Cr2O3,TiO2 and NiCr2O4.During oxidation at 700?-800?,densely distributed Cr2O3 was mainly formed on alloy surface.With the increase of temperature,massive TiO2 gradually appeared.The outer oxide film was mainly continuous Cr2O3 oxide,and the inner oxide was mainly strip-shaped Al2O3.At 700?-800?,the continuous and dense Cr2O3 film could hinder the outward diffusion of metal ions and the inward diffusion of O ions,effectively slowing down the rate of oxidation and inhibiting the oxidation of the matrix.At 900?and 1000?,the diffusion rate of Ti ions in the Cr2O3 layer increased,which promoted the formation of massive TiO2 and destroyed the continuity of surface oxide layer,resulting in an increase in the oxidation rate and a decrease in the oxidation performance.The high-temperature oxidation behavior of K4169 alloy has been studied comparatively,the oxidation resistance grade of K4169 alloy was essentially the same as that of K4750 alloy.The outer oxide film was divided into two layers,the outermost layer mainly contained Cr2O3,Fe2O3,Ni(Cr,Fe)2O4 and a small amount of nodular Nb2O5.he secondary outer layer was mainly continuous Cr2O3 layer,and the internal oxide was island-like connected Al2O3 and Cr2O3.It was found by comparison that Nb element in K4169 alloy promoted the formation of Cr2O3 film,and Nb was continuously enriched at the interface of outer oxide layer and substrate,which increased the adhesion of oxide film.Therefore,the short-term high temperature oxidation resistance of K4169 alloy was better than that of K4750 alloy.However,during long-term high temperature oxidation process,a large number of holes and looseness appeared in the outer oxide film,which destroyed the continuity of oxide film.Therefore,the long-term oxidation resistance of K4750 alloy was better than that of K4169 alloy.The hot corrosion behavior of K4750 and K4169 alloys coated with 75%Na2SO4+25%K2SO4 has been studied under the condition of 750? and 850? for 20-100h.The results showed that corrosion kinetic curves of two alloys had similar changing trends,namely a fluctuation phenomenon of first weight gain and then weight loss.At 750?,the deposited salt was solid,the degree of hot corrosion was relatively light,and the weight change of two alloys was less than 0.2 mg/cm2.At 850?,the deposited salt was molten,two alloys undergone high-temperature hot corrosion.Molten salt caused the aggravation of corrosion,but the weight gain was less than 0.5 mg/cm2,indicating that two alloys had good corrosion resistance to sulfate.In comparation,the protective Cr2O3 and Nb2O5 outer oxide film of K4169 alloy could delay the diffusion of elements,so the sulfate corrosion resistance of K4169 alloy was better than that of K4750 alloy.The hot corrosion behavior of K4750 and K4169 alloys coated with 75%Na2SO4+25%NaCl has also been studied under the condition of 750? and 850? for 20-100h.The results showed that the deposited salt was molten at 750? and 850?,and two alloys undergone high-temperature hot corrosion.At 750?,the weight change of two alloys was less than 2.0 mg/cm2.At 850?,K4169 alloy exhibited continuous weight loss,and the weight loss reached 30 mg/cm2 after corrosion for 100h.During high-temperature hot corrosion,the presence of NaCl reduced the protective performance of the Cr2O3 film,and K4169 alloy formed the outermost oxide mainly composed of NiO and Fe2O3,and the oxide layer was loose and porous,which accelerated the hot corrosion of the alloy.Therefore,the corrosion resistance of K4750 alloy to sulfate and chloride was better than that of K4169 alloy. |
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