Experiment And Theoretical Research On Improving The Oxidation Corrosion Resistance Of Ni-based Superalloy

Superalloys are most widely used in hot end parts of aero-engine,such as turbine blades,guide blades,turbine disks,combustors et al..With the rapid development of the aerospace industry,new requirements on high thrust and thrust-to-weight ratio of aero engines have been constantly put forward,it will lead to significantly increase the environment temperature of the turbine engines.This means that the superalloys might be fractured due to decreasing mechanical properties under the high under the extreme environments.Therefore,improving the capacity of the resistance to high-temperature will help to promote the development of aerospace engines.?1?The mechanism of high temperature oxidation corrosion of GH202 was studied.The isothermal oxidation kinetics curves Ni-based superalloy GH202followed the parabolic law at 800°C,900°C,1000°C,1100°C.Its oxidation activation energy is 247.8 KJ/mol,which is very closed to the diffusion activation energy of Cr³⁺(259 KJ·mol^-1).It is indicated that the oxidation process of the alloy is dominated by the diffusion of Cr³⁺to the surface oxide film.After high temperature oxidation a continuous and dense oxidation film on the surface of the alloy formed and protected effectively further oxidation of the alloy of the alloy.The oxide film is mainly composed of Cr₂O₃,a small amount of TiO₂ and spinel NiCr₂O₄.With the increase of temperature,TiO₂ and NiCr₂O₄ in the oxide film gradually grew up,and it resulted in decreasing the density of the oxide film.The internal oxidation was ggravated beneath the oxidation film at 900°C,and a large number of Topologically Close-Packed Phases?TCP?occurred.At 1000°C oxidation film was changed to two-layer structure,the outer was loose,inner was still dense,and TCP phase grew up significantly.After1100°C oxidation the outer oxidation film was almost spalled.?2?Experimental studies on laser shock process and high temperature oxidation were carried out.The surface hardness of GH202 was increased from 400HV to560HV by laser shock process.The residual compressive stress is generated on the surface,and the maximum residual compressive stress is-300MPa,where is at 2mm from the center of the spot.The obvious plastic deformation was produced on the the surface by LSP.The surface grains were refined and a large number of crystal defects were produced,such as twins,dislocation tangles,dislocation cells,dislocation walls,dislocation arrays et al..These crystal defects can prevent the slip of the dislocations and improve the mechanical properties of the alloy.The microstructures of the alloy with LSP have goog thermal stability at 800°C,900°C.The oxide film on the surface of the samples with LSP was denser,internal oxidation and TCP phase precipitation was significantly reduced.It is indicated that t the laser shock process can restrain the growth of the oxide on the alloy surface.The high temperature oxidation resistance performance of the alloy has been improved obviously.The corrosion products on the surface of the alloy with LSP mainly include Cr₂O₃?Al₂O₃?NiO?TiO₂?CrS?NiCr₂O₄?Ni₃S₂ in the molten salt of NaCl and Na₂SO₄ at 800°C,900°C.The results showed that weight reduction of the alloy with LSP was alleviated significantly,and thermal corrosion resistance of the alloy after LSP were improved.The cracks of the oxide film on the surface of the alloy were inhibited,the intercrystalline corrosion was obviously reduced,and the spalling zones of the oxide film was also significantly reduced.?3?The aluminizing process and laser shock process on the surface of the coatings were investigated.The effects of various parameters of NiAl coating thickness were investigated by pack cementation,such as temperature,content of Y₂O₃,Al powder,NH₄Cl et al..The results show that the mixed powder with the content of 4.5%Y₂O₃ at 850°C have a significant effect on the depth of NiAl coatings.The staged heating method during the aluminizing process was adopted to raise the thickness of the coatings to 170?m.The grains in the NiAl coating is not uniform,the grain boundary of NiAl phase is polygonal,and the diameter is about 500nm.There is existing a little of Cr-rich compound(Al₈₆Cr₁₄),and its grain diameter is about 150nm.A small amount of Ni₂Al₃ on the grain boundary of NiAl,and the grain diameter is less than or equal to 100nm.Some small sediments were also distributed in the grains or on the grain boundaries,and their scale was less than 20nm.A small number of dislocation lines and dislocation tangles were produced in the NiAl coatings by LSP.The effect of laser shock process on the depth of the coatings was weak.The dislocation tangles of the matrix beneath the interface were generated,and the grains were significantly refined.The shock wave induced by laser penetrates the NiAl coating,increase the impact force,and strengthen the alloy.The weight grain of the alloy with LSP after oxidation is lower than that of alloy without LSP.The laser process is help to form a denser oxidation film as quickly as possible,it can restrain the growth rate of Al₂O₃ and delay the phase transition to release thermal stress.The shock wave caused the alloy to produce a large number of crystal defects,provide channels for the diffusion of Al atoms,which contribute to protect the alloy for long-term.?4?The experimental study on the preparation of thermal barrier coating by laser shock and plasma spraying was carried out.The first,the CoCrAlYTaSi coating prepared by plasma spraying is a compact multilayer structure with many strips of Al₂O₃.The outer layer include rich Y,Ta and Si spheres with a thin Al₂O₃ film.After high temperature oxidation,the oxidation film of the CoCrAlYTaSi coating is mainly Al₂O₃,Cr₂O₃,CoO,CoCr₂O₄ and Ta₂O₅.The element of Ta can absorbe oxygen and form a large number of spheres Ta₂O₅,which grow up continuously with the increase of temperature.The plasticity and oxidation resistance of the coating are improved due to existing Ta.Second,The experimental study on the composite treatment of laser shock and plasma spraying was carried out.After oxidation at high temperature,the oxide film on the interface between the alloy and the coating after the composite treatment became denser.The thermal barrier coatings prepared by the laser shock compositing plasma spraying can delay the growth rate of the oxide film,hinder the oxygen diffusion into the alloy.The excessive deformation due to the rapid growth of the oxide on the alloy surface is avoided,it can extend the service life of the thermal barrier coating.