Microstructure And Oxidation Resistance Behaviour Of The NiCoCrAlY(Ta,Mo) Coatings On TiAl-Based Alloys

Titanium aluminide based alloys,owing to excellent properties such as low density,high specific strength and good creep resistance,have become the promising lightweight structural material in the field of aerospace.However,the insufficient oxidation resistance of the TiAl alloys above 800oC has limited their application as the hot end components of engine.The whole alloying and surface coating technologies are two main methods to improve the oxidation resistance of the alloys.In practical applications,the two methods are often used together.MCrAlY coatings are the most widely used at high temperature.But,the brittle interdiffusion layers will be formed at the interface between the MCrAlY coatings and the TiAl alloys during oxidation,which can result in the chemical failure and spalling failure of the coatings.Based on the above problems,the whole alloying and surface coating are combined in this paper.The effect of alloying elements and preparation technologies on the microstructure,mechanical properties,thermal shock resistance and high temperature oxidation resistance of the TiAl alloys and the NiCoCrAlY coatings were systematically investigated.And the failure mechanisms of the coating-alloy systems were deeply analyzed.The results are as follow.V?B and Y elements have the significant effects on the oxidation behaviors of the Ti-44Al-6Nb-1Cr alloy at 900oC.The addition of 2 at.%V results in the formation of V₂O₅.V₂O₅ is easily to volatilize at high temperature,which promotes the reaction of oxygen and metal ions,thereby resulting in the formation of a large number of TiO₂ and Al₂O₃ agglomerates in the oxide film.Finally,the oxide agglomerates deteriorate the oxidation resistance of the TiAl due to the poor bonding between oxide scale and coating.The synergistic addition of B and Y refines the microstructure of the Ti-44Al-6Nb-1Cr-2V alloy,thereby promoting the selective oxidation of Al element on the surface of the alloy.The selective of Al is beneficial to form an intact Al₂O₃ film in the short time.At the meanwhile,the segregation of B and Y at the grain boundaries inhibited the outward diffusion of V,which reduces the growth rate of V₂O₅.Therefore,the negative effect of grain refinement is impaired and oxidation resistance of Ti-44Al-6Nb-1Cr-2V-0.1B-0.15alloy is enhanced.Research results on the oxidation behaviors of the NiCoCrAlY/Ti-44Al-6Nb-1Cr?NiCoCRAlY/Ti-44Al-6Nb-1Cr-2V and NiCoCRAlY/Ti-44Al-6Nb-1Cr-2V-0.1B-0.15Y systems reveal that the alloying elements in the TiAl substrates have a great influence on the bonding property and the oxidation resistance of the NiCoCrAlY coatings.The V element in the substrate results in the formation of V₂O₅,AlVO₄ and NiCr₂O₄ on the surface of the coating.The fast growth of these oxides increases the internal stress,thereby resulting in the spallation of the oxide scale.A large number of O^2-diffuses into the coating,which promoted the serious internal oxidation at the interface.Finally,the bonding property and oxidation resistance of the coating are reduced.The inhibition effect of B and Y on the outward diffusion of V reduces the growth rate of the V₂O₅ and AlVO₄.As a result,the bonding between the coating and the substrate is enhanced.The good bonding of the oxide scale reduces the internal oxide scale.A Al₂O₃barrier is formed at the interface,which hinders the formation of the brittle interdiffusion layers.Therefore,the bonding property of the coating is improved.Refractory elements Ta and Mo have different effects on the bonding property and oxidation resistance of NiCoCrAlY coating.The addtion of 4.5 wt.%Ta or 2wt.%Mo has little effect on the bonding strength of the coating.However,as the transition group of elements,they increase the elastic modulus of the coating,which is conducive to improve the cohesive strength of the coating.The doping of Ta⁵⁺in the oxide scale can inhibit the growth of Al₂O₃ and Cr₂O₃ on the surface of the coating.As a result,the oxidation resistance of the coating reduces slightly.But,the doping of Ta⁵⁺improves the thermal shock resistance of the coating.This is because that the doping of Ta promotes the formation of interfacial active Al₂O₃ barrier.The diffusion barrier reduces the formation rate of interfacial brittle interdiffusion layers,thereby improving the bonding between coating and substrate.In NiCoCrAlYMo coating,the volatile MoO₃ is formed,which resulted in the serious internal oxidation of the coating.The serious internal oxidation results in the formation of a large number of Cr₂O₃+TiO₂+Al₂O₃ aggregates at the interface which increases the internal stress of the coating.Therefore,the oxidation resistance and the thermal shock resistance of the coating decrease significantly.Laser cladding technology reduces the defects of inclusions and pores in NiCoCrAlY coating,eliminates the layered structure of the coating and forms a metallurgical bonding between the coating and the substrate.The addition of Sm₂O₃changes the solidification structure of the coating.After adding Sm₂O₃,the width of the interfacial plane crystal in the coatings increases and the dendrite crystal melts.As a result,the growth of columnar crystals is promoted.Sm₂O₃ addition refines the structure of coating near the surface.And the grain size decreases with the increase of Sm₂O₃ content.Rare earth oxide Sm₂O₃ has a great influence on the mechanical properties and oxidation resistance of NiCoCrAlY coating.The fine grains of the NiCoCrAlY-x Sm₂O₃ coatings can reduce the crack sensitivity of the coatings.The fine grains near the surface of the coating promote the selective oxidation of Al.And with the increase of Sm₂O₃ content,the transition rate of?-Al₂O₃ to?-Al₂O₃ is accelerated.The fast transition rate is beneficial to reduce the oxidation weight of the coating.When the content of Sm₂O₃ is 3 wt.%,the laser cladding NiCoCrAlY coating has the best oxidation resistance.