Design And Oxidation Mechanism Of High Performance MCrAlYHf (M=Ni,Co,Fe) Bond Coat Material For Thermal Barrier Coatings

Thermal barrier coatings（TBCs）are widely used in aero-engines and land-based gas turbines to reduce the temperature of hot-section components and protect them from high tempeareture gas corrosion and oxidation,thus improving the operation efficiency and enhancing the service life.After long time service under high temperature,TBCs will spall off and leave superalloy components indirectly exposed to high temperature gas,resulting in terrible accidents.The TBCs durability is mostly dependent on the oxidation performance of bond coat.NiCoCrAl Y alloy is widely used as the bond coat for TBCs to protect the superalloy components from oxidation at high temperatures.Such application relies on its capability to form a continuous,adherent and slowly growing alumina scale.The oxidation resistance of NiCoCrAl Y alloy is directly determined by its chemistry and microstructure.Therefore,this work mainly investigates the effect of chemistry and microstructure with a focus on Y distribution,grain size and porosity in the NiCoCrAl Yon the oxidation behavior.In addition,this work develops a novel Y and Hf co-doped AlCoCrFeNi high-entropy alloy（HEA）with ultra oxidation and spallation resistance,which is expected to be applied as a high-performance bond coat material for TBCs.The main research content and conclusions are described as follows:Ⅰ.The NiCoCrAl Y alloy with uniform Y distribution was fabricated via powder milling and spark plasma sintering.Compared to the alloy prepared from the as-received powder,the alloy prepared from the milled powder exhibits a much lower oxidation rate and a significantly better spallation resistance.The factors affecting the oxidation performance,e.g.,the microstructure,oxidation rate,stresses in the oxide scale and interfacial chemistry,were systemically investigated.After powder milling,Y is uniformly distributed in the alloy in the form of Ni-Y precipitates（50 to 150 nm）and nano-sized Y₂O₃ particles（5 to 30 nm）.It was found that the uniform Y distribution could:1)inhibit the formation of Y-Al oxide inclusions in the oxide scale,leading to a lower oxidation rate;2)reduce the growth stress in the oxide scale and lower the driving force for scale spallation;3)suppress S segregation to the interface and the formation of interfacial imperfections（e.g.oxide intrusions,pores）,leading to an enhanced interfacial adhesion.Consequently,the spallation resistance of the oxide scale is significantly improved.Apart from the NiCoCrAl Y alloy prapred from the milled powder,the uniform Y distribution could be still obtained in the bond coat prepared from the milled powder,which also leads to the significantly improved oxidation performance including a lower oxidation rate and stronger spallation resistance compared with the bond coat prepared from the original powder.Ⅱ.The NiCoCrAl Y bond coat fabricated by high-velocity air fuel（HVAF）is subsequently processed through vacuum heat treatment（VHT）or spark plasma sintering（SPS）to obtain different grain size and porosity.The HVAF bond coat possessed nanostructure with a porosity of about 3.58%.After VHT,the porosity is reduced to about 1.96%,but the pores at the unmelted particle boundary can not be completely eliminated and instead increases the grain size.After SPS,HVAF bond coat shows a fully dense microstructure with a porosity of about 0.16%and limits grain growth due to the capability of fast sintering of SPS.The nanocrystalline structute in the HVAF coating can facilitate the earlier establishment ofα-Al₂O₃in the early time,which is beneficial to lowering the TGO growth rate.However,the porosity rather than grain size plays a predominant role on the oxidation performance of bond coat for the long-term oxidation.The porosity,especially the the pores at the unmelted particle boundary will induce the formation of local spinel and severe internal oxidation,which are detrimental to the stability of oxide scale and thereby led to the eventual failure of oxide scale.Ⅲ.A new type of Y-Hf co-doped AlCoCrFeNi high-entropy alloy（HEA）was designed and fabricated,while systematically investigating the effect of Al content on the microstructure and oxidation behavior of Y-Hf co-doped AlCoCrFeNi alloy.The ultra oxidation and spallation resistance of this alloy at 1100°C make it to be promising bond coat material for TBCs.First,AlCoCrFeNi YHf alloy exhibits an extremely oxidation rate,the main reasons are given as following:1)the featured nanostructure of alloy facilitates the earlier establishment ofα-Al₂O₃ and skips the transit oxidation stage;2)the primaryα-Al₂O₃columnar grain structure in oxide scale and the uniform distribution of Y₃Al₅O₁₂ and Hf O₂ atα-Al₂O₃ grain boundary indicates that the long-term oxidation process is dominated by the inward O diffusion,thus solwing down the oxidation rate;3)compared to the conventional NiCoCrAl YHf alloy,the larger columnar grain size ofα-Al₂O₃ for the AlCoCrFeNi YHf alloy reduces the paths of inward O diffusion,thereby further lowering the oxidation rate.Second,no any spallation of oxide scale could be observed after 1000 h oxidation,which ensures a strong interfacial bonding.Both the slow oxidation rate and the good interfacial adhesion contributes to the superior spallation resistance.Moreover,depending on the Al content,the HEAs show two-phase or triple-phase microstructure.All HEAs exhibit extremely slow oxidation rates and meanwhile the oxidation rate gradually reduces with the increasing Al contents resulting from the gradually increasing columnar grain size in theα-Al₂O₃ scale.