Study On The Microstructure Evolution And Interdiffusion Behavior Of Interface Between The PtAl Coating And The Fourth Generation Nickel-based Single Crystal Superalloy Under Geometric Constraint

Nickel-based single crystal superalloys are the prior choice material to manufacture turbine blades for aero-engines and industrial gas turbines,and β-(Ni,Pt)Al coating is widely used as a high-temperature protective coating for turbine blades for its excellent comprehensive performance of high-temperature oxidation resistance and thermal corrosion resistance.However,due to the notable difference of components between the coating and single crystal superalloy,the elemental interdiffusion between coating and substrate is inevitable under high-temperature thermal exposure or service conditions.It will lead to the degradation of the interface structure,the formation of the interdiffusion zone(IDZ)and secondary reaction zone(SRZ),which can dramatically deteriorate the high-temperature mechanical properties of superalloy and reduce the service life.On the one hand,with the addition of much more high-melting-point strengthening elements into the high generation single crystal superalloy,the mechanical degradation induced by interdiffusion between coating and substrate is more obvious in the high generation single crystal superalloy than in low generation single crystal superalloy.On the other hand,to meet the requirement of increasing turbine inlet temperature,the structure of blades becomes more complex and the thickness becomes thinner,even less than 1 mm.Thinner walls and complex shapes are typical structural characteristics of turbine blades for newly generation advanced aero-engine,and the interdiffusion between the coating and high generation single crystal superalloy which with complex shape and thinner walls will be more obvious.However,few researchers have studied the microstructural evolution of interface structure between Pt Al coating and single crystal superalloy associated with geometric constraints including wall thickness and curvature.In this paper,β-(Ni,Pt)Al coating and DD91 nickel-based single crystal superalloy was selected as the research object.Single crystal superalloy samples with different thicknesses(0.8 mm,1.0 mm and 1.3 mm)and surface curvature(plane,curvature radius 0.4 mm and curvature radius 0.65 mm)were processed by wire cutting,and β-(Ni,Pt)Al coating was prepared on the substrate surface via electroplating Pt and high-temperature low-activity gas aluminizing.Moreover,by changing the time of aluminizing,β-(Ni,Pt)Al coatings with different thicknesses were deposited on the samples with a substrate thickness of 1.0 mm.Combined with multi-scale experimental characterization and high throughput multicomponent interdiffusion coefficient calculating software Hit DIC,the effect of substrate thickness,coating thickness and substrate surface curvature on the elemental interdiffusion and microstructure evolution of β-(Ni,Pt)Al coating/single crystal superalloy interface during thermal exposure at 1100℃ were investigated.The main results are shown as followed:(1)The thickness of the substrate(0.8 mm～1.3 mm)has no obvious effect on the elemental interdiffusion between β-(Ni,Pt)Al and DD91 nickel-based single crystal superalloy during 200 h thermal exposure at 1100℃.The diffusion coefficients were calculated by Hit DIC software and the results shown that the main interdiffusion coefficients of each element in the β-(Ni,Pt)Al/DD91 nickel-based single crystal superalloy samples are mainly affected by Al,Co and Cr under this experimental condition.The element distribution at the interface of each sample and the thickness of IDZ and SRZ are similar.(2)After 100 h thermal exposure at 1100℃,the samples with different coating thickness obtained by changing gas aluminizing time showed significant differences in composition distribution and microstructure.The thickness of coating has an important effect on the elemental interdiffusion between β-(Ni,Pt)Al coating and DD91 nickel-based single crystal superalloy.There is no SRZ formed in the samples with a thinner coating(31 μm),while the SRZ formed in the samples with a thicker coating(40 μm).(3)During 200 h thermal exposure at 1100℃,the element distribution and microstructure evolution of the samples with different substrate surface curvature(plane,curvature radius 0.4mm and curvature radius 0.65 mm)were similar.Within the scope of this experiment,surface curvature has no obvious effect on the elemental interdiffusion and microstructure evolution between β-(Ni,Pt)Al coating and DD91 nickel-based single crystal superalloy.