Microstructure Evolution And Element Interdiffusion Behavior Of Interface Between PtAl Coating And Thin-walled Single Crystal Superalloy

Nickel-based single crystal superalloys have excellent high temperature mechanical properties and are widely used in advanced aero-engine and gas turbine turbine blades.The complex service environment and high thrust ratio design of a new generation of aero-engines and gas turbines have put forward higher requirements for single-crystal turbine blades.Nickel-based single crystal superalloy itself cannot meet the protection requirements of oxidation resistance and corrosion resistance at higher temperatures,and advanced gas film cooling technology and high temperature protective coating are the key technologies to improve its service performance.The wall thickness of advanced single crystal turbine blades is as low as 1.0 mm,and the film cooling technology drives the development of single crystal blades towards thinner wall thicknesses.PtAl coating has excellent anti-oxidation and anti-corrosion properties and is widely used as a high temperature protective coating for turbine blades.However,there is an obvious element concentration gradient between the PtAl coating and the nickel-based single crystal superalloy matrix.Under the condition of high temperature service,it is difficult to avoid the mutual diffusion of elements between the PtAl coating and the nickel-based single crystal superalloy matrix.The interdiffusion of elements makes the topologically close-packed phase(TCP)precipitate near the high temperature interface of the coating/Ni-based single crystal,forming an interdiffusion zone(IDZ)and a secondary reaction zone(SRZ),and the higher the generation of Ni-based single crystal superalloy,the higher the content of refractory elements,the increased tendency of TCP precipitation,which makes the PtAl coating/Ni-based single crystal superalloy system fail prematurely.In this thesis,the PtAl coating/Ni-based single crystal superalloy(second generation N5,third generation DD9 and fourth generation DD91)system is the research object.PtAl coating is prepared on the surface of nickel-based single crystal superalloy by electroplating Pt and high temperature and low activity gas phase aluminizing.Scanning electron microscope(SEM),X-ray diffraction(XRD),transmission electron microscope(TEM),X-ray energy dispersive spectrometer(EDS),electron probe(EPMA)and other characterization methods and high-throughput multi-element alloy diffusion simulation calculation software Hit DIC are used.The microstructure evolution mechanism and element interdiffusion mechanism at the interface of PtAl coating/Ni-based single crystal superalloy under different temperature,matrix thickness and different generation conditions are studied.The experimental results are as follows:(1)There are obvious differences in the interface microstructure evolution,element diffusion coefficient and diffusion flux of PtAl coating/DD9 nickel-based single crystal superalloy samples at different temperatures.After isothermal oxidation at 1000°C/52 h and1100°C/18 h,both samples formed an interdiffusion zone(IDZ)with?and R topologically close-packed phases(TCP)and a secondary reaction zone with?and P phases(SRZ).Compared with 1000?,the diffusion coefficient of each element is 1?2 orders of magnitude higher and the diffusion flux increases significantly at 1100?.During the isothermal oxidation process at 1100°C,the increase of the outdiffusion of refractory elements such as Re,W and Ta leads to the increase of the thickness of the SRZ,and the intensification of the interdiffusion of Al and Ni further promotes the growth of the SRZ and transformation of?-(Ni,Pt)Al phase to??-Ni₃Al phase.(2)During the isothermal oxidation process at 1100?,the thickness of the single crystal superalloy substrate has a significant effect on the microstructure evolution of the PtAl coating/DD9 nickel-based single crystal superalloy interface.The thicknesses of IDZ and SRZ increased during the isothermal oxidation process for samples with three substrate thicknesses(0.5 mm,1.0 mm,2.0 mm).The IDZ thickness of the samples with different substrate thicknesses is about 25?m.The content of TCP phase in the IDZ of the three samples decrease with the prolongation of the isothermal oxidation time,and only the sample with a substrate thickness of 0.5 mm formed a TCP phase“depletion zone”in the IDZ.The enrichment of Ta in IDZ and SRZ is the main reason for the precipitation of TCP phase and the formation of SRZ.After isothermal oxidation for 500 h,the SRZ thickness of the sample with a substrate thickness of 0.5 mm is significantly smaller than that of the other two samples,and the refractory elements such as W,Re,and Ta of the sample with a substrate thickness of 0.5 mm are homogenized earlier,and the element concentration gradient is low,decreasing the SRZ growth rate.(3)Different generations of nickel-based single crystal superalloys(N5,DD9,DD91)/PtAl coatings have significant differences in the interface microstructure evolution during the constant temperature oxidation process at 1100°C.After isothermal oxidation for1000 h,the IDZ thickness of the N5 and DD91 samples is about 30?m,the IDZ thickness of the DD9 sample is about 24?m,and the SRZ thickness of the DD91,N5 and DD9 samples is about 136?m,108?m,89?m,respectively.In the process of isothermal oxidation,the coatings of the three coating samples all transformed from?-(Ni,Pt)Al phase to??-Ni₃Al phase,but the phase transition rates in the three samples are DD9 sample,N5 sample,and DD91 sample in descending order.The difference in the diffusion flux and diffusion coefficient of each element in the three samples is the fundamental reason for the different phase transition rates.The enrichment of Ta element in IDZ and SRZ is the main reason for the precipitation of TCP phase and the formation of SRZ.With the prolongation of isothermal oxidation time,the refractory elements such as W,Re and Ta in the three samples are gradually homogenized,and the element concentration gradient is low,which reduces the growth rate of SRZ..