Microstructural Optimization And High Temperature Properties Of Novel Nickel Base Single Crystal Superalloy Turbine Blade

Nickel base single crystal superalloy has been widely used in advanced aeroengine hot end components for its superior performance.However,with the continuous improvement of the temperature in front of the turbine,the preparation of low rhenium and high performance nickel based single crystal hollow turbine blades with complex inner cavity structure has become the key to restrict the development of aviation industry.Therefore,the optimization and control of the microstructure of the new type of low rhenium nickel base single crystal alloy hollow turbine blade has become a key issue in the research of preparation technology and high temperature performance of single crystal blade.In the past,the ceramic mold shell for blade was mostly prepared by equiaxed crystal blade technology,but the design of composite ceramic mold shell for single crystal alloy was lacking.In addition,most of the previous microstructure studies focused on the solidification analysis of solid blade,and did not systematically study the blade hollow structure characteristics.Thirdly,it is difficult to control the casting defects of single crystal blade,low yield and lack of research on high temperature performance.Based on these problems,a new type of low rhenium nickel base single crystal alloy is selected as the research object in this paper.Through the preparation process of single crystal hollow turbine blade samples,the performance characteristics of carbon fiber reinforced composite mold shell and its influence on the single crystal structure are studied,and the growth law of nickel based single crystal hollow turbine blade structure is explored by combining with various characterization methods The microstructure growth and the effect of casting defects such as heterocrysts,broken arm crystals,freckles and so on.At the same time,the microstructure evolution law of the new type low rhenium nickel base single crystal alloy during heat treatment was analyzed,and its high temperature creep and oxidation behavior were investigated.The main conclusions are as follows:1.The results show that the thickness of carbon fiber reinforced composite mold shell is reduced by 32.26%,the temperature gradient at the front of solid-liquid interface is increased by 18.4%,the primary and secondary dendrite spacing of blade is reduced by 6.4% and 11.9%,eutectic phase content is reduced by 0.56%,micro shrinkage cavity is refined and dendrite segregation degree is reduced.2.The solidification behavior of Ni based single crystal hollow turbine blade shows that when the blade cross-section changes slightly,the isotherm moves along the axial direction,and there is a thermal barrier zone at the junction between the blade edge plate and the blade body.The dendrites in the blade deflect in the form of derived high-order dendrite arms and grow around the thermal barrier area,forming four distinct morphologic regions.The primary dendrite spacing in the blade increases with the increase of the height,and the dendrite spacing is larger at the coarse section.At the same position in the blade,the size,volume ratio and cubic degree of the main?? phase in the dendrite are lower than those in the interdendritic region.3.The research on casting defects of single crystal blade shows that shrinkage porosity is easy to form at the blade platform,and the formation of heterocrysts at this position is related to temperature gradient,platform length and inclination angle,and critical nucleation undercooling degree of alloy,and broken arm crystal is easy to form at the position with higher undercooling degree.In addition,the application of the technology can eliminate the supercooling at the far end of the platform,eliminate the broken arm crystal and inhibit the formation of heterocrystal,and significantly reduce the dendrite inhomogeneity at both ends of the platform.At the same time,the number of freckles increases first and then decreases with the increase of inclination angle.If the local contour is convex arc or edge angle,it is easy to form freckles.4.The results show that the residual eutectic phase decreases with the increase of solution temperature.With the increase of high temperature aging temperature,the size of ?? phase increases gradually,and the morphology of ?? phase changes from round to cubic,and then becomes unstable.In addition,the two-stage aging at high temperature can effectively avoid the coarsening of microstructure and obtain the?? phase with high cubic degree.The optimized heat treatment system is 1285 ?/ 1h + 1300 ? / 2h + 1310 ? / 2h + 1320 ? / 2h + 1330 ?/ 12 h,AC;1160 ?(1H)+ 1120 ?(4h);870 ? / 24 h,AC;the final volume ratio of ?? precipitates is 70.3%,and the average size is 0.42 ? M.5.The high temperature creep and fracture mechanism of Ni base single crystal alloy show that the creep life of the alloy is 133 h at 1100 ? / 137 MPa.At 15 h,a??-phase depletion layer with a depth of 10 ? m appeared on the surface of the sample,equiaxed recrystallization occurred at 40 h,and petal like structure similar to eutectic was formed at 65 H.At 95 h,needle like TCP phase appeared in the tissue and enriched W,re,Ta and other elements.For the fracture mechanism,the directional coarsening of ?? phase is basically completed at 65 h,diamond shaped crack appears near the micro shrinkage cavity at 95 h,and then the fracture surface is formed by crack aggregation.The fracture is composed of dimple and tear edge,and there is no obvious slip surface.There is recrystallization in the internal crack near the fracture.6.The high temperature oxidation behavior of a new type of low rhenium nickel base alloy was studied.The results show that the oxidation weight gain curve at900 ? satisfies the parabolic law.The thickness of the oxide film is 30 ? m at 200 h.the outer layer is cylindrical Ni O and the inner layer is dense Al2O3.At 1100 ?,the internal oxidation is significant,and the outer oxide layer is loose and easy to fall off.The weight gain curve deviates from the parabolic law,and the oxidation layer depth is 80 ? m at 200 H.Cr element is dispersed in the oxide layer,while Al is densely distributed.The oxide layer is relatively dense,which blocks the further diffusion of O element.The high temperature oxidation behavior has little effect on the distribution of Re.