| Nickel-based single crystal superalloys become the preferred materials for aeroengine turbine blade due to their excellent elevated temperature mechanical properties,outstanding oxidation resistance and wonderful corrosion resistance.The addition of refractory elements W and Re can significantly improve the high temperature mechanical properties of the alloy.Understanding the effects of W and Re on the microstructure evolution in the superalloy is of great significance to improve the service temperature of nickel-based single crystal superalloy.In the actual production process of single crystal blade,the control and elimination of stray grains can significantly improve the qualified rate of blade,thus greatly reducing the cost of single crystal blade material.In this paper,we use the OM,SEM,EPMA,SEM in-situ tensile,spherical aberration corrected TEM,EBSD and Pro CAST simulation to study the microstructure evolutions of model single crystal superalloys during thermal exposure and room temperature tension,and the formation of stray grain in the transition region between airfoil and platform of a single crystal turbine blade.The main conclusions are summarized as follows:1.In the casting of Ni-Al-W and Ni-Al-W-Re model alloys,W and Re segregate in the dendrite core,?' precipitates are coarse and irregular.After standard heat treatment,the segregation of W is basically eliminated and the segregation of Re is obviously reduced,and the cubic degree of ?' phase is better.In the Ni-Al-W and Ni-Al-W-Re model alloys,W is preferentially distributed in ?' phase and occupies Al site,Re is preferentially distributed in the ? phase.2.With the addition of W and W-Re,the growth rate of ?' phase is obviously reduced,and the thermal stability of ?/?' interface is improved.At the same time,the room temperature strength of the alloy is improved,and the dislocation configuration changes into dislocation pairs coupled with APB and dislocations with stacking fault in the ?' phase.3.The stray grains in the transition region between airfoil and platform are banded.And their length direction is parallel or perpendicular to the dendrite growth direction of airfoil and platform in macro scale;the dendrites tilt to the same direction in the defect region in micro scale.And the composition of the stray grain is not significantly different from that of the airfoil and platform.Due to the poor heat dissipation condition and low undercooling,the transition zone between airfoil and platform becomes the last solidification zone on the platform,and the dendrite also transforms into spindle shape.The solidification shrinkage stress at the edge of the first solidified platform is large,which leads to the fracture of the third dendrite in the transition zone.furthermore,the broken dendrites growth and become stray grains.The innovation of this paper is as follows:1.For ternary Ni-Al-W and quaternary Ni-A-W-Re model superalloys,1)the direct experimental evidence of phase preference and site occupation of W and Re is given by the STEM-HAADF mode of TEM with spherical aberration correction.W prefers distribution in ?' phase and occupies Al site,Re prefers distribution in ? phase.2)It is found that the addition of W or W/Re can promote the stability of ?/?' interface through the heat exposure experiment.3)It is clear that W or W/Re can reduce the stacking fault energy of ?' phase by SEM in-situ tensile test and TEM characterization.2.The formation mechanism of a new stray grain defect was studied by combining various experimental characterization techniques and finite element simulation.It is found that the transition zone of airfoil and platform becomes the last solidification area because of the poor heat dissipation,and the shape of dendrite become spindle as the low undercooling.The solidification shrinkage stress,which is caused by the first solidified edge of platform,is large and results the fracture of the third dendrite in the transition zone between airfoil and platform.And the broken dendrites become the stray grains. |
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