| Nickel-based single crystal superalloy has excellent high temperature oxidation resistance and high temperature mechanical properties.These excellent properties are mainly due to the solid solution strengthening ofγphase and precipitation strengthening ofγ′phase,which can be obtained by adjusting the type and content of alloying elements and controlling the microstructure of the alloy.The proper addition of rare earth elements is of great benefit to improve the serviceability of nickel base single crystal superalloys,especially the oxidation resistance at high temperature.In this paper,a typical second generation single crystal superalloy CMSX-4 was selected as the base alloy,and three kinds of single crystal cylindrical bars with different contents of rare earth were prepared by adding rare earth elements La and Y.The heat treatment system of CMSX-4 alloy and rare earth modified alloy was investigated,and the cyclic oxidation experiments were carried out at 1000℃and 1100℃,and the cyclic oxidation behavior of the alloy was comprehensively analyzed.The main research contents and relevant conclusions are as follows:（1）CMSX-4 single crystal superalloy test rod with rare earth elements Y and La was designed and prepared.By means of scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）,electron probe（EPMA）and Image J image analysis software,the initial molten pool,eutectic phase dissolution rate,porosity and element diffusion in the alloy were analyzed,and the appropriate solution heat treatment system was optimized.The morphology and size ofγ′phase in the alloy were analyzed and the appropriate aging heat treatment system was obtained.（2）200 cycles of oxidation experiments were carried out at 1000℃and 1100℃respectively,and the corresponding oxidation kinetics curves were obtained.With the increase of cycle times,the weight of the four alloys increased gradually,among which the oxidation kinetics curve of CMSX-4+17 ppm Y alloy was the lowest,and that of CMSX-4+12 ppm Y alloy and CMSX-4+3 ppm La alloy was close.The oxidation kinetics curve of CMSX-4 base alloy is the highest.The results show that the proper addition of rare earth elements Y and La can effectively improve the high temperature cycle oxidation performance of CMSX-4 alloy.（3）The oxidation products and oxide layers were analyzed by X-ray diffraction（XRD）,SEM and EDS.The results show that:At 1000℃,the cross section oxide layer of CMSX-4+17 ppm Y alloy is divided into two layers,the outer layer is the complex spinel layer embedded with Ni O,Ti and Ta oxide particles,the inner layer is the Al2O3layer,and the outer layer of the other three alloys is more than the CMSX-4+17 ppm Y alloy,one layer of dense Ni O layer.Theγ′-free region of CMSX-4+17ppm Y alloy is the smallest,and the thickness is only 2.83μm,which is significantly lower than 5.13μm of CMSX-4 base alloy.At 1100℃,the cross-sectional oxide layers of the four alloys are both inner and outer layers,the outer layer is complex spinel layer embedded with Ti and Ta oxide particles,and the inner layer is Al2O3layer.The cross section oxidation layer of CMSX-4+3ppm La alloy is regular and uniform,while the cross section oxidation layer of CMSX-4 alloy,CMSX-4+17ppm Y alloy and CMSX-4+12ppm Y alloy is discontinuous and cracks exist.Theγ′-free region of CMSX-4+3 ppm La alloy is about 4.32μm,which is significantly improved from 9.92μm of CMSX-4 base alloy. |
