| With the continuous improvement of the propulsion ratio of aeroengines,the heat-carrying capacity of engine hot-end components needs to be further improved,and a comprehensive balance matching of various performances such as high temperature strength,creep resistance,room temperature toughness,oxidation resistance and density needs to be achieved.In recent years,the Nb-Si-based ultrahigh temperature alloy has the potential to be used under the new target temperature,which is 200?300? higher than the service temperature of Ni-based superalloy.Nb-Si-Ti ultrahigh temperature alloy contains hard strengthening phase(Nb,Ti)5Si3 and toughness Nb-based solid solution(Nb,Ti)ss.Silicide Nb5Si3 has better high temperature strength and oxidation resistance,and the solid solution(Nb,Ti)ss has good toughness at room temperature.The results show that new strengthening phases can be obtained and the content of strengthening phase and toughness solid solution can be changed to improve the overall performance of the alloy by adding alloying elements.Laser direct deposition technology is based on the principle of rapid prototyping,combining the solidification principle of CAD,laser technology and materials to form a new material preparation technology,which has the advantages of flexible production process,short cycle,fine structure and so on.The Nb-16Si-20Ti ultrahigh temperature alloy is prepared by laser direct deposition technique.The alloy composition is optimized by adding a certain amount of alloying elements Al and B in the Nb-16Si-20Ti ultrahigh temperature alloy,and the microstructure and properties are preliminarily characterized and evaluated.In this paper,the laser process parameters are optimized by laser direct deposition of single channel Nb-16Si-20Ti material.The optimized parameters contains laser power 550W,laser scanning speed 5.40mm/s and overlap rate 40%.The bulk samples of Nb-16Si-20Ti,Nb-16Si-20Ti-x Al(x=0,2,3,4at.%)and Nb-16Si-20Ti-3Al-yB(y=5,10,15at.%)are successfully prepared using optimized process parameters.The microstructure and mechanical properties are investigated by optical microscope,scanning electron microscope,X-ray diffraction,micro vickers hardness tester,friction and wear tester and high temperature resistance furnace.The effects of Al element and B element for microstructure and properties of Nb-16Si-20Ti ultrahigh temperature alloy are as follows.The Al element not only promotes the formation of Nbss.Nb5Si3 eutectic structure but also suppresses the formation of the Nb3Si phase.With the increase of Al content,the eutectic structure is refined and the microhardness of the upper region of the alloy deposit is on the rise,indicating that the formation of a large amount of eutectic structure can improve the microhardness of the alloy.With the increase of Al content,the oxidation resistance of the alloy increases.The oxidation rate decreases from 11.08?m/h to 4.99?m/h with the Al content from 2at.%to 4at.%.Nb-16Si-20Ti-4Al alloy consists of a large amount of eutectic and a small amount of toughness(Nb,Ti)ss phase,which effectively improve the oxidation resistance of the alloy.B element promotes the formation of Nb3Si phase and reduces the formation of Nbss/Nb5Si3.A large amount of dendritic and large-size Nb3Si phase appear in the microstructure of Nb-16Si-20Ti-3Al alloy with 5at.%?10at.%B element.A large amount of tetragonal(Nb,Ti)ss and hexagonal ?(Nb,Ti)5(Si,B)3 phases appear in the microstructure of Nb-16Si-20Ti-3Al alloy with 15at.%B element.With the increase of B content,the microhardness of the prepared alloy shows a sharp upward trend,and the wear resistance is effectively improved,but the oxidation resistance of the alloy shows a downward trend due to the presence of a large amount of high temperature unstable(Nb,Ti)3(Si,B)phase.B element content should be controlled below 5at.%. |
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