Effect Mechanism Of B,C And Co On Microstructure And Mechanical Properties Of K4750 Alloy

With the continuous improvement of thrust-weight ratio and thermal efficiency of advanced aero-engines,higher requirements were put forward for the temperature bearing capacity and high-temperature service stability of materials used in the key hotend parts of aero-engine.For example,the skew plate bearing frame of domestic aeroengine was faced with problems such as insufficient temperature bearing capacity of materials in service and processing difficulties of imported foreign alloys.Therefore,a nickel-based cast superalloy K4750 with temperature bearing capacity up to 750℃ was developed by the Institute of Metal Research,Chinese Academy of Sciences.Because the skew plate bearing frame had to withstand the interaction of high temperature and complex stress for a long time,K4750 alloy must have both excellent high temperature strength and performance stability under long-term service.However,preliminary study showed that the microstructure of K4750 alloy degraded and premature fracture mainly occurred in intergranular mode after aging at 750℃ for 3000 h.In this paper,aiming at the problems of microstructure degradation and grain boundary weakening of K4750 alloy after long-term aging,taking advantage of the potential advantages of B and C in improving grain boundary strength and Co in improving microstructure stability,the influence of B,C and Co on the microstructure and properties of K4750 alloy was studied,and the action mechanism of these three key elements in K4750 alloy was clarified.The reasonable control range of B,C and Co was proposed to provided reliable basis for obtaining K4750 alloy with excellent comprehensive properties.The effect of B on microstructure and mechnical properties of K4750 alloy was systematically studied.It was found that M5B3 boride precipitation in as-cast microstructure of K4750 alloy containing B,and the increase of B content could promote the precipitation of M5B3 and the transformation of boride morphology from lamellar to strip.With the increase of B content from 0 to 0.010 wt.%and then to 0.014 wt.%,the morphology of M23C6 carbide in K4750 alloy after heat treatment changed from continuous to granular and then to chain,the stress rupture life increased from 89 h to 190 h and then decreased to 135 h,and the elongation increased from 7%to 23%and then decreased to 16%.The difference of microstructure and properties of K4750 alloy with various B content was related to the existence state of B.In 0B alloy,continuous M23C6 carbide at grain boundary could promote the initiation of intergranular micro-cracks.In 0.010B alloy,the segregation of B primarily at grain boundaries could fill lattice defects and impeded grain boundary migration and element diffusion,which could improve grain boundary bonding and optimize the morphology of M23C6 carbides,and thus inhibited micro-void formation and micro-crack propagation.In 0.014B alloy,as borides precipitated and grew up along grain boundary,the density of intergranular M23C6 carbide increased and presented a chain distribution,which weakened the improvement effect on stress rupture properties.After aging at 800℃ for 1000 h,the segregation of B primarily at grain boundaries in 0.01OB alloy delayed M23C6 carbides agglomeration and η phase precipitation,which improved grain boundary microstructure stability and stress rupture properties.The effect of C on microstructure and mechnical properties of K4750 alloy was studied.It was found that the solidification path of K4750 alloy did not change with the rising C content,but with the increase of carbon content,the liquidus temperature of K4750 alloy decreased,while the solidus temperature and MC carbide precipitation temperature increased.When C content increased from 0.07 wt.%to 0.10 wt.%and then to 0.12 wt.%,the size and quantity of MC and M23C6 carbide increased gradually and their morphology changed,but the grain size decreased gradually.The stress rupture life of 0.07C,0.10C and 0.12C alloys was 162 h,222 h and 184 h,respectively,and the elongation was 5%,12%and 10%,respectively.The difference of stress rupture properties was related to the characteristics of carbides.In 0.07C alloy,relatively few carbides had limited binding effect on grain boundaries,so grain boundaries were prone to slip and deformation at elevated temperature.The increase of carbide in 0.10C alloy could effectively nail grain boundaries and inhibited the formation and connection of micro-voids,thus improved the stress rupture properties of 0.10C alloy.The size and quantity of carbide in 0.12C alloy increased further,which promoted the initiation and propagation of intergranular micro-cracks,thus accelerated the failure of 0.12C alloy.The effect of Co on microstructure and mechnical properties of K4750 alloy was studied.It was found that adding 4 wt.%Co could increase the contents of Ti and Nb in γ’ phase and MC carbides,especially Nb,which was related to the decrease of solid solubility of Ti and Nb in γ matrix.The increase of Ti and Nb contents in γ’ phase could improve the precipitation strengthening effect of γ’ phase,and thus increased the stress rupture life of K4750 alloy from 155 h to 251 h.After aging at 750℃ for 3000 h,it was found that the stability of γ’ phase and MC carbides in 4Co alloy was better than that of OCo alloy,which could not only ensure the precipitation strengthening effect ofγ’ phase,but also reduce the tendency of micro-crack initiation.Therefore,after longterm aging,the stress rupture life of 4Co alloy(189 h)was significantly higher than that of 0Co alloy(45 h).The study of this paper not only provided an important theoretical basis for optimized B,C and Co content and improved service stability of K4750 alloy,but also extended the understanding of action mechanism of B,C and Co elements in nickelbased superalloy.