Impact Of Refractory Elements On Microstructure And Properties Of Ni-based Single Crystal Superalloys

Nickel-based single crystal superalloys are widely used in the turbine blades for industrial gas and aerospace turbine engines owing to their excellent high-temperature creep strength,fatigue crack growth resistance and corrosion resistance.The addition of Re has the most striking effect on improving the creep resistance of nickel-based single crystal superalloys.However,the increment of Re content remarkably degrades the castability of single crystal superalloy blades,as well as deteriorate long-term microstructure stability at high temperature corresponding to of the precipitation of TCP phases.To overcome this,it was widely accepted that the addition of Ru can delay the precipitation of TCP phases,but it is believed that Ru cannot completely avoid the precipitation of TCP phases.The reason why Re can significantly improve the high-temperature properties of ni-based single crystal superalloys is that Re has a very low diffusion coefficient in Ni matrix at high temperatures.Previous computational and experimental studies have shown that Os has a lower diffusion coefficient than Re at high temperature,and Os has the same number of electron vacancies as Ru.In this research,Os was used to replace the Re element of nickel-based single crystal superalloys,and two different Os-containing nickel-based single crystal superalloys(CSU-Os-1 and CSU-Os-2)are designed by tuning Cr:Co ratio.The heat treatment procedures of two novel alloys were optimized,the corresponding microstructure and elemental segregation behavior of ascast and heat-treated alloys were investigated.Oxidation behavior at1100 ℃/200 h,the microstructure stability at 1100 ℃/1000 h and the creep properties at 980 ℃/200 MPa/100 h were tested,and results were used to compare with those of the second-generation nickel-based Re-containing single crystal superalloy CMSX-4.The conclusions are made as follows:(1)Os and Re segregate in the dendrite core and γ phase.Compared with Re,Os can reduce the percentage of γ/γ′ eutectic and primary dendrite spacing,different Cr:Co ratios have great influence on the percentage of eutectic and primary dendrite spacing of the alloys.(2)After the oxidation experiment at 1100 ℃/200 h,no Os-containing oxides were found in the oxide layer of the alloy,which is consistent with the results of CMSX-4 alloy that has no Re-containing oxides.Till to 150 h,the oxidation weight gain rate of CSU-Os-1 alloy(high Cr:Co ratio)was higher than that of CSU-Os-2 alloy(low Cr:Co ratio),from 150 to 200 h,the oxidative weight gain rate had the opposite trend.(3)The long-term aging experiment at 1100 ℃ and the high temperature creep experiment at 980 ℃/200 MPa/100 h show that the creep strains of CSU-Os-1,CSU-Os-2 and CMSX-4 alloys at 100 h are 0.464%,0.062%and 0.085%,respectively.It is confirmed that replacing Re with Os can improve the creep resistance and microstructure stability of ni-based single crystal superalloys.At the same time,reducing the Cr:Co ratio is beneficial to improve the high temperature microstructure stability and creep properties of nickel-based single crystal superalloys.