Phase Diagram Determinations And Thermodynamic Optimizations Of The Co-Ni-X(X=Ta,Ti,Nb) Ternary Systems For Novel Co-based Superalloys

The traditional cobalt-based superalloys exhibit excellent hot corrosion and thermal fatigue resistance,but further application as components in the hot end of aeroengines and gas turbines is limited because their high-temperature strength is not as good as that of the ?'-Ni3Al hardened nickel-based superalloys.However,it is hardly permissible to raise the operating temperature of Ni-based single-crystal superalloys because it is already very close to the melting point.In recent years,the discovery of ?'-Co3(Al,W)in the Co-Al-W ternary system breaks a new ground for the development of novel Co-based superalloys.Given that the melting point of Co is 40? higher than that of Ni,novel Co-based superalloys are highly promising to be a new generation of high-temperature structural materials.Because of the novel Co-based superalloys' lower ?' solvus temperature compared to their Ni-based counterparts,how to raise the y' solvus temperature has become the primary problem to solve during the development of novel Co-based superalloys.Thermodynamic database based on the CALPHAD method plays an irreplaceable part in the prediction of basic properties,such as ?' solvus temperature,?' volume fraction,etc.,which still requires relevant phase equilibria and thermochemical information.However,the lack of phase equilibria data seriously hold back the further development of novel Co-based superalloys.Nickel is a ?' phase forming element,which can increase the ?' solvus temperature with little influencing on the solidus temperature.Tantalum,titanium and niobium are strong ?' phase forming elements,which can significantly improve the ?' phase solvus temperature,and also greatly reduce the solidus temperature.However,an excessive amount of them can lead to the precipitation of the topologically close-packed(TCP)phases,which will deteriorate the mechanical properties of superalloys.Therefore,the paper focuses on the phase diagram determinations and thermodynamic optimizations of the Co-Ni-Ta,Co-Ni-Ti and Co-Ni-Nb ternary systems for novel Co-based superalloys.The main research work are as follows:(1)The liquidus surface projection and 1473K isothermal section of the Co-Ni-Ta ternary system were constructed based on the analysis of several as-cast and equilibrated alloys.The Co-Ta and Ni-Ta binary system were optimized with the help of the key experiments and the first-principles calculations.On the basis of the reported Co-Ni and the modified Co-Ta and Ni-Ta binary thermodynamic parameters,combined with the measured and reported experimental data,a reliable thermodynamic assessment of the Co-Ni-Ta ternary system was achieved and a set of reasonable and consistent thermodynamic parameters was obtained as well.(2)The liquidus surface projection and 1373/1273K isothermal sections of the Co-Ni-Ti ternary system were established by analyzing some as-cast and equilibrated alloys.Based on the reported Co-Ni and Ni-Ti and the modified Co-Ti binary thermodynamic parameters,combined with the present and reported experimental information,the Co-Ni-Ti ternary system was assessed,and the calculated results turned out to be in good agreement with the available experimental data.(3)The 1373/1173K isothermal sections of the Co-Ni-Nb ternary system were experimentally determined.On the basis of the reported Co-Ni and the modified Co-Nb and Ni-Nb binary thermodynamic parameters,along with the measured and reported experimental data,the Co-Ni-Nb ternary system was optimized,and a set of thermodynamic parameters was obtained as well,which can reproduce most of phase equilibria information.(4)Some deficiencies of the PanCobalt thermodynamic database were pointed out by the comparison between the optimized results at present and the calculated ones from the database.