Study On The Thermodynamics Of The Ni-Co-Al-Mo-W System And The Diffusion Kinetics Of Its Fcc Phase

With the development of nickel based and novel cobalt based superalloys,how to improve the stability of y' strengthening phase at high temperature,how to control the precipitation of TCP phases caused by adding of large amounts of refractory alloying elements,how to solve the current problems of materials thermodynamics which have not been solved properly and explore the universal research methods to design key experiments and calculation tools combined with diversified materials,such as CALPHAD method,first principle calculation and design of key experiments with accurate and efficient construction of thermodynamic phase diagram,how to investigate the diffusion behavior of the alloying elements to further explore the evolution process of microstructures and its influence on the mechanical properties at high temperature,will undoubtedly become the primary solutions to the problems of developing a new generation of superalloys.Therefore,it is very urgent to establish and improve the thermodynamic and kinetic database of the Ni-Co based superalloys.The focus of this work is on the thermodynamics of Ni-Co-Al-Mo-W system and the diffusion kinetics of its fee phase.The main work is as follows:(1)Thermodynamics:thermodynamic modeling and thermodynamic parameters of Ni-Co,Ni-Al,Ni-Mo,Ni-W,Co-Al,Co-Mo,Co-W,Al-Mo,Al-W,Mo-W,ten binary systems in all,have been respectively established and reassessed based on the existing experimental data and thermodynamic assessment in the literatures;isothermal sections of Ni-Co-Al,Ni-Al-W and Co-Al-Mo systems at different temperatures have been measured,thermodynamic modeling and thermodynamic parameters of Ni-Co-Al,Ni-Co-Mo,Ni-Al-Mo,Ni-Co-W,Ni-Al-W,Ni-Mo-W,Co-Al-Mo,Co-Al-W and Co-Mo-W ternary systems,have been respectively established and reassessed based on the existing experimental data and thermodynamic assessment in the literatures;isothermal sections of Ni-Co-Al-Mo and Ni-Co-Al-W systems at different temperatures have been measured and their thermodynamic modeling has been established based on available experimental data,isothermal sections of Ni-Co-Al-Mo and Ni-Co-Al-W systems at given temperatures were calculated by the established thermodynamic database of Ni-Co-Al,Ni-Co-Mo,Ni-Co-W,Ni-Al-Mo,Ni-Al-W,Co-AI-Mo,and Co-Al-W systems and compared with experimental results,the reliability of the constructed thermodynamic database were subsequently verified and thermodynamic database of Ni-Co-Al-Mo-W system were thus established;(2)Diffusion kinetics:parameters of self diffusion mobility of pure fcc-Ni,fcc-Co,fcc-Al,fcc-Mo and fcc-W,as well as the mobility parameters of the fee phase in the Ni-Co,Ni-Al,Ni-Mo,Ni-W,Co-Al,Co-Mo,Co-W,Al-Mo,Al-W and Mo-W systems,have been assessed based on the existing experimental data in the literatures,the kinetic database of 10 sub binary systems of the Ni-Co-Al-Mo-W system has been further constructed;concentration profiles of each diffusion couples prepared with fee alloys of Ni-Co-Al,Ni-Co-Mo,Ni-Co-W,Ni-Al-Mo,Ni-Al-W,Ni-Mo-W,Co-Al-Mo and Co-Al-W systems were measured and the corresponding interdiffusion coefficients at the crossing point of every two diffusion couples were calculated based on the measured concentration profiles and then the mobility parameters of fee phase of the above 8 sub ternary systems were optimized based on constructed thermodynamic database and the calculated interdiffusion coefficients together with the existing experimental data in the literatures,and meanwhile,the mobility parameters of fee phase of Co-Mo-W system were reassessed based on the experimental data in the literatures,and the kinetic database of 9 sub ternary systems of Ni-Co-Al-Mo-W system has been thus constructed;concentration profiles of each diffusion couples prepared with fee alloys of Ni-Co-Al-W and Ni-Co-Al-Mo-W systems were measured and well reproduced by the established thermodynamic database of Ni-Co-Al-Mo-W system and kinetic database of its fee phase,and the reliability of the established kinetic database were thus verified.(3)Based on the thermodynamics of the Ni-Co-Al-Mo-W system and its fee phase kinetics database established in this work,TC-PRISMA software was used to simulate the precipitation and growth process of y'-Co3(Al,W)phase in Co-9.3Al-7.4W(at.%)alloy after heat treated at 1173K for 5 min to 200 h,combined with thermophysical properties,such as molar volume,interface,etc.The results show that the simulated results are highly consistent with the experimental data and further highlight the application value of the database established in this work.