Phase Diagram Determination And Thermodynamic Optimization Of The Systems Related To Co-based Superalloys Containing Refractory Elements

Recently,more and more novel Co-based superalloys with γ’(L12)and γ(fcc)phases have been found and fabricated,which provided guidance for the design ofγ/γ’ Co-based superalloys.Most of the novel Co-based superalloys are composed of ternary,quaternary,and quinary systems.The comprehensive properties of novel Co-based superalloys with low component system are inferior and can not meet the complex service environment.Multicomponent alloying is used to improve the comprehensive properties of superalloys.The constituent phases,constituent microstructures,and phase transition temperatures of multicomponent superalloys in equilibrium and non-equilibrium states can be obtained by using CALPHAD method,which provides a theoretical basis for composition design and process parameter selection of superalloys.This study focuses on the phase diagram determination and thermodynamic optimization of the Co-Hf-Mo,Hf-Mo-Ni,and Ni-Ta-W systems for Co-based superalloys containing refractory elements by means of alloying method,diffusion couple technique,and CALPHAD method.According to the advantages and disadvantages of the current method for preparing diffusion triples,two kinds of graphite molds were designed,and the diffusion triples(Co-Hf-Mo,Hf-Mo-Ni,and Ni-Ta-W)have been successfully fabricated by SPS with reasonable process parameters.Compared with HIP,the method for preparing diffusion triple using SPS is high-efficiency in process and low in price.The main work is as follows:(1)The phase equilibria of the Co-Hf-Mo system at 1373,1223,and 1123 K were established according to the analysis of diffusion triples and equilibrated alloys.Three isothermal sections of the Co-Hf-Mo system were constructed,and the structure and composition range of ternary compound τ was determined.Based on the data reported in literature and the experimental results in this study,the temperature range of Co2Mo3 was determined and the Co-Mo system was optimized.To keep the consistency of Co-Hf system parameters with the database of Co-based superalloys of our group,the Co-Hf system was optimized.Based on the optimized Co-Hf and Co-Mo systems and the reported Hf-Mo system thermodynamic parameters,combined with the experimental data determined in this work,the Co-Hf-Mo ternary system was optimized.A set of self-consistent thermodynamic parameters of was obtained and the calculated isothermal sections were basically consistent with the experimental data of this work.(2)The phase equilibria of the Hf-Mo-Ni system at 1373,1223,and 1123 K were established according to the analysis of diffusion triples and equilibrated alloys.Three isothermal sections of the Hf-Mo-Ni system were constructed and the ternary compound τ with Hf2Rh structure was determined.The Hf3Ni was proven to be stable according to the analysis of as-cast and annealed Hf-Ni alloys.Based on the data reported in literature and the experimental data in this study,the phase equilibrium relationships of Hf-rich in the Hf-Ni system was corrected and the Hf-Ni system was optimized.Based on the optimized Hf-Ni and the reported Hf-Mo and Mo-Ni systems thermodynamic parameters,combined with the experimental data determined in this work,the Hf-Mo-Ni system was optimized,and the calculated isothermal sections were basically consistent with experimental data of this work.(3)The primary phases and solidification paths were determined by as-cast alloy,and the liquidus surface projection of Ni-Ta-W system was constructed.The phase equilibria of the Ni-Ta-W system at 1473 and 1273 K were established according to the analysis of annealed diffusion triples and equilibrated alloys,and two isothermal sections of the Ni-Ta-W system were constructed.According to the Ni-Ta,Ni-W,and Ta-W systems thermodynamic parameters reported in literature,combined with the experimental data determined in this work and reported in the literature,the Ni-Ta-W system was optimized,and the present experimental results and literature data were well reproduced.