Research On The Sub-Rapid Solidification Process Of CoCrNi Cast Alloys

The Superalloys are a kind of metal materials working at a high temperature of 600 ?under certain stress.They have good overall performance which are used in aerospace,nuclear power,chemical and other industries.High-temperature alloy castings prepared by traditional casting methods have some serious defects such as coarse grains and segregation.The castings prepared by rapid solidification technology can overcome the defects produced of conventional casting and improve the properties.However,rapid solidification technology requires high equipment and complex processes,making it difficult to industrialize mass production.The sub-rapid solidification technology has the general characteristics of rapid solidification technology and has broad application prospects.At present,there are a few researches on the sub-rapid solidification technology of superalloys at home and abroad,and the microstructure evolution of cobalt-based alloys in the range of sub-rapid solidification is also unclear.Therefore,in this paper,the cobalt-chromium-nickel casting alloy is prepared by arc melting and vacuum suction casting,and the microstructure evolution of the sub-rapid solidification process is studied in detail.Combined with the numerical fitting method,cobalt-chromium-nickel cast crystal growth model and its pseudo binary alloy phase diagram were established.The detailed research contents and results are as follows:The casting process of cobalt-chromium-nickel alloy was studied by Pro CAST software,and the solid-liquid crystallization rate of the cobalt-chromium-nickel alloy was calculated.The reliability of the calculation was verified by the analysis of the dendrite spacing of the casting alloy.The solidification crystallization rate is 61K/s?120K/s,which belongs to the range of sub-rapid solidification.As the plate thickness decreases,the cooling rate increases,and the dendrite spacing decreases.The microstructure evolution of the cobalt-chromium-nickel casting alloy in the range of sub-rapid solidification rate was studied.The results showed that the sub-rapid solidification structure of the cobalt-chromium-nickel cast alloy was cellular dendrites.In a wide range of composition,the structure of the cast alloy is two-phase structure,the dendritic stem is FCC phase,and the interdendritic is HCP phase.The main solute elements Cr and Ni segregate between the two phases,the solute element Cr is concentrated in the HCP phase between the dendrites,and the Ni element is concentrated in the FCC phase of the dendrites.The concentration of the solute elements of Ni and Cr in the two phases is uniform,and there is an obvious concentration difference between the two phases.As the plate thickness increases,the cooling rate decreases,and the degree of Ni and Cr segregation between the two phases decreases.The pseudo-binary phase diagram of Co-Cr-Ni alloy was drawn by thermodynamic calculation.There is a big gap between the thermodynamic calculation results based on the existing data and the experimental results,which is caused by the influence of alloying elements.The chromium-nickel component equivalent method should be used for cobalt-chromium-nickel alloy microstructure composition and phase analysis,with carbon as the nickel equivalent element and Mo and Fe as the chromium equivalent element.The phase diagram of the formed pseudo-binary alloy and the experimental results show higher consistency.When the Creq/Nieq equivalence ratio is greater than 1.87,the alloy has a dual-phase structure of FCC and HCP.When the Creq/Nieq equivalence ratio is 0.18,the alloy structure is a FCC single-phase structure.The volume fraction of the FCC phase decreases with the increase of the chromium-nickel equivalent ratio,and the alloy solidification interval increases accordingly.