Hot Deformation Induced Preparation Of Semi-solid Billets And High Temperature Short Time Oxidation Behavior Of CoCrCu_1.2FeNi High Entropy Alloy

The proposal of high entropy alloys breaks through the design concept of traditional alloys.Multi principal component design makes the alloy exhibit a series of excellent properties,such as high strength,high hardness,high specific strength,high wear resistance,excellent high temperature softening resistance,oxidation resistance and corrosion resistance.The application of semi-solid forming technology to high entropy alloys has been reported by scholars in recent four years.In this paper,the semi-solid billets of CoCrCu_1.2FeNi high entropy alloy with dual phases of face centered cubic structure were prepared by hot deformation induced spherulization method.The oxidation experiment was designed by simulating the heating state of the alloy during high temperature processing.The oxidation mechanism of the alloy was revealed,and the prevention strategy was provided to avoid oxidation.Based on the above researches,the semi-solid billets of the high entropy alloy were prepared by isothermal heat treatment in semi-solid state,and the whole process evolution mechanism of semi-solid microstructure formation was revealed.Hot deformation behavior,processing maps and microstructure evolution of CoCrCu_1.2FeNi high entropy alloy were studied.The results show that the flow stress decreases with the increase of temperature and the decrease of strain rate.The activation energy,constitutive model and processing maps of the alloy were obtained.The optimized processing parameters are(730-900?,0.04-1s^-1)and(900-950?,0.001-0.01 s^-1).Dynamic recrystallization occurs during hot deformation.The degree of dynamic recrystallization increases with the increase of temperature,and the grains are refined continuously.The mechanism of dynamic recrystallization is a mixture mechanism of DDRX and CDRX,and DDRX is the main mechanism at low temperature.At high temperature,CDRX mechanism is dominant.At low temperature and high strain rate,the two phases form strong textures respectively.With the increase of temperature,the textures strength decreases and the orientations tend to be random.The asynchrony of the dynamic recrystallization of the two phases is the main reason of alloy failure.The short-time oxidation behavior of CoCrCu_1.2FeNi alloy at high temperature was studied by simulating the short-time heating process of the alloy at high temperature.With the increase of temperature and time,the oxidation of the alloy becomes severe.The weight gain increases with the increase of temperature and time.Cu O and Cu₂O are mainly formed on the oxidized surface at high temperature solid state,and Co Cr₂O₄,Ni Cr₂O₄,Ni Fe₂O₄,Ni O and Co O are formed after further oxidation.In the semi-solid state,the oxidation is intense.The oxidation occurred in all the main elements,and the spherical particles are formed on the surface.The oxides on the surface are mainly composed of Ni O,Co O,Cu O and Cu₂O,and a large number of Co Cr₂O₄,Ni Cr₂O₄ and Ni Fe₂O₄ are produced in the oxide layer.The oxide layer thickens greatly in semi-solid state.At high temperature solid state,the Cu in the FCC₁phase diffuses outward along the FCC₁ phase to the surface of the alloy.Consequently,an outer oxide layer composed of Cu O and Cu₂O was formed.Ni O and Co O were formed between the interface of the inner oxidation layer and the inner diffused O.With the increase of temperature and oxidation time,the outer oxidation layer falls off,and more elements were oxidized.In the semi-solid state,the formation of the liquid phase led to the external diffusion of various principal components through the liquid phase to participate in the oxidation.The microstructure evolution and formation mechanism of CoCrCu_1.2FeNi semi-solid billets were studied.The temperature range of semi-solid was determined by melting behavior of the alloy.The semi-solid microstructure has high roundness,and the solid grains coalesce and break during heat preservation.The average grain size of spherical solid phase increases with the increase of isothermal time and temperature.The analysis of composition and structure show that semi-solid treatment is beneficial to improve the segregation of Cu in FCC₁and reduce the difference of lattice constants between FCC₁ phase and FCC₂ phase.The kinetic equation of semi-solid grain coarsening was established.Coarsening grows slowly.During the heating process,the recrystallization of the all FCC₁ phase was formed,while at higher temperature,most of the FCC₂ phase recrystallized and the phase morphology spheroidized to a certain extent.When the temperature is raised to the semi-solid temperature,FCC₁phase melts to form liquid phase,and the grains spheroidize to form semi-solid structure,and the preferred orientation of grains disappears.The above results show that it is very important for microstructure and defect controlling to adopt appropriate process parameters when the intermediate billet is prepared by hot deformation.The oxidation of semi-solid alloy is severe in a short time.In order to reduce the oxidation of semi-solid billet,gas protection is needed in the process of semi-solid billet preparation.The semi-solid microstructure with good microstructure can be obtained by appropriate isothermal heat treatment parameters.