Study On High Temperature And Cold Rolling Annealing Behavior Of CoCrNi Medium Entropy Alloy

In recent years,the development of metallic materials has tended to be in a gentle state.With the development of human society,people have put forward higher requirements in terms of quality of life.The development of science and technology has put more demands on materials.As a new type of alloy(including medium entropy alloy and high entropy alloy),there are many excellent properties compared to conventional alloys,such as high hardness,high strength and corrosion resistance.The typical CoCrNi-based medium-entropy alloy with single-phase face-centered cubic structure is easy to be processed and formed because of its simple composition and excellent toughness,but it still has low strength and resistance.Shortcomings such as poor wearability,it is difficult to meet the needs of practical engineering applications.Based on this,the research on the microstructure and mechanical properties of the CoCrNi alloy based on face-centered cubic structure.In this paper,the medium entropy alloys of CoCrNi and(CoCrNi)94Ti3Al3 were prepared by vacuum arc melting furnace.The mechanical behavior and microstructure evolution of the alloy after high temperature deformation and cold rolling annealing were systematically studied.The research results provide a theoretical basis and technical support for the development of CoCrNi series medium-entropy alloys.The main research contents are as follows:(1)The thermal deformation behavior of CoCrNi alloy was studied,the high temperature equation is established,and the evolution of microstructure and the distribution characteristics of crystal boundary in the process of thermal deformation are analyzed.The experimental results show that the activation energy of the experimental alloy is 401.626 kJ/mol.The complete recrystallization temperature during the thermal deformation was determined to be 950℃.Dynamic recrystallization is nucleated by "expansion mechanism",as a typical discontinuous recrystallization nucleation mechanism,the continuous dynamic recrystallization mechanism becomes active as the strain rate decreases.As the strain rate decreases,the proportion of the ∑3 grain boundary also increases.The frequency of the orientation angle difference between 10° and 15° remains almost constant,and the rotation of the continuous sub-crystals around the original grain boundary will result in nucleation of dynamic recrystallization in the substructure.(2)The thermal deformation behavior of the(CoCrNi)94Ti3Al3 alloy is studied,the high temperature constitutive equation was established,and the evolution of microstructure and grain boundary distribution characteristics during hot deformation were analyzed.The experimental results show that the activation energy of the experimental alloy is 622.771 kJ/mol.The complete recrystallization temperature during the thermal deformation was determined to be 1000℃.Dynamic recrystallization is nucleated by the "expansion mechanism",and dynamic recrystallization complements the formation of twins.When the deformation temperature is lower than the complete recrystallization temperature,it is a discontinuous dynamic recrystallization mechanism.The continuous dynamic recrystallization mechanism is gradually activated when the strain rate is lowered.When the deformation temperature exceeds the complete recrystallization temperature,the continuous recrystallization mechanism is weakened.At the same deformation temperature,the proportion of the ∑3 grain boundary increases as the strain rate decreases.At all applied strain rates,the proportion of twins increases with the development of dynamic recrystallization.The low strain rate and high deformation temperature are the most suitable conditions for twin formation because there is sufficient time for thermal energy to drive the grain boundary migration under this condition.(3)The evolution of microstructure and properties during CoCrNi cold rolling annealing was investigated.The results show that after cold rolling annealing at-50℃,the experimental alloy has fine grain size and average grain size was 0.90-0.99 μm.The strain is accumulated and exhibits excellent mechanical properties,and its tensile strength was 1168 MPa,the yield strength was 1040 MPa,and the elongation rate was 37%.After annealing,a large number of CSL grain boundaries are formed,showing a quantitative relationship of∑3>∑9>∑27.The critical twinning stress of the experimental alloy during the deformation process was calculated to be 1100 MPa.