Research On Microstructure And Properties Of CoCrFeNi(Ni₃Ti/Ni₃Al)_x High Entropy Alloy

Multi-principal high-entropy alloys breaks the design concept of traditional alloys based on one or two elements,and is a new type of alloy with multiple elements as major elements.It has excellent physical,chemical and mechanical properties,such as high strength and high hardness,high wear resistance,high corrosion resistance,etc.,and has potential broad industrial application prospects and important theoretical research values.In this paper,CoCrFeNi high entropy alloy with good toughness is selected as the matrix,and Ni₃Ti and Ni₃Al are used as strengthening phases to strengthen the matrix.OM,XRD,SEM,EDS,tensile and compression,and hardness tests were used to characterize the microstructure and mechanical properties of CoCrFeNi(Ni₃Ti)_xand CoCrFeNi(Ni₃Al)_x(x=0.5,0.75,1,1.25)high entropy alloys,The effects of the type of strengthening phase,the amount of strengthening phase,and heat treatment on the microstructure and properties of high-entropy alloys were analyzed.Main conclusions are as follows:(1)Ni₃Ti can be precipitated from the matrix in the form of slabs in the as-cast state,while Ni₃Al is not precipitated in the as-cast state.After aging treatment,it can be precipitated from the matrix as nanoparticles.All phases can effectively improve the strength of CoCrFeNi matrix.(2)CoCrFeNi(Ni₃Ti)_x(x=0.5,0.75,1,1.25)high-entropy alloy has a simple dendrite+slab-like Ni₃Ti structure in the as-cast state,and Ni₃Ti can be largely removed from the matrix during the as-cast state.The four alloys are composed of FCC+Ni₃Ti two phases.A certain amount of Co is dissolved in the Ni₃Ti phase.When the addition of Ni₃Ti is small,the Ni₃Ti phase mainly precipitates between the dendrites.After increasing the amount of Ni₃Ti,a large amount of Ni₃Ti phase is precipitated on the dendrites.With the increase of Ni₃Ti,the tensile strength of the alloy increased significantly.When the molar ratio of Ni₃Ti to the substrate reached 0.75:1,the tensile strength of the alloy was the highest,which was 1.5 times higher than the tensile strength of the CoCrFeNi substrate.It reached 1057MPa and the elongation was 2%.Continue to increase Ni₃Ti,but the tensile strength decreased.The analysis shows that because the mixing enthalpy of Ni and Co and Ti are the most negative and have a strong interaction with each other,Ni₃Ti can be precipitated in the as-cast state,and a certain amount of Co element is dissolved in Ni₃Ti.(3)The phase composition of the CoCrFeNi(Ni₃Ti)_x(x=0.5,0.75,1,1.25)high-entropy alloy has not changed after being subjected to a solution treatment at 1000°C for 3h.It still consists of two phases,FCC+Ni₃Ti.After solution treatment,the plasticity of the alloy is significantly improved.Among them,the tensile strength of CoCrFeNi(Ni₃Ti)_0.75is still the highest,which is 801MPa,and the elongation is increased from 2%to 5%.(4)CoCrFeNi(Ni₃Al)_x(x=0.5,0.75,1,1.25)high entropy alloy has a simple dendrite structure in the as-cast state,and the difference between the dendrite and the dendrite composition is small.All four alloys in the as-cast state are single solid solution of FCC structure.No precipitation of Ni₃Al-ype L1₂phase was found.The strength and hardness of the alloy increased with the increase of Ni3Al.CoCrFeNi(Ni₃Al)_1.25has the highest tensile strength,reaching 867MPa,Vickers hardness of 254HV,and still has an elongation of 21%.(5)After aging at 780°C for 4h,CoCrFeNi(Ni₃Al)_xhigh-entropy alloy precipitates a large number of nano-scale L1₂phase particles in which certain Co,Cr,Fe elements are dissolved,and the particle size is between 40nm-50nm.When the content of Ni₃Al is low,the L1₂phase mainly precipitates between the dendrites.As the amount of Ni₃Al continues to increase,a large amount of L1₂phase particles begin to precipitate on the entire dendrite.With the proper composition,the precipitation of the L1₂phase can effectively increase the strength of the material,and with the increase of the precipitation phase,the strength of the alloy gradually increases.CoCrFeNi(Ni₃Al)₁has the highest tensile strength,and the strength reaches 828MPa and The elongation is 22%.After Ni₃Al continued to increase,the strengthening effect of the precipitated phase began to decrease.Ni₃Al does not precipitate in the as-cast state because the difference in mixing enthalpy between Al and Co,Cr,Fe,and Ni is small.The alloy forms a single-phase FCC solid solution due to high entropy effect.After aging,Al and Ni form the Ni₃Al-type L1₂phase due to the most negative and preferential combination of mixing enthalpy,and a certain amount of Co,Cr,Fe elements are also dissolved in the precipitated L1₂phase.