Microstructure And Mechanical Properties Of Al-Co-Fe-Ni Eutectic High-entropy Alloys By Directionally Solidification

Eutectic high entropy alloy is a new kind of multi-phase and multi-principal component metal material,which has attracted much attention due to its good casting fluidity,excellent comprehensive mechanical properties and high temperature application potential.High temperature gradient directional solidification technology is an advanced preparation method of metal materials,which can not only regulate its structure,but also change the withdrawal rate and temperature gradient to study its solidification behavior.But so far,the research on directional solidification of eutectic high entropy alloys is still relatively few.In this study,the microstructure,solidification behavior and mechanical property of Al_xCo Fe Ni_2.05alloy was systematically analyzed under the conditions of arc melting and directional solidification by means of optical metallography,SEM,TEM,EBSD and XRD.As hyper-eutectic in this system has brittle and hard B₂phase with large volume fraction and poor mechanical properties,hyper-eutectic and eutectic were selected for directional solidification.The main conclusions are as follows:Under arc smelting,Al_0.95Co Fe Ni_2.05alloy is a eutectic high-entropy alloy,and the microstructure is an irregular lamellar eutectic composed of B₂phase and L1₂phase,in which B₂phase is rich in Al and Ni elements,L1₂phase is rich in Co and Fe elements,the yield strength of the alloy is 650MPa,the fracture strength is 1100MPa,and the plasticity is 11%.The microstructure of Al_0.85Co Fe Ni_2.05hypo-eutectic high-entropy alloy is L1_2primary+?L1₂+B₂?_eutectic,the yield strength is 450MPa,the fracture strength is 1030MPa,and the plasticity is 29%.The microstructure of Al_1.05Co Fe Ni_2.05hyper-eutectic high-entropy alloy is B_2primary+?L1₂+B₂?_eutectic,the yield strength is 708.09MPa,the fracture strength is 1237.57MPa,and the plasticity is 14.03%.Under the condition of directional solidification with a temperature gradient of 200K/cm,the solid-liquid interface evolution process of Al_0.95Co Fe Ni_2.05eutectic high-entropy alloy along with the increase of the withdrawal rate is as follows:planar interface?cellular interface?dendritic interface.The microstructure transformation process is as follows:Zigzag instability+lamellar?a small amount of rod+lamellar?completely lamellar?lamellar+irregular lamellar eutectic cell?completely irregular lamellar eutectic cell.The relationship between lamellar thickness and withdrawal rate in regular lamellar eutectic is?=15.90V^-0.41and the two-phase directional relationship is:[^—112]_FCC//[^—113]_BCC,?1^—11?_FCC//?110?_BCC.Regular lamellar eutectic increases strength while irregular lamellar eutectic increases strength and plasticity simultaneously.Al_0.85Co Fe Ni_2.05hypo-eutectic high-entropy alloy obtains a planar solid-liquid interface after directional solidification at 10?m/s,transforms its microstructure into a pseudo-eutectic structure of L1₂+B₂,and increases the tensile strength by 318.53MPa compared with the as-cast state,but the plasticity decreased by 11.53%.When the withdrawal rate is increased to30?m/s,the flat solid-liquid interface is destroyed,forming a composite structure of L1_2primaryand?L1₂+B₂?_eutecticregular arrangement,which achieves the best match of strength?692.06MPa?and plasticity?36.21%?.The dendritic interface is obtained at 50-200?m/s.With the increase of the withdrawal rate,the volume fraction and horizontal interface of the primary L1₂phase increases,so the strength and plasticity of the hypo-eutectic high entropy alloy gradually gradually decreases.