Effect Of Directional Solidification And Heat Treatment On Microstructure And Mechanical Properties Of AlCoCrFeNi_2.1 Eutectic High Entropy Alloy

Recent studies report that eutectic high entropy alloys which possess both high strength and high ductility have potential industrial applications.In the present work,the solidification behaviors and mechanical properties of directionally solidified Al Co Cr Fe Ni_2.1eutectic high entropy alloys obtained at different growth velocities are investigated.The effects of heat treatment temperature and holding time on the phase composition,microstructure and mechanical properties of directionally solidified and as-cast Al Co Cr Fe Ni_2.1eutectic high entropy alloys are studied.The microstructure of the as-cast Al Co Cr Fe Ni_2.1eutectic high entropy alloys is composed of bulky dendrites（Ni Al phase）and lamellar eutectic structures.The lamellar eutectic consists of light gray FCC phase enriched with Co,Cr and Fe elements and dark gray BCC phase enriched with Al and Ni elements.Although the actual composition of the alloy is shown to slightly deviate from the eutectic point,it is interesting to observe that the full lamellar structure of this alloy is obtained through directional solidification at different growth rates（5μm/s-200μm/s）.The maximum interface temperature criterion and the interface response function（IRF）theory are applied to calculate the velocity range of the transition from the primary phase to the eutectic,which is 1.2μm/s-2×10~4μm/s.In this range,the interface temperature of the eutectic is higher than that of the primary phase,and the eutectic will grow preferentially.Therefore,the final microstructure of Al Co Cr Fe Ni_2.1is lamellar eutectic structure after directional solidification.The lamellar spacing gradually decreases with the increasing growth velocity.It changes from 6.31μm at5μm/s to 1.21μm at 200μm/s.The mechanical properties of the alloy are significantly improved after directional solidification,the tensile test indicates that the directionally solidified Al Co Cr Fe Ni_2.1eutectic high entropy alloys possess a good combination of strength（1340MPa）and ductility（30.5%）at 100μm/s.Since the microstructure of Al Co Cr Fe Ni_2.1eutectic high entropy alloy is arranged in a specific direction after directional solidification,the mechanical properties of the alloy are anisotropic.At the same growth velocity,the compressive strength of axial specimen is greater than that of radial specimen.Therefore,the alloy have the best mechanical properties in parallel with the growth directions.From the microhardness test results,it can be seen that the microhardness increases with the decrease of lamellar spacing as the growth velocity increases.Through heat treatment,it is found that both the directionally solidified and as-cast Al Co Cr Fe Ni_2.1eutectic high entropy alloys exhibits good thermal stability,and the phase structure of the alloy does not change with the change of heat treatment temperature and holding time.At different growth rates of 10μm/s and 100μm/s,Al Co Cr Fe Ni_2.1eutectic high entropy alloys maintained stable lamellar eutectic structure and good mechanical properties at 800℃.After heat treatment at different temperatures and holding time,the primary Ni Al phase decomposes and fine Co Cr Fe phase precipitates in as-cast Al Co Cr Fe Ni_2.1eutectic high entropy alloys,while the lamellar eutectic structure always maintains good thermal stability.In addition,after heat treatment,the change in the tensile properties,compressive properties and microhardness of Al Co Cr Fe Ni_2.1is not obvious,which indicates that Al Co Cr Fe Ni_2.1eutectic high entropy alloys can still maintain good microstructure stability at high temperature.