Study On Microstructure Evolution Of Al_xCoCrFeNiCu_0.5 High Entropy Alloys During Directional Solidification

Al_xCo Cr Fe Ni high-entropy alloy series is one of the most intensively studied HEAs systems to date.The lattice structure of the alloy depends on Al content.With the increase of Al content,the original single fcc phase gradually transformed into the dual-phase structure composed of fcc and bcc.At the same time,the Young’s modulus,hardness and yield strength of the alloy are greatly improved.The single-phase fcc high-entropy alloy is relatively soft and not conducive to industrial applications;single-phase bcc is too hard and brittle to process.While the composite phase fcc+bcc produces a superimposed effect that can simultaneously improve strength and plasticity.In addition,literatures find adding Cu element to Al_xCo Cr Fe Ni improves Therefore,the article studies the solidified microstructure corresponding to the x=0.5,0.6,0.7,0.75 in Al_xCo Cr Fe Ni Cu_0.5 high-entropy alloy,laying the foundation for expanding the practical application of high-entropy alloy.The microstructure and mechanical properties of the alloy depend on the solidification parameters.For directional solidification,there are the drawing speed v,temperature gradient G,and alloy composition C₀,all the parameters can be adjusted as variables independently.In this paper,bridgeman directional solidification technology is used to study the directional solidification behavior of Al_xCo Cr Fe Ni Cu_0.5 high entropy alloy.This paper studys the microstructure evolution of Al_xCo Cr Fe Ni Cu_0.5（x=0.5,0.6,0.7,0.8）alloys during directional solidification at 1-200μm/s.The samples are prepared by vacuum arc melting furnace and high temperature gradient directional solidification furnace（temperature gradient G=20 K/mm）.The XRD diffractometer,scanning electron microscope and energy spectrometer were used to study the directional behavior.The paper aims to analyze the influence of the Al content and drawing velocity on the solid-liquid interface morphology,element segregation and microstructure evolution.The increased Al content increases the instability of the solid-liquid interface.When the pulling velocity increases,and the morphology of the directional solid-liquid interface goes through the flat-cell-branch process.The directional macrostructure is divided into quenching zone,paste zone,and solid phase zone.The changes of directional solidification parameters（alloy composition C₀ and pulling velocity v）have an important influences on the evolution of the macro/micro structure.The Al_xFe Cr Ni Co Cu_0.5（x=0.5,0.6）solidified structure consists of fcc phase and Cu-rich phase.The solid phase zone of Al_xFe Cr Ni Co Cu_0.5（x=0.7,0.75）undergo an extra phase transition such as precipitation phase transformation and Spindol decomposition from the high temperature region to the low temperature region.The high temperature solid phase zone and the low temperature solid phase zone are retained by rapidly quenching.The structure of Al_0.7Fe Cr Ni Co Cu_0.5 includes amplitude modulation matrix,fcc precipitated phase,and Cu-rich phase in the low temperature region.Spindol decomposition is inhibited when pulling velocity is too high.For Al_0.75Fe Cr Ni Co Cu_0.5 alloy,although the solidification reaction is different under different pulling velocity,the final solidification structure consists of the modulation structure and fcc phase.