| Eutectic high entropy alloys is a new type of alloy that has emerged from recent years.It has the advantages of both high entropy alloys and eutectic alloys.It not only has excellent comprehensive mechanical properties,but also has good casting fluidity,so it has received widespread attention.High temperature gradient directional solidification technology,as an advanced method for preparing metal materials,can not only precisely control the structure and improve the mechanical properties,but also study the solidification behavior of alloys by changing the drawing speed and temperature gradient.This topic uses the arc melting method to prepare AlCoCrFeNi2.i as-cast master alloy,on this basis,through directional solidification technology,combined with OM,XRD,TGA,SEM,TEM,universal tensile testing machine and other test methods,comprehensive analysis of its solidification behavior,microstructure,mechanical properties,deformation behavior and deformation mechanism.The main research results and conclusions are as follows:The as-cast AlCoCrFeNi2.1 eutectic high-entropy alloy is composed of L12 phase and B2 phase,and the microstructure is composed of regular lamellar regions in the center and irregular regions on both sides.Under 400K/cm temperature gradient,after directional solidification at different drawing speeds(2-800?m/s),the alloy always consists of L12+B2 eutectic two phases.The interface relationship between the two phases is:[011]L12//[001]B2,(111)L12//(110)B2.The transformation process of solid-liquid interface and solidified structure is:planar interface?cellular interface?dendritic interface;Zigzag instability+lamellar-rod eutectic?a small amount of rods+lamella eutectic?complete lamellar eutectic?center Regular lamellar+irregular lamellar eutectic on both sides.Under three different temperature gradients of 110K/cm,200K/cm,and 400K/cm,the relationship between the thickness of the regular eutectic layer and the drawing speed is:?=13.276V-0.34,?=10.174V-0.33,?=9.266V-0.33.Under a temperature gradient of 400K/cm,the fully lamellar eutectic structure alloy obtained at a drawing speed of 10?m/s has the highest yield strength of 900MPa.The alloy with the best comprehensive mechanical properties is obtained at 600?m/s,the yield strength is 610MPa,the tensile strength is 1050MPa,and the plasticity is 49.1%.The structure is composed of a regular lamellar eutectic structure in the center and an irregular eutectic structure on both sides.The former provides strength to the alloy,while the latter maintains the two-phase synergistic deformation.In the process of tensile deformation,because the B2 phase in the coarser full-lamella structure at 100?m/s is not easy to deform,it leads to poor plasticity.At 600?m/s,the alloy's lamellar structure synergistic deformation ability is significantly improved,and the plasticity of the alloy is greatly improved due to the structure of irregular eutectic on both sides has the good ability to hinder crack propagation and the effect of the wavy strain field.In terms of the evolution of the substructure during the compression deformation process,the deformation of the structure at 10?m/s is mainly carried out by the phase boundary activation of the dislocation movement of the L12 phase and the slip bands in two different directions on the close packed surface {111}.At 600?m/s,in the L12 phase,it is from dislocation outcrop and short dislocation of the planer deformation characteristics,a small amount of dislocation entanglement and dislocation pileup,to dislocations networks,dislocation intersection and the long and straight slip band,and then to the dense dislocation wall and dislocation entanglement,and finally deformed by generating a large number of stacking faults along the closely packed {111} slip surface.The B2 phase produces a small amount of dislocations in the early deformation,and the dislocation density increases significantly in the later stage. |
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