Research On Microstructures And Properties Of Three Special High-Entropy Alloys

In recent years,as a new class of multi-principal element alloys,high-entropy alloys(HEAs)have attracted widespread attention due to their unique microstructure and physical and chemical properties.According to the design concept of HEAs,three different types of HEAs,i.e.YErCuAgAlx(x=0,2,and 4 at.%),ReTaWNbMo,and TiScZrNbV,were selected in order to clarifry the correlation between the microstructure,mechanical properties,and corrosion resistance of as-cast and annealed states.In this work,the present HEAs were prepared by arc melting suction casting methods.The phase formation and the microstructures of three types of HEAs were studied by X-ray diffraction technique,scanning electron microscopy,and transmission electron microscopy,respectivelyThe mechanical properties and corrosion resistance of these HEAs were studied using the universal mechanical testing machine(NEW SANSI,CMT 5305)and the electrochemical workstation(CHI660E).Firstly,rare earth metals riched HEAs containing transition metal elements were studied.The YErCuAgAlx HEAs were prepared by vacuum arc melting with the addition of A1 elements(i.e.0 at.%,2 at.%,and 4 at.%).It was found that the microstructures of YErCuAgAlx HEAs are composed of dendrite,while the matrix phase is BCC(Cu,Ag)(Y,Er)phase.The corresponding crystal structure is similar to BCC CuY phase and other elements are dissolved into the CuY crystals.The interdendritic phase is CU2Y phase,while a limited amount of Y2O3 phase also precipitates.With the increase of A1 content,the dendrite crystals gradually increase and connect with each other,while the Y2O3 phase distribute randomly within the matrix.When the Al content was 4 at.%,the amorphous phase can be observed in the matrix.During deformation,the yield strength increases from 624±30 MPa to 1032±34 MPa,and the plastic strain decreases from 18.3±0.6%to 8.9±0.33%.Compared with the CuY,AgY,CuEr,and AgEr intermetallic alloys,the present HEAs exhibit better mechanical properties.Secondly,the refractory ReTaWNbMo HEAs at different states were studied in detail.The as-cast ReTaWNbMo HEA rods with an equal atomic ratio prepared by rapid solidification displays a BCC crystal structure.After annealing the as-quenched samples at 673,873,1073,and 1273 K for 12 h,respectively,the crystalline phase keeps its BCC structure,but the crystalline morphology transforms from coarse dendrites to fine equiaxed crystals together with gradual grain refinement.The intragranular regions are enriched in W and Ta,while the crystals in the intergranular regions are enriched in Nb,Mo,and Re.With the increase of the annealing temperature,the yield strength increases,and the compressive plasticity and fracture strength are obviously enhanced.The failure mode gradually changes from transgranular to intergranular fracture.Furthermore,the corrosion behaviors of the as-cast ReTaWNbMo HEA and the annealed states in a 3.5 wt.%NaCl solution were also studied.The samples annealed at 1273 K exhibit the best corrosion resistance due to the elemental redistributions within grains.Thirdly,the microstructures and mechanical properties of the TiScZrNbV HEAs at different states were investigated.The as-cast rods were prepared by vacuum melting and suction casting,and then were annealed at 673K,873K,1073K for 12h.The as-cast TiScZrNbV high-entropy alloy crystal phase is composed of two BCC and one HCP phase structures.The microstructure of as-cast sample is equiaxed dendrite structure,and the intergranular region is two-phase mixed region.With the increase of annealing temperature,HCP phase precipitates in the equiaxed dendrites,and the intergranular structure becomes coarse.The microstructure of the alloy gradually changes into a dual phase structure composed of BCC and HCP.As the annealing temperature increases,the yield strength increases first and then decreases,and the compressive plasticity decreases first and then increases.Furthermore,the fracture strength does not change significantly.The fracture mode is a combination of two kinds of ductile fracture modes:shear fracture and dimple fracture.All in all,the present HEAs can be a good candidate for the industrial application as a new structural material.