Preparation, Microstructure And Mechanical Properties Of Multiceomponent High-entropy Alloys

High-entropy (HE) alloys with multi-principal elements were developed based on a novel alloys design strategy of equal atomic ratio and high entrophy. It contains over five major constituents, and breaks through the tranditional strategy for developing alloys, which has lead to many multi-component alloys based on one or two main components. HE alloys have been found to have many unique proerties, and draw strong attention in recent years.In this thesis, FeCoxNiCuAl (x=0.2,0.5,1,1.5,2,3), FeCoNiCuAlNd、FeCoNiCuAlTi、 FeCoNiCuAlSi、FeCoNiCuAlZr and FeCoNiCuAlYx(x=0,0.1,0.3,0.5,1) HE alloys were prepared by the arc melting and suck-casting method. Meanwhile, the FeCoNiCuAl high-entropy alloys were heated for2h at700℃and different intensity of high magnet field. The effect of Co content, the sixth elements, Y content and high magnetic field on the phase constituent, microstructure and mechanical properties were investigated using the optical microscopy, scanning electron microscope, universal mechanical test and microhardness test.It has been found that the amount of Co in the FeCoxNiCuAl HE alloys has greatly effect on the phase constituent, microstructure and mechanical properties. The solid solutions are formed in the HE alloys. When x≤2, the FeCoxNiCuAl HE alloys consist BCC and FCC phases, while the FeCo3NiCuAl is composed of only FCC phase. The volume fraction of FCC phases increases with the increase of the Co content, suggesting that the Co element promotes the formation of FCC phase. It has also been observed that the lattic constant of FCC phases decreases with the Co content increasing, which might be attributed to the smaller atomic radius of Co element. On the other hand, the amount of Co in the FeCoxNiCuAl HE alloys obviously affect the mechanical properties of the alloys. With the Co increasing, the compress yield strength gradually decreases, which the plasticity at room temperature gradually increases. Meanwhile, they have different fracture pattern. The alloys display the brittle fracture at x≤1, and show the fracture along the the direction of45°at x is1.5and2; while the FeCo3NiCuAl alloy has excellent plasticity, and is pressed into bread shape. The hardness is reduced with the increase of the Co content.The addition of Si, Ti, Zr, Y and Nd in the FeCoNiCuAl alloys forms the alloys with different phase constituent, microstructure and properties. It has been found that the addition of the Si and Ti, which have smaller or similar atomic radius compared to the average atomic radius of the FeCoNiCuAl, has no influence on the phase constituent of the alloys, which mainly are BCC and FCC phases. However, it affacts the latttic constant and volume fraction of the BCC and FCC phase. The yield strength, hardness and plasticity are enhanced by the addition of Si and Ti. The complex compounds are formed in the FeCoNiCuAlZr alloy, which reduce the mechanical properties of the alloys. When the rare earth element Y and Nd are added into the alloys, the brittle rich-rare earth phases are formed, which are respoinsible for the poor performance of the alloys. Due to the different mixed enthalpy between elements, it has been observed that the element enrichment usually happen, the interdendrite is normally enriched with Cu element.The microstructure and properties show a series of regular changes with different Y content in FeCoNiCuAlYx alloys. The alloy with lower addition of Y (x=0.2) still consist simple BCC and FCC phases, and show a little enhancement of the yield strength and obvious decrease of the plasticity at room temperature. The Cu and Y tend to enrich in interdendrite region. Except for the FeCoNiCuAlY alloy, Y is found to appear only in the interdendrite region. In the FeCoNiCuAlY alloy, Y homongenously distribute in the drendirte and interdendrite regions, where the rich-rare earth phase is formed. The properties of the alloys is significantly deteriorated by the addition of the Y.FeCoNiCuAl high-entropy alloys were heated for2h at700℃and different intensity of high magnet field ranging from OT to6T. The effect of high magnetic field on the phase constituent, microstructure and mechanical properties has been studied. The results show that (110) and (211) of BCC arrange along the direction of the magnetic fields at0.5and6T, respectively, though the reason is still unclear. The magnetic fields could refine the microstructure of alloys under the experimental condition used. The yield strength is enhaced a little for the alloys heated at0.5and6T. The maximum compressive strength of the alloys heated at different intensity magnetic field shows similar value, and is larger than that of the alloys heated without magnetic field.