| High entropy alloys is a novel concept for alloy design, which breaks through the traditional strategy for developing alloys based on "one or two major alloy elements". High entropy effects in the high entropy alloys, which are caused by the mixture of multi-principal elements, promote the formation of solid solution with simple structures. The high entropy alloys have been found to have many unique proerties. As a result, high entropy alloys have great potential applications.In this thesis, FeCoNiCuAl, FeCoNiCuAlCrx (x values in molar ration, x=0.5,1,1.5,2and3) and FexCoNiCuCr(x=0.2,0.5,0.8,1, and2) high entropy alloys were prepared by the arc melting and suck-casting method. Meanwhile, the FeCoNiCuAl high entropy (HE) alloys were heated at various temperatures and time, and the FeCoNiCuAlCr high-entropy alloys were heated at700℃for2hours under high magnetic field, respectively. The effect of Cr and Fe content, heated-treatment and high magnetic field on the phase constituent, microstructure and mechanical properties were investigated using X-ray diffraction (XRD), optical microscopy, scanning electron microscope, compressive tests and microhardness test.It has been found that the as-cast FeCoNiCuAl HE alloys consist of simple BCC and FCC structures. The phase constituent in the annealed FeCoNiCuAl HE alloys remained unchanged after heated treatment on5hours at different temperature ranging from600℃to1000℃. α1particles with FCC structures were observed in the alloys annealed at800℃, which is attributed to the different solubility of elements at high temperature. The shape of α1particles gradually becomes needle and round with the rising heat treatment temperature. Distributions of elements in dendrite and interdentrite regions in the FeCoNiCuAl heated at1000℃became hoomogenous. On the other hand, the heat treatment retards compressive yield strength of the alloys, and improves the room temperature ductility.The volume fraction of the α1particles precipitated in the FeCoNiCuAl high entropy alloys heated at800℃for different time gradually increased with the increased time. The compressive-yield strength remained almost same for all alloys heated for different times. Secondary yield phenomenon was observed in the FeCoNiCuAl high entropy alloy aged at 800℃for24hours.The FexCoNiCuCr high entropy alloys are composed of two solid solutions with FCC structure. All the alloys exhibit a typical dentritic microstructure. When the molar ratio of Fe in the alloy is less than1, the XRD results show that the peak intensity of the (200) increased with the addition of the Fe. Due to the large positive mixingenthalpy between Fe and Cu, the Cu enrichment in the interdendrite region was founded. The microhardness of the high entropy alloys is reduced with the increase of the Fe content.It was found that the as-prepared (FeCoNiCuAlCrx HE alloy) have simple BCC+FCC solid solution structure. The formation of BCC phase is enhanced by the addition of the Cr. On the other hand, the solidification mode of the alloys were modified by adding Cr element. When x=0.5, the alloy solidified as the hypoeutectic reaction. As Cr content increasing, the solidification mode of the high entropy alloy changed to eutectic and hypereutectic. The compressive strength gradually increased and plastic deformation decreased by the addition of the Cr. When x=3, the high entropy alloy became brittle.The FeCoNiCuAlCr high-entropy alloys were heated at700℃for2h under different intensity of high magnet field ranging from OT to6T. The high magnetic filed has no obvioius effect on the phase consitutent of the alloy. In the alloy anneaed under4T, the grains have similar orientation on the surface the direction of magnetic fields, though the reason is still unclear. The alloys have similar the compressive yield strength no matter how strong the magnrtic filed is. |
PDF Full Text Request