Rare Earth High Entropy Alloys In Hexagonal Close-packed Structure And The Mechanical Properties

Since the new alloying concept of high entropy alloys(HEAs)was proposed,high entropy alloys have attracted wide attention in the materials community.These excellent properties of HEAs are mainly ascribed to the reason that they form into simple solid solutions.Up to now,the study of high entropy alloys has been mainly concentrated on the alloys in face-centered cubic structure and body-centered cubic structure,and high entropy alloys in hexagonal close-packed structure have been rarely reported.In this paper,the main research content is the preparation of high entropy alloys in hexagonal close-packed structure,and their mechanical properties at room temperature are tested.At room temperature,the majority of rare earth elements have stable hexagonal close-packed structure or double hexagonal close-packed structure.Many aspects including atomic size,electronegativity,and chemical properties,are extremely similar.The solid solubility among different elements is very large,and they often form isomorphous solid solutions.So this study selects the rare earth elements to prepare high entropy alloys.During the experiment,the high entropy alloys were prepared using the vacuum arc furnace.The morphology and microstructure of high entropy alloys were analyzed by the metallographic microscope,X-ray diffractometer,and scanning electron microscopy(SEM).The microhardness was calculated by Vickers hardness tester.The mechanical properties under low strain rate and high strain rate were conducted using the materials testing machine and the Split Hopkinson Pressure Bar(SHPB)device,respectively.The results are as follows:The quinary high entropy alloy,GdHoLaTbY,is a single crystalline phase with hexagonal close-packed structure.The lattice constant meet the rule of mixture,and the alloy is approximately an ideal solid solution.The mechanical test results show that strength and hardness of the alloy obey the rule of mixture.The alloy doesn't behave the solid solution strengthening effect,and the entropy has little effect on the mechanical properties of the alloy.Compared with the as-cast GdHoLaTbY alloy,the tensile yield strength of the 10%-deformed alloy increases by 74.6%,and ultimate strength increases by 34.6%.Rolling can significantly improve the mechanical properties of the alloy.But when the deformation is too much,cracking phenomenon appears at the edge of the alloy.After adding the element Al into the GdHo LaTbY alloy,intermetallic compounds form in the interdendrits.With increasing the content of the element Al,the volume fraction of second phase gradually increases.The yield strength of the alloy is improved,but the tendency of the brittleness increases,which leads to the gradual decrease of the plasticity.Senary high entropy alloys,GdHoLaTbYEr and GdHo LaTbYDy,are single-phase solid solutions in hexagonal close-packed structure.The lattice constants of the alloys obey the rule of mixture.The alloys show slightly solid solution strengthening effect.Strain rate has a significant effect on the mechanical properties of the GdHoLaTbYDy high entropy alloy.The mechanical performance under low strain rate is superior to that under high strain rate.The constitutive equation is established under different strain rates using Johnson-Cook(J-C)model,and the experimental result is in accord with the fitting result.The J-C model can reasonably describe the plastic deformation of the alloy.