Preparation And Properties Of High Entropy Silicide Ceramics

High-entropy alloys are new multi-principal alloys developed in recent years,It consists of five or more equimolar or near equimolar elements.With the development of research,the concept of high entropy is extended to the field of ceramics gradually.High-entropy ceramics are composed of five or more metal elements and one non-metallic element with simple crystal structure?body-centered cubic,face-centered cubic and hexagonal closepacked?.High-entropy ceramics have excellent mechanical properties,thermophysical properties,radiation resistance and corrosion resistance.However,most of the research focus on high-entropy oxide ceramics,high-entropy carbide ceramics and high-entropy boride ceramics.And there are few studies on high-entropy silicides.Mo Si₂ is the most widely used in transition metal silicide,and can used as resistive heating elements up to 1800? in air.In this article,according to the design idea of high-entropy ceramics,transition metal elements are selected as the raw material to prepare the different high-entropy silicides systems via SPS,and the effects of the phase,microstructure and properties of high-entropy silicides by adding different second phase.high-entropy silicide(Ti_0.2Zr_0.2Nb_0.2Mo_0.2W_0.2)Si₂ was successfully prepared at 1300? for 15 mins by Ti,Zr,Nb,Mo,W five transition metals and Si as raw materials.The study shows that the high-entropy silicide was a hexagonal close-paked crystal structure at room temperature,which indicates that b-Mo Si₂ that can only be stabilized at high temperature phase is stabilized to room temperature through high-entropy.However,part of zirconium elements combined with the oxygen impurities that are difficult to avoid in the material to form a small amount of zirconia.The hardness of the obtained high-entropy silicides was 12.09±0.5GPa,which was between the hardness of each single component silicides.The hardness of the material was affected by many factors,such as porosity,grain size,impurity content and interfacial characteristics etc.Considering that zirconium element combine with oxygen elemnet in the system to form zirconia heterophases,so we added a small amount of active element Al with oxygen to form alumina,so that the zirconium element can enter the high-entropy phase and improve the microstructure and performance of the high-entropy silicides.The results show that aluminum absorbed oxygen impurities in the system,but the zirconium element reacted with silicon to form a single phase Zr Si₂,and did not increase the content of zirconium in the high-entropy phase.A supercell of 2×3×1 with hexagonal close-paked crystal structure was constructed by DFT theory.The lattice parameters predicted by DFT were matches well with the lattice parameters obtained by XRD results.Moreover,when the five metal elements in the crystal structure randomly change the position of the cation,the lattice parameters and total energy level change were very small,which also shows that such an atomic arrangement is desirable.Meanwhile,the hardness of the high-entropy silicides doped with aluminium increased from 12.09±0.5GPa to 13.58±0.6GPa.Since metallic simple powder,especially zirconium powders,is dangerous during the experimental operation,we had also carried out the study of preparing high entropy silicide(Ti_0.2Hf_0.2Nb_0.2Mo_0.2W_0.2)Si₂ by using disilicide powders as raw material,and further add 20vol%Si C to prepare composite high-entropy silicide material.The results show that both materials with hexagonal close-packed crystal structures were synthesized at 1400?.XRD and SEM results show that the oxygen impurities in this system combine with the hafnium to form the hafnium oxide phase,which is preliminarily judged that this is related to the electronegativity of the elements.The presence of Si C significantly improved the Young's modulus,Vickers hardness,and fracture toughness of high-entropy silicides,in which fracture toughness increased by nearly 40%.