Development and characterization of high entropy (HfTiZrMn/Cr)B2 based ceramics

Abstract

Materials are divided into four groups: metals/alloys, ceramics, polymers and composites. Materials science includes the study of the physical, mechanical, thermal, chemical and many other properties of materials and the development of new materials. Advanced ceramic materials, including transition metal borides, carbides and nitrides, have attracted attention in recent years compared to traditional ceramics. Transition metal borides are characterized by a high melting point, high strength, high hardness, high wear and corrosion resistance, good thermal shock resistance, high chemical and thermal stability and high transmission stability. Thanks to all these superior properties, transition metal borides can be used as catalysts, refractory parts, sensors in high resolution detectors, decorative coatings, abrasive materials, coatings on cathodes, neutron absorption materials, sanding and polishing processes and in the aerospace industry, defense industry and nuclear technology. Various methods have been used to synthesize transition metal borides until today. The thermal plasma method, self-propagating high-temperature synthesis, metallothermic or carbothermic/borothermic reduction, autoclave synthesis, molten salt electrolysis and solid-state synthesis methods are the main ones. Another method used in the synthesis of metal borides, different from the methods mentioned, is the mechanochemical synthesis process, which has been used for other material groups for the last 20 years and is still being developed. Mechanochemical synthesis is a powder metallurgy production method that allows for the production of composite metal powders with small crystal grains and controlled microstructures at room temperature, using a cold welding-fracturing-rewelding mechanism and starting from easily accessible raw materials, as opposed to high reaction temperature production methods. In recent years, it has been necessary to develop new materials to meet the needs of many sectors, such as medicine, biomedicine, energy, aerospace technologies, automotive and electronics. High-entropy alloys (HEA) are one of the materials developed to meet these needs. Traditional alloying includes combining two or more elements. In high-entropy alloys, four or more elements are combined in equimolar ratios. Contrary to expectations, solid-solutions are formed instead of intermetallic compounds. In this way, HEAs have a single-phase structure even though they contain more than one element. Although there are many elements in the structure, high-entropy alloys mostly have body-centered cubic or face-centered cubic crystal structures. Recent studies have shown that such alloys may also have a hexagonal close-packed structure. Along with solid-solutions, high-entropy alloys also show four different core effects. The high-entropy effect explains its relationship with thermodynamic properties. The sluggish diffusion effect explains the kinetic state. The severe-lattice distortion effect represents both the crystal structure and the formation of mechanical properties. The effect of all the elements added to the alloy is examined under the cocktail effect. High-entropy alloys have high thermal and chemical resistance, good wear, oxidation and corrosion resistance, and mechanical properties such as high hardness, fracture toughness, and strength due to the elements in the alloy, the solid-solutions formed, and the four core effects. Thanks to its superior properties, it is used in the nuclear industry, shipping, the production of refractory materials, the aerospace industry, and cutting tool tips. Many methods are preferred in the production of high-entropy alloys, but arc melting, mechanical alloying, pressureless sintering and pressure sintering are the most common ones. The production of high-entropy ceramics, which is a new class based on high-entropy alloys, is a subject that has been studied in recent years. High-entropy ceramics include oxides, borides, carbides, nitrides and silicides. The idea of producing high-entropy metal borides, which is considered a new type of high-entropy materials and a new class of ultra-high-temperature ceramics, also has been emerged in 2016. High-entropy diboride ceramics have a P6/mmm space group and a hexagonal close-packed structure. In this structure, there are metal-boron, boron-boron and metal-metal bonds. It is characterized by superior properties as it contains metallic, ionic and covalent bonds together. High-entropy metal borides have the combination of superior properties of ceramics, such as low density, excellent high temperature strength, high hardness and strength, high wear and corrosion resistances and specific physical (optical, electrical and magnetic) properties. Due to these superior properties, it can be used in aviation, the solar and nuclear energy sectors, cutting edges and microelectronic systems. Many methods are used in the synthesis of high-entropy metal boride ceramics, a material group that has attracted attention recently due to its high thermal stability, improved mechanical properties, high oxidation resistance, and radiation damage tolerance. Mechanical alloying, boro/carbothermal reduction, self-propagating high-temperature synthesis, pressureless sintering, pressure sintering like spark plasma sintering or hot pressing are the main ones. In cases where a single-phase high-entropy diboride structure cannot be obtained, two consequent methods can be used. Mechanical alloying is a powder metallurgical production method and has the advantages of being carried out at room temperature, using cheap starting materials and inexpensive equipment. In the spark plasma sintering method, single-phase structure can be obtained with high temperature and high pressure. Within the scope of this study, HfB2, TiB2, ZrB2, TaB, Mn boride, Cr boride, Mo boride and W boride powders were synthesized by a mechanochemical route and purified by leaching in the lab-scale using the optimum conditions. Boride powders synthesized without any by-products were synthesized from optimum ones. The reproduced powders were blended in an equimolar ratio of consisting three to eight components. The three-component (Hf0.33Ti0.33Zr0.33)B2 medium-entropy alloy was chosen as the main alloy. The selected composition was first synthesized in a planetary ball mill for 30 h, 60 h or 100 h at ball-to-powder weight ratios of 10:1, 20:1 and 30:1. Then, the same composition was milled in a high-energy ball mill at a ball-to-powder weight ratio of 10:1 for 6 h, 10 h, 15 h and 20 h. In the high-energy ball mill, a ball-to-powder weight ratio of 10:1 and a milling time of 6 h were chosen as the optimum conditions. All prepared compositions were synthesized under optimum situation. For the characterization of powder samples, X-ray diffractometry, particle size measurement and density measurement with pycnometer were performed. Single-phase high-entropy diboride could not be obtained after mechanical alloying. The highest density was observed at 7.1379 ± 0.0057 g/cm3 (Hf0.142Ti0.142Zr0.142Mn0.142Cr0.142W0.142 Ta0.142)B2 composition, while the lowest density was observed in the (Ti0.25Zr0.25Mn0.25Cr0.25)B2 compositions at 4.9708 ± 0.005 g/cm3. A single phase high-entropy structure was synthesized by spark plasma sintering after milling. In addition, low intensity (Hf, Zr) oxide phases were observed. Again, secondary phases with low intensity were formed in five different compositions. X-ray diffractometer, scanning electron microscope/energy dispersive spectrometer, hardness measurement with the Vickers method, dry-sliding wear test and density measurement with the Archimedes method were used for characterization of sintered samples. The composition (Hf0.125Ti0.125Zr0.125Mn0.125Cr0.125Mo0.125W0.125 Ta0.125)B2 has the highest density value of 7.4794 ± 0.0065 g/cm3, while the composition (Ti0.25Zr0.25Mn0.25Cr0.25)B2 has the lowest density value of 4.7517 ± 0.0015 g/cm3. When all samples were examined, the hardness values ranged from 17.08 ± 2.32 GPa to 26.74 ± 1.85 GPa. The average hardness value of all samples was calculated at about 24 GPa. (Hf0.125Ti0.125Zr0.125Mn0.125Cr0.125Mo0.125W0.125Ta0.125)B2 has the lowest wear resistance and (Hf0.166Ti0.166Zr0.166Mn0.166Cr0.166Mo0.166)B2 has the highest wear resistance.