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  • Öge
    Development and characterization of high entropy (HfTiZrMn/Cr)B2 based ceramics
    (Graduate School, 2022-12-26) Süzer, İlayda ; Ağaoğulları, Duygu ; 506201403 ; Materials Engineering
    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.
  • Öge
    Wear behaviour analysis of different metals by the finite element method
    (Institute of Science And Technology, 2020-06-15) Demir, Canay ; Baydoğan, Murat ; 506171407 ; Materials Engineering ; Materials Engineering
    Material losses occur because of the damage caused by friction between materials relatively moving in contact with each other. Wear damage can significantly reduce the life cycle of the materials and can significantly affect their operating performance. To prevent or minimize this damage, wear mechanisms of material and material pairs must be determined under certain service conditions. Accordingly, wear testing and wear prediction have gained great importance. Wear is a very common type of damage in systems operating in motion. Wear can take place with more than one different mechanism. These are mainly classified as adhesive wear, abrasive wear, fatigue wear and corrosive wear. There are many factors that affect the wear phenomenon: crystal lattice structure, hardness, elasticity modulus, work-hardening, plastic deformation behavior, surface roughness of the materials etc. and they depend on the properties of materials. Additionally, the service or ambient conditions (temperature, humidity, etc.) very effective for the wear behavior. In order to minimize wear damage, wear behavior must be carefully examined. However, the most common is the method of determining the friction coefficient by the wear of the pin or ball, which is constantly under a certain force on the rotating disk with the pin-on-disk assembly, or vice versa. With this method, the wear loss is determined by measuring the wear traces on the wear disc or pin / ball. This experiment can be carried out under different loads, at different sliding speeds and distances, even at different temperatures. In all cases, it may not be possible to access all materials or wear surfaces can be complex geometries. In such cases, it is possible to obtain an approach to experimental results in cases where it is not possible to experiment using the Finite Element Method (FEA) as a numerical analysis method. Studies on wear modeling have been developed taking into account the classical wear theory put forward by Archard. In wear analysis using finite element analysis, Archard wear theory is still the most commonly used method today. The aim of this study is to obtain ball-on-disc type wear test results carried out in a laboratory environment via modeling in 3-dimensional in finite element analysis software. In this context, Inconel 718, 316L stainless steel, grey cast iron, spherical graphite cast iron, Zamak, Ti6Al4V, 7075 and 6082 aluminum alloys, AZ91 magnesium alloy and pure copper as metals with different crystal structure, hardness and microstructure have been subjected to wear test against alumina (Al2O3) ball. It is expected to verify that the validity of the finite element model used by comparing the results obtained from these experiments with the 3-dimensional wear model created with ANSYS Workbench and the results obtained by using Archard theory. In this way, it is aimed to make accurate predictions about the results of the wear analysis by using the finite element method. In line with the determination of wear loss in the specified materials, Inconel 718, 316L stainless steel, grey cast iron, spherical graphite cast iron, Ti6Al4V, 7075, AZ91, Zamak, 6082 and pure copper metals were tested under different loads in ball-on-disc wear test configuration. The wear loss is used in Archard`s wear equation to calculate the wear coefficient K and the coefficient of friction is used as an input to the simulation with hardness of material. SEM and Raman spectroscopy analysis of wear tracks were done. Using the 3-dimensional model of the ball-on-disc test setup was used to perform numerical analysis. Results from the numerical analysis were compared to the experimental analysis. There was a good correlation with the results in general. However, relatively higher error values were recorded for some metals like 7075 alloy and grey cast iron. The difference between these results were investigated both experimentally and numerically. First, the simulation is accepting that all surfaces are perfect. Secondly, the contact pressure was calculated as constant during the simulation. However, the in experiments the contact area is changing throughout the sliding thus, the contact pressure is expected to decrease. Furthermore, the contact pressure values calculated at the numerical model is differs from the Hertzian contact theory. Because in simulation assumes that bodies are elastic. Another reason is that oxide formations were found in wear tracks on sliding surfaces. The oxides created lubrication effect for the coefficient of friction of grey cast iron; however, it was kept constant during the simulation. Similarly, the metallic layer formation on the alumina ball against the Ti-6Al-4V resulted to metal-metal wear and the experimental K values was became different than the K value calculated from the Archard's equations. There are any many factors that can be found for accuracy of the simulation. Despite all that, the results were very promising to create a simulation tool for wear analysis of different materials.