| High-entropy ceramics has attracted more and more attention because of its unique composition,micro-structure,regulation performance and various potential applications.In this paper,high-entropy ceramics?TiZrHfNbTaMe?C?Me=V,Cr,Mo,W?with four different components were prepared successfully for the first time by direct hot pressed sintering and assisted hot pressed sintering.And the phase,organizational structure and mechanical properties of four high-entropy ceramics with six-element carbide were compared in the thesis.Meanwhile,the paper also studied the effects of heat treatment temperature on organizational structure of four six-element high-entropy ceramics and the effects of element Me on oxidation resistance of high-entropy ceramics.A single-phase high-entropy ceramic?TiZrHfNbTaMe?C?Me=Cr,Mo,W?with face-centered cubic structure was prepared from hot pressed sintering on carbide powders by pressurizing 30 Mpa and preserving heat for an hour at 1850?,and an hour at 2200?.High-entropy ceramic?TiZrHfNbTaMo?C has the optimal mechanical properties among them,and its modulus of elasticity,bending strength,Vickers hardness,and fracture toughness were 507±9.38GPa,403±10MPa,23.3±0.39GPa,4.1±0.35MPa?m1/2/2 respectively.Other elements are uniformly distributed in the ceramic except the element Zr,with a small amount of enrichment.The findings of experiments show that the desired carbide powders prepared by oxide and carbon black can effectively avoid oxygen pollution.Firstly,the thermodynamic software FactSage7.2 was adapted for calculation.The results show that under the condition of 100MPa,carbon thermal reduction temperature of ZrO is the highest,which needs to be higher than 1204?and needs to be preserved heat for a period of time before all oxide powders completely transformed into carbide composite powders.Considering actual experiments and kinetics,it can be found that carbide composite powders without oxides can be obtained eventually after carbon thermal reduction temperature reached 1500?and preserved heat for an hour.The particle size of powders ranges from 150nm to 300nm.With carbide composite powders from carbon thermal reduction,high-entropy ceramics?TiZrHfNbTaMe?C?Me=V,Cr,Mo,W?was successfully prepared by the same process of hot pressed sintering.The results show that the four carbide high-entropy ceramics are face-centered cubic structures with densities of 99.4%,99.6%,99.2%,98.4%respectively and the crystalline grain sizes of 3.24±0.22?m,2.17±0.31?m,1.75±0.43?m,2.57±0.47?m respectively.Transmission analysis of high-entropy ceramics?TiZrHfNbTaV?C shows that metal elements are uniformly distributed on the nanometer scale without any chemical aggregation,indicating that mechanical properties of high-entropy ceramics?TiZrHfNbTaMo?C are optimal,its modulus of elasticity,bending strength,Vickers hardness,and fracture toughness are 538±10.03GPa,465±12.5MPa,25.4±1.1GPa and 4.2±0.52MPa?m1/2.espectively.The raw powder has a certain effect on micro structures and mechanical properties of the prepared high-entropy ceramics.It can be found by experiments that the high-entropy ceramic that prepared by carbon thermal reduction assisted hot pressed sintering has better mechanical properties,more compact material,more uniform distribution of metal elements.High-entropy ceramics?TiZrHfNbTaMe?C?Me=V,Cr,Mo?has good phase and thermal stability after heat treatment at 12002000?.Effects of element Me on the oxidation resistance of three high-entropy ceramics were studied in the thesis.According to the curve of cross-section thickness varying with temperature,high-entropy ceramic?TiZrHfNbTaV?C has best oxidation resistance.The oxidation behavior of high-entropy ceramics?TiZrHfNbTaV?C was also investigated in this section,and the curve of its weight gain conforms to the parabolic law at 800?for diffusion-controlled oxidation.At the beginning of oxidation,complex rod-shaped oxides were formed,and the oxidation surface was not dense.But the oxidation layer became denser with the increase of oxidation time,effectively preventing further oxidation.High-entropy ceramics?TiZrHfNbTaV?C has good oxidation resistance. |
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