Synthesis Of Multi-principal Element Carbide Particles And Fabrication Of Aluminum Matrix Composites

With the rapid development of the aerospace,mechanical industry,national defense technology and other fields in our country,the requirements for the comprehensive properties of materials are becoming higher and higher.The metal matrix composites,especially particle-reinforced aluminum matrix composites are of interest to researchers due to their high cost performance and excellent comprehensive properties.Transition metal carbides have many promising physical and chemical properties,such as high hardness,high melting point,high modulus,and good stability.They are often used as reinforcements for aluminum matrix composites.As we all know,introducing the high-entropy effect into alloys design can give the alloys more outstanding comprehensive properties,and the high-entropy effect also plays a similar role on ceramic materials.At present,the study on high-entropy ceramics is still in its infancy,and there are few studies on high-entropy ceramic particles reinforced composites.Therefore,the investigation on high-entropy carbide particles reinforced aluminum matrix composites has important significance for expanding the application range of aluminum matrix composites.In this thesis,the selection rules of high-entropy carbide component elements was explored,and the reaction mechanism of Al-(Ti,V,Nb,Ta)-C and Al-(Ti,V,Zr,Ta)-C systems during combustion synthesis were investigated for the first time.The nano-(Ti,V,Nb,Ta)C particles reinforced aluminum matrix composites have been produced by a process consisting of combustion synthesis followed by hot pressing in a vacuum.The effect of Al content,pressure on the synthesis products of Al-(Ti,V,Nb,Ta)-C system was studied,and the hardness,density,electrical conductivity and compression properties of composites with different Al contents were tested.Results of the present study are:(1)The forming possibility of high-entropy carbides has been analyzed based on the thermodynamic theory:?.A simplified thermodynamic model based on thermodynamic calculations and geometric models was proposed for the first time.This simplified model can be used to analyze the forming possibility of multi-principal element carbides and intermetallics.?.According to the calculation of thermodynamics,the synthesis of high-entropy carbides in Al-(Ti0.25V0.25Nb0.25Ta0.25)-C system and Al-(Ti0.25V0.25Zr0.25Ta0.25)-C system is feasible.The driving force of the reaction in Al-(Ti0.25V0.25Zr0.25Ta0.25)-C system is smaller than that in Al-(Ti0.25V0.25Nb0.25Ta0.25)-C system.(2)The selection rules of high-entropy carbide components has been explored.The combustion reaction path of Al-(Ti,V,Nb,Ta)-C and Al-(Ti,V,Zr,Ta)-C system were studied.The specific conclusions are as follows:?.Combustion synthesis experiment of single metal element and 2-metal element systems shows that Ti,V,and Ta elements have strong affinity for each other,and it is possible for these elements to form multi-principal element carbides.These elements can be used as the preferred elements for high-entropy carbide elements.?.Single-phase high-entropy carbides can be synthesized in Al-(Ti,V,Nb,Ta)-C and Al-(Ti,V,Zr,Ta)-C systems.(Ti0.25V0.25Nb0.25Ta0.25)C nano-particles with uniform solute distribution were successfully synthesized by heating Al-Ti-V-Nb-Ta-C powder mixtures at 1500? for 10 min.In Al-(Ti,V,Zr,Ta)-C system,higher temperature are required for the synthesis of high-entropy carbide particles.?.The addition of Al changed the reaction path of(Ti,V,Nb,Ta)-C system and(Ti,V,Zr,Ta)-C system.Aluminum not only reacts with transition metal elements to form the multi-principal element(MPE)intermetallics,which act as intermediates to the formation of MPE carbides,but also acts as a liquid solvent and facilitated the diffusion of transition metal elements.(3)Both the Al content and pressure have a significant impact on phase composition of combustion synthesis product of Al-(Ti,V,Nb,Ta)-C system:?.When the Al content is less than 30 vol.%,single-phase high-entropy carbides cannot be synthesized.When the Al content is 45?60 vol.%,the product is composed of intermetallics and(Ti,V,Nb,Ta)C.When the Al content is 75?90 vol.%,the product is composed of a-Al phase and(Ti,V,Nb,Ta)C particles?.During the formation of a single-phase solid solution,the application of pressure is not conducive to the progress of the high-entropy carbide synthesis reaction.After forming the high-entropy carbides,the application of pressure does not affect the phase composition of product(4)The effect of the content of(Ti,V,Nb,Ta)C particles on the properties of(Ti,V,Nb,Ta)C/Al composites was revealed.It is found that the hardness and strength of(Ti,V,Nb,Ta)C/Al composites increases whilst the electrical conductivity decreases with the increase of the content of addition of(Ti,V,Nb,Ta)C particles.