Solution Doping Effects On Ti₂AlC Structure And Performance At Different Atomic Sites

The ternary layered compounds with the general formula Mn+1AXn(n=1 to 3), where M is an early transition metal, A is an A-group(mostly IIIA and IVA) element, and X is either C and/or N, have attracted considerable attention because of their unusually combination of ceramic and metallic properties. As a class, they are readily machinable, good electrical and thermal conductors, relatively soft, thermal shock resistant, damage tolerant, stiff, and have relatively low coefficients of thermal expansion.Up to now, approximately 70 MAX phases have been synthesized and more than a dozen have been widely studied on their structure, physical and mechanical properties. More and more interest is turned to the exploration of the role of chemistry in controlling physical properties through systematic investigations of the materials where the elemental constituents can be varied, while maintaining the same crystallographic structure. The study of solid solution is one of the methods.Ti2AlC is the typical 211 phases, its structure and properties have been deeply studied. So it would be helpful ti discuss the change of structure and propertied between Ti2 AlC and its solid solutions. In this paper, we fabricate a series of TixM2-xAlyA1-yCz N1-z by hot pressing, the introduction of energy of mixing is to judge the easy of synthesis of solid solutions. We systematically analysis the density of states near the Fermi level based on first principles to judge the stability of the structure. Elastic properties can be calculated by the elastic constants, the electrical and thermal physical properties of solid solution have been tested.The energy of mixing data indicates the unstable of TiCrAlC compared to other compounds. On synthesis, TiTaAlC as well as TiNbAlC are proved to be exist while TiCrAlC not. A wide range of TixTa2-xAlC solid solutions have also been successfully synthesized as theory predicted. It is worth noticing that the DOS of TiCrAlC changes significantly near the Fermi level, mainly attributing to the fact that both Ti d bands and Cr d bands affect the DOS. The shape of the PDOS of Ti and Cr are very different. However, in TixTa2-xAlC solid solutions, the shape of the PDOS of Ti and Ta are similar near the Fermi level, resulting in the fact that the DOS of solid solutions change little compared to Tix Cr2-xAlC.Several kinds of solid solutions such as TiZrAlC,TiVAlC,CrVAlC and CrTaAlC are selected to analysis, showing that he computing of Emix is relatively correct on the synthesis of solid solution, the density of states provide the visual angle of observation on the stability of solid solution.Element S is to substitute Al in Ti2 AlC, XRD show that the expected Ti2A0.5S0.5C solid solution is not observed but the phenomenon of two-phase coexisting of Ti2 AlC and Ti2 SC appeared. Using in-situ reaction method, we have studied the sinter process of the layered ternary Ti2 SC. By XRD and thermodynamic calculations we know that Ti2 SC is the final result of the reaction between TiS and TiC. TiS is an important intermediate which is more possible the result of TiS2 and Ti rather than the decomposition of TiS2. DSC curves are used to investigate the reaction kinetics of the whole sintering process. By Kissinger and Ozawa method the activation energy are separately calculated to be 125.4 kJ/mol and 134.9 KJ/mol. The special of Ti2 SC can be seen in DOS which shows that the connection between Ti and S are not the metallic bonding as other MAX compounds, but the covalent bond. As the difference between Ti-S and Ti-Al, the formation of solid solution may destroy the stability of the structure.By the study of X site solid solution, it is shown that solid solutions can be synthesized with well-controlled chemical compositions in the entire composition range. Solid solution show its little change of density of states.By the research of the reaction process, the synthetic mechanism of solid solution is investigated. XRD reveal the formation of TiNbAlC is possible the reaction between Nb-Al intermetallic compound and TiC.The calculated elastic constants show that the majority of the MAX-phase compounds have C11 greater than C33.However by the substitute with A for Al, as showed in the case of Ti2 GeC, Ti2 SnC, and Cr2 AlC, C11 and C33 are comparable or a little small which mean that Al-containing compounds have a larger strain deformation resistance along the axis. Most MAX compounds have a higher C44 values. By comparison, the elastic constants of M site solid solution and pure substance is not a linear relation. This means that in terms of structure, mechanical properties of solid solution will be changed.There is a direct relationship between the hardness and the elastic constants C44 in this kind of ternary compounds. The greater the C44 is, the greater the hardness of materials. Solid solution doping effect on C44 is different, some C44 value become larger and some become smaller. Among M2 AlC compounds, the elastic performance of Cr2 AlC is superior to other substances. The elastic performance of M site solid solution has improved significantly. By the substitution in A sites, elastic performance will decline. The elastic properties have a direct relationship with M-C bond strength, The stronger the bond strength, The high elastic modulus. The elastic property of the X site solid solution is between in Ti2 AlC and Ti2 AlN.According to mechanical properties test, the performance of M site solid solutions has a certain degree of decline which may due to the quality of the sample preparation. Similar situation is on the Electrical performance of the research shows that the value of DOS near the Fermi level has no direct relationship with the electrical conductivity of the material. The more value means higher electronic concentration, but also could reduce the electron transfer rate. Heat conductivity of solid solution increase with the rise of temperature, thermal conductivity at room temperature reduce obviously. This is mainly because the increase of resistivity reduces the electron thermal conductivity, the increase of the defect formed by the solution reduces the phonon thermal conductivity.