SHS Reaction Path And Electronic Structure Calculation Of (Ti, Cr)C_x

SHS reaction path and electronic structure calculation of (Ti,Cr)CxTiC ceramic has become one of the most widely used ceramics in aerospace, mechanics and other applies due to its excellent physical and chemical properties, such as low density, high melting temperature, high hardness and modulus, as well as good thermal stability.In the past decades, self–propagation high temperature synthesis (SHS), has been widely used in fabricating ceramics, intermetallic compounds and other high temperature materials, due to its specific advantages including energy and time saving, high purity of products and low cost. Generally, TiC ceramic is produced by Ti and C powder mixture through SHS process. Recently, the third metal element (Al, Cu, Fe or Ni) of low melting point is added to the Ti–C system. The SHS reaction ignition temperature is effectively decreased because metal liquid is formed within a shorter time and a lower temperature. Also, the added metal could cost a large part of energy and increase the distance of atom diffusion preventing the growth of TiC particle. After reaction, the added metal servers as a binding agent between particles. As a result, the density and properties of the compound is improved. However, this material may work out at a relatively low temperature because the added low–point metal.The mechanical properties and formation of the materials depend on the synthesis mechanism. Until now, the SHS reaction of Me(Al, Cu, Fe and Ni)–Ti–C systems have been widely studied with low–melting metal added. Unfortunately, the researches on the SHS reaction of Cr–Ti–C system are limited. Most of these researches are forced on the products of SHS reaction, combustion temperature and ignition temperature of Cr–Ti–C system; very few works are forced on the SHS reaction path of Cr–Ti–C system. Compared with the other metal elements, Cr has a much higher melting (2130K), offering the materials higher working temperature. Also, Cr atoms can enter in to TiC lattice as the replacement of Ti atoms or in the form of Cr3C2, forming a different compound of NaCl–structure (Ti,Cr)C. Also, several chromium carbides such as Cr3C2, Cr7C3 and Cr23C6, which all have high melting point and hardness, can be formed and serve as binding agents between particles. The chromium carbides can work together with TiC and increase the work–temperature of the materials formed by Cr–Ti–C system.Due to the solid solution of Cr in TiC, the lattice parameters of TiC formed by the SHS reaction of Cr–Ti–C system vary as the Cr content changed. But the reason is still unclear. For one side, the solid solution of Cr in (Ti,Cr)Cx can affect the lattice parameters. For another side, the presence of vacancies and ordering in TiCx can also change the lattice parameters. So, these two factors should be considered to study change of TiC lattice parameter. Therefore, the system of Cr–Ti–C with high–temperature melting Cr metal added is studied in our research. The SHS reaction path of Cr–Ti–C system is deeply studied by combustion front quenching technique, the effects of Cr atoms on the lattice parameter of (Ti,Cr)Cx is also studied by first–principle method. It is hoped these researches can lay some theoretical foundation on the fabrication of TiC composites.The results are mainly concluded as followed:1. the SHS reaction path of Cr–Ti–C system can be described as: Firstlyα–Ti is transformed toβ–Ti at the temperature of 882℃. Thenβ–Ti and Cr also react via solid-diffusion and formed (βTi,Cr) layer, which is posited at the interface of Cr and Ti particles and grows gradually. As the temperature increase,β–Ti and C react via solid-diffusion and formed sub-stoichoimetic TiCx, some (Ti,Cr)Cx is also expected to form in the (βTi,Cr) layer. As the temperature increases, (Ti,Cr)Cx and TiCx increase. Due to the increased temperature of the system and high energy released from the formation of (Ti,Cr)Cx, (βTi,Cr) firstly melt and form Ti–Cr liquid at the temperature which is lower than the melting point of (βTi,Cr). After that, Large amount of C atoms diffuse into the liquid forming Ti–Cr–C liquid and then larger amount of (Ti,Cr)Cx precipitate from the liquid. When the SHS reaction is completed, the residual Ti–Cr liquids transform in to CrTi4 and Cr2Ti. The ignition mechanism of Cr-Ti-C SHS reaction is (βTi,Cr) and C solid-solid reaction.2. It is found that the CrTi4 and Cr2Ti phases in the products are not formed between Ti and Cr reaction. (βTi,Cr) phase is first formed via solid-diffusion of Ti and Cr, and CrTi4 and Cr2Ti phases are formed during the quenched process. The Cr content in (βTi,Cr) determines the formation of CrTi4 and Cr2Ti.3. The presence of vacancies result in increased formation energy and decreased of the stabilities of TiCx and (Ti,Cr)Cx. To a certain extent, more vacancies TiCx and (Ti, Cr)Cx have, less stable the TiCx and (Ti,Cr)Cx are. The presence of Cr atoms can decrease the formation of TiCx and (Ti,Cr)Cx, resulting in a better stability. More Cr atoms in (Ti, Cr)Cx, more stable the (Ti, Cr)Cx is.4. In the TiC crystal, the Ti atoms nearest to vacancy move away from the vacancy and Ti-C bond becomes shorter and stronger, resulting in the decrease of TiC crystal volume and lattice parameters. The crystal volume and lattice parameters of TiCx and (Ti,Cr)Cx shrink with the increasing of vacancies. When Cr atoms replace Ti in TiCx, the length of Cr–C bond is smaller than that of Ti–C bond. At the same time, the other Ti–C bonds become shorter. As a result, The crystal volume and lattice parameters of (Ti, Cr)Cx are decreased, and further decrease with the increase of Cr content in (Ti,Cr)Cx crystal.