Combustion Synthesis Path And Electronic Structure Calculation Of Ti₅Si₃ Doped With Ni

Refractory silicide Ti₅Si₃ is the new generation super-high-temperature candidate material, since it possesses low density, high specific strength and good high temperature oxidation resistance. However, given its low fracture toughness and roon-temperature brittleness, the investigation on Ti₅Si₃ is far lagged behind. Research shows that reasonable alloying can improve the fracture toughness of Ti₅Si₃. Results of first-principle calculation indicate that some alloying elements can effectively reduce the CTE (Coefficients of Thermal Expansion) anisotropy of Ti₅Si₃, and hence lower the microcracks in amounts during the cooling process from high temperature to room temperature. Besides, proper alloying elements can act as ductile phase distributed in boundaries of grains, and thus enhance the capacity of plasticity deformation of Ti₅Si₃; furthermore, the addition of alloying elements can serve as a diluent which decreases the reaction temperature of Ti₅Si₃, and therefore refines the grains of products. All these effects can enhance the fracture toughness of Ti₅Si₃, inspiring a growing amount of research on Me-Ti-Si system, which genetates some considerable advancement.Nevertheless, recent studies on Me-Ti-Si system have been mainly focused on various technologies of sintering synthesis and testing of property, only minor research has dedicated to combustion synthesis of the system, and the limited work was mainly focused on the analysis of the phase compositions and microstructures of products, while the investigation on the reaction path and mechanism of Ti₅Si₃ formation has hardly been conducted. In short, without a scientific and systematic understanding, will in turn, restrict the development of high temperature Ti₅Si₃-Me materials, and makes it imperative and meaningful to conduct research on this field.Since Ni atoms may replace Ti in Ti₅Si₃ lattices, and the remaining Ni can distribute between the grains, it is expected that the addition of Ni might improve the fracture toughness of Ti₅Si₃ and therefore establish value of the present study. In the present paper, we chose Ni as an alloying element to fabricate the Ti₅Si₃-Ni material via combustion synthesis. Particular attention was paid to the reaction path of Ti₅Si₃ in Ni-Ti-Si system; besides, the effect of Ni on the electronic structure of Ti₅Si₃ was also investigated. It is expected that the present work could lay some theoretical foundation for the effective implementation of Ti₅Si₃ alloying.Results of the present study are:(1) The reaction path of Ni-Ti-Si system under DTA condition can be described as: Ni and Si firstly take reaction via solid-diffusion reaction and form the metastable NixSiy phase. When the temperature exceeds theαT i→βTi phase transformation temperature,βTi and Ni as well asβTi and Si react via solid-diffusion reaction to form metastable NixTiy and TixSiy phases, respectively. As the temperature further increases, NixTiy and TixSiy melt via eutectic reaction, forming local binary melts and subsequently spraying rapidly on the surface of powders and particles. Simultaneously, the unreacted powders and metstable phases will dissolve into the melts and form local ternary melts of Ni-Ti-Si. When the concentration of [Ti] and [Si] in the ternary melts become saturated, the stable Ti₅Si₃ phase begins to precipitate out of the melts.(2) The reaction path of Ni-Ti-Si system under TE mode can be described as: Ni and Si take reaction via solid-diffusion reaction and form the metastable NixSiy phase releasing much reaction heat, which ignite the TE reaction among reactants Ni, Ti and Si to form the preliminary phase Ti₅Si₃. Simultaneously, heat generating from TE reaction makes the unreacted powders and NixSiy metstable phases melt and form ternary melts of Ni-Ti-Si. When the concentration of the [Ti] and [Si] in the ternary melts become saturated, the stable Ti₅Si₃ phase begins to precipitate out of the melts.(3) With the increase in the Ni content, the ignition temperature of TE reaction decreases, and the ignition time reduces; when Ni content increases from 10 to 30 wt.%, all the products of TE consist of the stable Ti₅Si₃ phase and metastable NiTiSi and Ni3Si2 phases, and the size of Ti₅Si₃ grains gradually decreases. The maximum combustion temperature and the combustion velocity of SHS reaction gradually reduce as the Ni content increases; fine Ti powders and high preheating temperature both can facilitate the reaction system to reach a higher combustion temperature, and further prolong the high temperature duration of liquid phases, which can lead to the precipitation of Ti₅Si₃ grains more sufficient and the reduce of the remaining liquid phases.(4) According to the first-principles calculation results, compared with the Si and Ti6g sites, when Ni occupies the Ti4d sites, the electronic orbits have more overlaps, and bond at lower energy level, which decreases the energy of the system significantly, and therefore, Ni has the priority to occupy the Ti4d cites. When Ni replaces one of the Ti atoms located in Ti4d cites, the strong interaction between Ni-Si can reduce the energy of Si, and the distance between Ni-Si will decrease (compared with Ti4d-Si), which will result the distortion of lattices and reduce the lattices parameters; simultaneously, the distortion of lattices reduce the equilibrium distance between Ti6g-Si, and the repulsion effect caused by which lead the increase in energy of Ti6g. The two adverse process of energy variation lead to the total energy of the system enhance and hence to the instability of Ni-doped Ti₅Si₃.