Formation of titanium silicides Ti5Si3 and TiSi2 by self-propagating combustion synthesis

Preparation of titanium silicides Ti5Si3 and TiSi2 from elemental powder compacts of their corresponding stoichiometries was conducted by self-propagating high-temperature synthesis (SHS) in this study. Effects of the sample green density, preheating temperature, and starting stoichiometry on combustion characteristics, as well as on product composition were studied. Experimental evidence indicated that a self-sustained combustion front was established upon ignition and subsequently traversed the entire sample in a steady manner. After the passage of the flame front, further phase transformation taking place in the sample led to the emergence of afterburning glows. As a result of the combustion temperatures exceeding the lowest eutectic point (1330 deg C) of the Ti-Si binary mixture, formation of Ti5Si3 from the stoichiometric powder compact is primarily dominated by the solid-liquid mechanism, which involves the dissolution of solid reactants and the precipitation of silicide products. Moreover, complete conversion yielding a single-phase silicide Ti5Si3 was achieved in this study. On the contrary, the interaction between reactant elements within the Ti+2Si compacts is governed by a solid-state mechanism, due to their low reaction temperatures less than the Ti-Si eutectic point 1330 deg C. The XRD analysis identifies the disilicide TiSi2 as the major composition in the final products of Ti+2Si samples. In addition to TiSi2, however, small amounts of TiSi and Si were detected in the products obtained from the samples of Ti:Si=1:2. It is believed that different reaction mechanisms are responsible for the significantly higher propagation velocity of the reaction front observed in the 5Ti+3Si compacts than in the samples of Ti:Si=1:2. Based upon the dependence of flame-front velocity on combustion temperature, the activation energies in association with formation of Ti5Si3 and TiSi2 by SHS were determined to be 205.2 and 165.4kJ/mol, respectively.