Experimental and Numerical Studies on Self-Propagating High-Temperature Synthesis of Ta 5 Si 3 Intermetallics
Formation of Ta5Si3 by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of Ta:Si = 5:3 was experimentally and numerically studied. Experimental evidence showed that the increase of either sample density or preheating temperature led to the increase of combustion wave velocity and reaction temperature. The apparent activation energy, Ea ≈ 108 kJ/mol, was determined for the synthesis reaction. Based upon numerical simulation, the Arrhenius factor of the rate function, K0 = 2.5 × 10 s, was obtained for the 5Ta + 3Si combustion system. In addition, the influence of sample density on combustion wave kinetics was correlated with the effective thermal conductivity (keff) of the powder compact. By adopting 0.005 ≤ keff/kbulk ≤ 0.016 in the computation model, the calculated combustion velocity and temperature were in good agreement with experimental data of the samples with compaction densities between 35% and 45% theoretical maximum density (TMD).
Formation of titanium silicides Ti 5Si 3 and TiSi 2 by self-propagating combustion synthesis
Abstract Preparation of titanium silicides Ti 5 Si 3 and TiSi 2 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 °C) of the Ti–Si binary mixture, formation of Ti 5 Si 3 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 Ti 5 Si 3 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 °C. The XRD analysis identifies the disilicide TiSi 2 as the major composition in the final products of Ti + 2Si samples. In addition to TiSi 2 , 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 Ti 5 Si 3 and TiSi 2 by SHS were determined to be 205.2 and 165.4 kJ/mol, respectively.
Quantities of grains of aluminum and those of TiB2 and Al3Ti particles added in the grain-refining processes
Abstract Al–Ti–B alloys are the most commonly used grain-refiners in the casting of aluminum alloys; however, the grain-refining mechanisms are still not conclusive. Quantities of the two kinds of phase particles, TiB2 and Al3Ti, introduced into the molten aluminum by the addition of Al–Ti–B grain refiners are determined experimentally. It is found that the quantities of Al3Ti particles introduced are much lower than those of grains in the as-cast aluminum alloys, while those of introduced TiB2 are higher. Thus it is concluded the Al3Ti particles in the Al–Ti–B grain-refiners cannot be the primary nucleation sites during solidification of aluminum.