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).
SHS (Self-sustained high-temperature synthesis) of intermetallic compounds: effect of process parameters by computer simulation
Abstract The very wide temperature ranges and the many different chemical and phase transformation steps experienced by an SHS process make the search for a unifying theory impractical and a numerical approach highly desirable. The work describes an investigation of the Al+Ni→AlNi SHS by a computer simulation method developed by the Authors and based on a finite difference numerical engine coupled to a detailed description of several assumed reaction steps. The model includes Al melting, Ni diffusion-controlled dissolution (and possibly Ni melting), nucleation and precipitation of AlNi, and eutectic deposition. The results are discussed from the point of view of feasibility of the process, transition from self-propagating to thermal explosion behavior, and influence of process variables such as reactant grain sizes, diffusion coefficient, initial composition, and thermal conductivity.
Reactive growth of niobium silicides in bulk diffusion couples
Abstract The diffusion-controlled growth of niobium silicides (NbSi 2 and Nb 5 Si 3 ) was studied in Nb/Si and Nb/NbSi 2 bulk diffusion couples annealed at 1200–1350 °C for 2–24 h. Both compounds were found to grow as parallel layers, according to the parabolic rate law. The concept of the integrated diffusion coefficient is used to describe the growth kinetics of the two silicides. The corresponding activation energy is 263 kJ/mol for Nb 5 Si 3 and 304 kJ/mol for NbSi 2 . The activation energy (in eV) scales as 0.98 T m (K)/1000 for Nb 5 Si 3 and as 1.4 T m (K)/1000 for NbSi 2 in agreement with the general behavior observed for many transition metal silicides. The position of the Kirkendall plane inside the Nb 5 Si 3 layer developed in Nb/NbSi 2 couples indicates that, in the present temperature range, the diffusion of Si in Nb 5 Si 3 is considerably faster than that of Nb.