Silvia Gennari

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.

Dynamic behaviour and chemical mechanism in the self-propagating high-temperature reaction between Zr powders and oxygen gas

The self-propagating high-temperature reaction between zirconium powders and oxygen gas has been investigated by computer simulated experiments using a one-dimensional model which accounts for (a) solid state diffusion in the oxide layer growing at the surface of each reacting zirconium grain and (b) the gaseous flux of oxygen molecules towards the reacting grain. Two process parameters—dilution with additional zirconia, and oxygen pressure in the reaction environment—have been considered. The results show that the reaction can propagate under steady conditions with constant wave velocity and maximum temperature or under various unsteady conditions. The main process parameter controlling the dynamic behaviour is the dilution degree, with oxygen pressure playing a minor role. Diffusion through the product layer and pressure flux can both be found as the rate determining step under steady propagation conditions, while unsteady propagation is always under diffusion control. Lowering oxygen pressure has a stabilizing effect on unsteady propagation.