Modeling heterogeneous interaction during SHS in the Ni-Al system: A phase-formation-mechanism map

Abstract

Heterogeneous reaction Ni + Al → NiAl in strongly non-isothermal conditions typical of SHS has been modeled for planar geometry (unit bilayer Ni/Al film). The model used for simulation is based on the traditional quasi-equilibrium concept of the diffusion-controlled growth of a continuous solid product layer, NiAl, between molten Al and solid (or liquid) Ni-based solution. The basic idea is to analyze the competition between the growth of NiAl and its dissolution in parent phases due to variation in solubility limits with temperature. In calculations, a fragment of the equilibrium Ni-Al phase diagram is used along with real values of the interdiffusion coefficient in NiAl instead of determining the activation energy and preexponent by fitting the calculated and experimental results of the SHS wave temperature profile (as was done in other modeling works). The temperature dependence of phase densities is also accounted for. Simulation has revealed the existence of critical heating rates that correspond to the onset of the quasi-equilibrium dissolution-precipitation route with an intermediate solid NiAl film separating two liquid phases and then to non-equilibrium crystallization (when this film dissolves completely). Thus, the existence of non-equilibrium interaction mechanisms during SHS in the Ni-Al system, which were observed experimentally, is confirmed theoretically by the ex adverso method. A map of phase formation mechanisms is plotted which permits determining the reaction route in non-isothermal conditions.