Modeling nonisothermal interaction kinetics in the condensed state: A diagram of phase formation mechanisms for the Ni–Al system

Abstract

Nonisothermal interaction in condensed Ni–Al system takes place during combustion synthesis (CS) or the so-called self-propagating high-temperature synthesis, which is characterized by a high heating rate, high final temperature, fast accomplishment of interaction, and unconventional, nonequilibrium phase formation mechanisms. In this work, a kinetic model for phase formation during CS of nickel monoaluminide in a layered Ni–Al system is developed basing on the diffusion-controlled growth of a solid product layer in strongly nonisothermal conditions. The model includes competition between the growth of NiAl and its dissolution in the parent phases (Al-base melt and solid or liquid Ni-base solution). For numerical calculations, real values of diffusion parameters in NiAl and the Ni–Al phase diagram are used. Simulation has revealed the existence of critical heating rates that correspond to a transition from the common diffusion-controlled growth mechanism to quasiequilibrium dissolution-precipitation pattern and then to nonequilibrium crystallization route. Thereby the existence of uncommon interaction pathways during CS of intermetallics, which were observed experimentally and debated in literature, is proved theoretically ex contrario. A map of phase formation mechanisms for reaction Ni+Al→NiAl in nonisothermal conditions is constructed, which defines the applicability domains of previously known theoretical approaches and permits predicting the interaction route.