Thermodynamic interpretation of reactive processes in Ni-Al nanolayers from atomistic simulations

TL;DR

This study uses atomistic simulations and thermodynamic calculations to understand the early reaction processes in Ni-Al nanolayers, revealing a consistent reaction pathway involving disordered and B2 phases.

Contribution

It introduces a thermodynamic framework combined with molecular dynamics to interpret short-timescale reactive processes in Ni-Al multilayers.

Findings

Disordered phase forms upon mixing as a precursor.

Reaction pathway always involves disordered then B2 phase.

Simulated times up to 30 ns reveal new phenomena.

Abstract

Metals which can form intermetallic compounds by an exothermic reaction constitute a class of reactive materials with multiple applications. Ni-Al laminates of thin alternating layers are being considered as model nanometric metallic multilayers for studying various reaction processes. However, the reaction kinetics at short timescales after mixing are not entirely understood. In this work, we calculate the free energies of Ni-Al alloys as a function of composition and temperature for different solid phases using thermodynamic integration based on state-of-the-art interatomic potentials. We use this information to interpret molecular dynamics (MD) simulations of bilayer systems at 800 K and zero pressure, both in isothermal and isenthalpic conditions. We find that a disordered phase always forms upon mixing as a precursor to a more stable nano crystalline B2 phase. We construe the reactions observed in terms of thermodynamic trajectories governed by the state variables computed. Simulated times of up to 30 ns were achieved, which provides a window to phenomena not previously observed in MD simulations. Our results provide insight into the early experimental reaction timescales and suggest that the path (segregated reactants)$\rightarrow$(disordered phase)$\rightarrow$(B2 structure) is always realized irrespective of the imposed boundary conditions.