  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.
