Shock compression of reactive Al/Ni multilayers - Phase transformations and mechanical properties

Abstract

Reactive multilayers store large amounts of chemical energy, which can be released through a self-sustaining reaction. One way of triggering the self-sustaining reaction is mechanical ignition, which is a prerequisite for designing a self-healing system. For potential integration into various devices, it is important to understand how Al/Ni reactive multilayers behave under shock compression. In this study, molecular dynamics (MD) simulations are employed to investigate Al/Ni reactive multilayers under shock compression. MD simulations allow for the understanding of what is happening at the atomistic level. Furthermore, they give access to bilayer heights that are difficult to study otherwise. This allows studying the shock wave propagation from bilayer heights of 100 down to 5 nm, while at the same time observing what is happening atomistically. Shock compression is studied both, for interfaces parallel and normal to the shock wave. It is shown that when the shock wave is parallel to the Al-Ni interfaces, there is a clear relationship between bilayer height and effective elastic modulus, which is not true when the interfaces are normal to the shock wave. Furthermore, intermixing of Al and Ni, as a prerequisite for ignition, strongly depends on the bilayer height as well as the impact velocity. Behind the shock wave, a phase transformation occurs, which strongly depends on the impact velocity, with a weak dependence on the bilayer height. Furthermore, void nucleation and fracture are observed, where the voids start nucleating in the Al layers.