The kinetics of Ni/Al reactive intermetallic composites

Mathew J Cherukara, Purdue University

Abstract

Molecular dynamics (MD) simulations have been used to study the underlying physics and atomistic mechanisms of the reaction progression in Ni/Al reactive intermetallic composites. Preparation of these composites, either through deposition techniques or through the process of mechanical ball milling, gives rise to a periodic ordered, nanolaminated structure and in the first part of this thesis, the effects of this laminate period, ignition temperature and volumetric defects are studied. The presence of defects not only speeds up the reaction by as much as 5 times, but changes the nature of mass transport from diffusive to partly ballistic. Subsequently, the feasibility of using amorphous energetic materials is studied. The use of amorphous precursors is found to speed up the reaction as well as increase the heat of reaction, starting as it does from a higher energy state. Amorphous Ni recrystallizes at elevated temperatures and this process has been investigated (both thermal and shock induced recrystallization). The results presented herein, hint at the possibility of nanostructural tiling and the building of hierarchal nanostructures, starting from amorphous rather than liquid or chemical precursors. Through the use of large-scale, massively parallel simulations, the reaction pathways in Ni/Al systems that are close to equilibrium (through low temperature ignition studies) and the reaction paths in systems far from equilibrium (under dynamic mechanical loading) are described. To study the reaction mechanisms in the Ni/Al system close to equilibrium, thermal ignition studies on a core/shell structure of Ni/Al are performed. The reaction path is found to be dependent on ignition temperature, with nucleation of the NiAl intermetallic phase observed in samples ignited at 900 K or less. This nucleation of NiAl, which occurs under a large concentration gradient, slows mass diffusion and adversely affects the reaction rates. To study the far from equilibrium response, a granular Ni/Al composite system, built to mimic the experimentally observed structure, is studied under shock loading. A transition in the mechanism of compaction under shock loading was observed, from plastic deformation mediated pore closure at low impact strengths, to fluid flow and atomistic jetting into the pores at higher shock strengths. This is found to localize thermal as well as mechanical kinetic energy due to the formation of vorticity in the samples, leading to accelerated mixing in the pores, from where the reaction front initiates and propagates. Finally, an attempt is made to tie reaction kinetics obtained from MD simulations with continuum models plasticity models. Such a framework would provide the ability to model physically relevant length and time scales.

Degree

Ph.D.

Advisors

Strachan, Purdue University.

Subject Area

Materials science

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