Date of Award

Fall 2014

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Mechanical Engineering

First Advisor

Steven F. Son

Second Advisor

Lori J. Groven

Committee Chair

Steven F. Son

Committee Co-Chair

Lori J. Groven

Committee Member 1

Alejandro H. Strachan

Committee Member 2

I. Emre Gunduz


Metal-based reactive composites are a class of materials that consist of at least one metals, such as Ni/Al, that have high-energy densities and can produce significant energy output during exothermic reaction after thermal or mechanical initiation. However, conventionally these materials typically have slow reaction rates and are difficult to ignite at typical micron particle size ranges limiting their application. Therefore, mechanical activation techniques have been used to create materials with high surface areas and smaller characteristic dimensions in order to increase combustion velocity and ignition sensitivity. Their combustion and mechanical impact behavior is being studied to develop the understanding needed so that the materials can be ultimately developed for applications such as multi-functional energetic materials, blast enhancement and synthesis of novel metastable non-equilibrium materials. ^ In this work Ni and Al powder is mixed by High-Energy Ball Milling (HEBM) to produce a mechanically activated (MA) Ni/Al reactive composite. A two-step process is adopted that includes dry milling followed by wet milling using hexanes as a process control agent. The microstructure of the resulting powder contains layered Ni and Al laminates that have micron to nano-scale dimensions depending upon the dry milling time and particle size of the material. The mechanical impact response and combustion behavior of these materials was studied through a series of experiments. ^ Mechanical impact experiments were performed using a modified Asay shear experiment where properties such as mechanical impact ignition threshold, ignition delay time, and combustion velocity were identified. It was found that the mechanical impact ignition threshold decreases as the dry milling time increases. The material with the longest dry milling time considered (97% tcr where tcr is the critical milling time that results in combustion during milling, which was 17.5 minutes, ignited at impact energy of about ∼50 J or higher (projectile speed of about ∼65 m/s). Ignition delays due to the formation of hotspots ranged from 1.2 to 6.5 msand were observed to be in the same range for all milling times considered less than tcr. ^ Combustion velocities ranged from 25-31 cm/s for impacted samples at an impact energy of 200-250 J. Combustion experiments on MA Ni/Al pressed into cylindrical pellets shows that the combustion velocities increase as the milling time increases from ∼9.4 cm/s at 25% tcr to ~20 cm/s at a milling time of 97% tcr. Maximum combustion temperatures were measured to be ∼1870 ± 35 K for samples milled up to 50% tcr, whereas combustion temperatures for materials milled for 97% tcr were on average of 100 K lower. It was also shown that hydrocarbon contaminants are milled into the MA Ni/Al composite particles during the wet milling step and result in the expansion of the pellets during combustion. It was shown that the concentration of hydrocarbon contamination decreased as the dry milling times increased, which suggests particle structure and mechanical property evolution during dry milling also play a role in contamination during wet milling. ^ Mechanically activated Ni/Al composite powders were also annealed at two different temperatures to observe the effect of intermetallic formation and strain relaxation on the reaction kinetics. Williamson-Hall analysis suggests that recovery occurs at an annealing temperature of 403K, resulting in lower strains for both Al and Ni. At an annealing temperature of 460 K, both recovery and Al grain growth was observed along with the growth of NiAl3 phase, which was also detected using scanning electron microscopy. The morphology and characteristic laminate dimensions were not affected significantly by annealing. The 403 K annealing had little effect on combustion velocities and temperature, but the 460 K annealing significantly reduced the combustion velocities and temperatures. This shows that the combustion velocities and temperatures are not significantly affected by strain relaxation but are largely influenced by the formation of NiAl3 during annealing. ^ Lastly, it was shown that cleaning the milling jar with hexanes, as opposed to water, decreased the amount of cold-welding on the milling jar walls and media. This reduced the finial yield of fine particles (< 106 μm) and increased the concentration of solid solutions, and intermetallics. As a result of this loss in available heat release, combustion temperatures, and combustion velocities decreased.