MSE PhD Defense - Jennifer Breidenich

Date:  Friday, September 18

Time:  3:00 pm

Location:  Room 3515, MRDC (Hightower Conference Room)

Committee:

Dr. Naresh Thadhani, Advisor, MSE

Dr. Sunil Dwivedi, MSE 

Dr. Arun Gokhale, MSE 

Dr. Tom Sanders, MSE

Dr. Suhithi Peiris, DTRA

Title:  Combustion of Aluminum Under High Strain Rate Conditions

Abstract:  This work focuses on the understanding of impact-initiated combustion of aluminum powder compacts.  Aluminum is typically one of the components of intermetallic-forming structural energetic materials, such as Al+Ni, Al+Ta, and Al +W.  Intermetallic-forming structural energetic materials (SEMs) are highly desirable for several applications due to their rapid energy release characteristics and mechanical properties.  In order to better understand the process of reaction initiation in intermetallic-forming SEMs, this work focuses on the understanding of impact-initiated combustion of compacts of one of the intermetallic-forming SEM constituents, aluminum powders.  Aluminum powders of various sizes and different levels of mechanical pre-activation are investigated to determine their reactivity under uniaxial stress rod-on-anvil impact conditions, using a 7.62 mm gas gun.  The compacts reveal light emission due to combustion upon impact at velocities greater than 170 m/s.  Particle size and the level of mechanical pre-activation influence combustion initiation on the particle-level by controlling the localized friction, strain, and heating between particle surfaces, as well as on the continuum-scale, by controlling the amount of energy required for compaction and deformation of the powder compact during uniaxial stress loading.  Compacts composed of larger diameter aluminum particles (approximately 70µm) are shown to be more sensitive to impact initiated combustion than those composed of smaller particle diameters.  Additionally, mechanical pre-activation by high energy ball milling (HEBM) of the aluminum powders increases the propensity for reaction initiation at low velocities.

The mechanistic processes leading to reaction initiation in the Al samples are investigated via high speed and IR imaging of light associated with the reaction.  Images captured during compaction and deformation, revealing light emission and temperature rise, were correlated with meso-scale CTH simulations performed using real microstructures of aluminum powder compacts.  The results of mesoscale CTH simulations reveal that initiation of combustion reactions in aluminum powder compacts is closely tied to mesoscale processes, such as particle-particle interactions, pore collapse, and particle-level deformation.  These phenomena could not be measured directly because traditional pressure and velocity sensors are large in surface area and spatially average out the effects of these mesoscale processes, rendering them incapable of probing the heterogeneous processes taking place at this length scale.  In order to address this issue, quantum dots (QDs) are investigated as possible meso-scale pressure sensors for probing the shock response of heterogeneous materials directly. Impact experiments were conducted on a QD-polymer film using a laser driven flyer setup at the University of Illinois Urbana-Champaign (UIUC).  In situ time-resolved spectroscopy is used to monitor the energy shift and intensity loss as a function of pressure over nanosecond time scales.  Shock compression of a QD-PVA film results in an upward shift in energy (or a blueshift in the emission spectra) and a decrease in emission intensity.  The magnitude of the shift in energy and the drop in intensity is a function of shock pressure; this relationship can be used to track the differences in the shock pressure at the voids and interfaces of granular materials, such as aluminum powders.  Quantum dot-based mesoscale diagnostics pave the way for a better understanding of the mesoscale mechanisms involved in the high strain rate-initiation of intermetallic reactions.