Combustion of mechanically activated Ni/Al reactive composites with microstructural refinement tailored using two-step milling.

Metal-based reactive composites are high energy-density materials that have potential uses as multifunctional energetics. However, when composed of micron size particles they can be difficult to ignite and have slow reaction rates. Recent work has shown that mechanically activated (MA) materials can have increased ignition sensitivity and reaction rate, yet the role of microstructure refinement (i.e., mechanical activation duration) in controlling combustion behavior is not well understood. In this work, the combustion velocities and flame temperatures were measured for equiatomic MA Ni/Al reactive powders produced using different milling durations in a two-step dry/wet milling process. For MA Ni/Al pellets pressed to 70% of the theoretical maximum density, it was shown that the combustion velocities increase as the milling time increases from ∼9.4 cm/s at 25% of the critical reaction milling time (t cr ) to ∼20 cm/s at a milling time of 97% t cr . For the cases considered, the average maximum flame temperatures were measured to be ∼1873 ± 30 K for samples milled for 25% t cr to 1786 ± 30 K at 97% t cr . It was also found that hydrocarbon contaminants are milled into the MA Ni/Al composite particles during the wet milling step and result in expansion of the pellets during combustion. Differential scanning calorimetry coupled with Fourier transform infrared spectroscopy showed that the release of hydrocarbon contaminants occurs at a temperature of ∼630 K. It was also shown that the concentration of hydrocarbon contamination decreased as the dry milling times increased, which suggests particle structure and mechanical property evolution during initial dry milling also affects contamination during subsequent wet milling. [ABSTRACT FROM AUTHOR]