The role of fracture in the impact initiation of Ni-Al intermetallic composite reactives during dynamic loading

Mechanical initiation of reactions in powder systems involve complex interactions that can yield unexpected results. Two materials that exhibit similar thermal responses can behave very differently under the same loading conditions due to differences in their mechanical properties. Reactive composite powders with small characteristic dimensions can exhibit short ignition delays and reduced thermal ignition thresholds; however, a full understanding of the response of these powders to rapid mechanical loading is still unclear. This work seeks to clarify the role of mechanical properties in impact induced ignition by considering the response of nanolaminate (NL) powders and high energy ball milled (HEBM) Ni-Al powders subjected to impact loading. The selected materials have nearly identical thermal ignition temperatures, but contrast a relatively complex microstructure (HEBM) with a simple ordered microstructure (NL) while maintaining comparable length scales (approximately 50-100 nm diffusion distances). The powders were placed into a windowed enclosure and mechanically loaded using a light gas gun, which allowed the resulting reactions to be observed using high-speed imaging. Even though the thermal ignition temperatures for the two powders are within 30 degrees C of each other, it was observed that the NL powders reacted on the microsecond timescale, immediately following the compaction wave for a short distance before decoupling from the compaction front. In contrast, the HEBM powders reacted after a several millisecond delay and clearly propagated as a deflagration front. Microindentation showed that the HEBM powders are much more ductile than those of NL. This suggests that the primary difference between the behavior of these materials on impact results from the ability and degree of the material to fracture, illustrating that the mechanical properties of a reactive material can have a dramatic effect on ignition during impact loading. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.