High-pressure shock activation and mixing of nickel-aluminium powder mixtures

Abstract

The adiabatic chemical reaction behaviour of shock-compressed Ni-Al powder mixtures of varying morphology and different volumetric distributions has been investigated by microstructural and differential thermal analysis (DTA) to understand the mechanistic changes responsible for chemical reactions occurring during shock treatment. Mechanically mixed Ni-Al powders undergo exothermic chemical reactions at temperatures close to the melt-temperature of AI. In contrast, shock-treated Ni-Al powder mixtures exhibit a “pre-initiation” exothermic event, before the main exothermic reaction. Different forms (reaction start and peak temperatures) of the preinitiation exotherm are observed depending on the degree of macroscopic mixing, contact intimacy and activation, accomplished during shock compression of the powder mixtures of different morphology and volumetric distribution, all shock-treated under the same conditions. Mixtures containing equimolar volumetric distribution of powders of more irregular (flaky) morphologies undergo a significant extent of configuration change during shock-compression, resulting in the formation of an activated, intimately mixed and close-packed state. In such a state, chemical reaction is readily initiated by external thermal stimulation, such as heating during DTA. In fact, a greater degree of configuration change, activation and more intense mixing occurring during shock-compression can even lead to reaction initiation and completion in the shock duration itself.