HIGH-PRESSURE SHOCK ACTIVATION AND MIXING OF NICKEL-ALUMINUM POWDER MIXTURES

The adiabatic chemical reaction behaviour of shock-compressed Ni-Al po wder mixtures of varying morphology and different volumetric distribut ions has been investigated by microstructural and differential thermal analysis (DTA) to understand the mechanistic changes responsible for chemical reactions occurring during shock treatment. Mechanically mixe d Ni-Al powders undergo exothermic chemical reactions at temperatures close to the melt-temperature of Al. 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, ac complished during shock compression of the powder mixtures of differen t 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 significa nt extent of configuration change during shock-compression, resulting in the formation of an activated, intimately mixed and close-packed st ate. In such a state, chemical reaction is readily initiated by extern al thermal stimulation, such as heating during DTA. In fact, a greater degree of configuration change, activation and more intense mixing oc curring during shock-compression can even lead to reaction initiation and completion in the shock duration itself.