Electron microscopy of shock-synthesized silicides and aluminides

Abstract

The effects of shock‐wave passage through porous (powder) materials are complex, since intense and non‐uniform plastic deformation is coupled with the shock‐wave effects. Attempts to synthesize niobium and molybdenum silicides and nickel aluminides by the passage of shock waves through elemental powder mixtures resulted in an array of different microstructures. Extensive electron microscopy was employed to characterize both unreacted and reacted regions, and revealed only equilibrium phases. Convergent beam electron diffraction combined with x‐ray microanalysis was used to determine the crystal structure and compositions of the reacted products. The extent of reaction was found to vary to different degrees with shock pressure, initial temperature, morphology of the powder particles, elemental system and mixture ratios, among other features. Electron microscopy of partially‐reacted regions has led to the development of a new reaction mechanism based on the formation of a liquid‐disilicide reaction product at the metal‐silicon interface. For the nickel‐aluminides the extent of reaction was considerably less for comparable shock pressures, and the nickel particle morphology influenced the reaction kinetics leading to the formation of Ni‐aluminide compounds. Comparison of the microstructures produced in each of these systems has led to a better understanding of the role of elemental mixtures on the nature and extent of the reaction.