Influence of the high energy ball milling on structure and reactivity of the Ni + Al powder mixture

Investigation of the micro- and atomic structures for the planetary ball-milled Ni + Al mixtures has revealed existence of intermediate nano-crystalline and amorphous phases, which are not detectable by XRD analysis, but can be observed by means of HRTEM. Annealing of the milled mixtures at moderate temperature, 205-280 °C, transforms the nano-phases into crystal state and makes them XRD-detectable. These nano-scale structures may serve as nucleus for the intermetallic phases produced via reaction between Ni and Al and, therefore, decrease the activation energy of this reaction and diminish the temperature of the reaction initiation. Apparently, the active nano-phases are formed due to sliding of Al and Ni layers during the ball milling. The simple model is suggested, which allows non-contradictory explanations of the nucleus formation and their influence on the reactivity of the mixture.