Criteria of the Critical State of the Ni-Al System during Mechanical Activation

The influence of mechanical activation on the structural state and reactivity of a Ni-Al powder mixture is studied. Mechanical activation of the Ni-Al mixture is demonstrated to have an induction period that involves accumulation of deformation defects of the crystal structure of the species; the NiAl phase of the reaction product is not yet registered in this period. Partial amorphization of aluminum increasing the system reactivity is revealed. Based on results of a diffraction analysis, a quantitative criterion of the state of the species during their activation is proposed: the value of stresses of the second kind. The system becomes unstable and autogenerates a synthesis reaction when microstresses are accumulated in the initial material to the level of the yield stress of the corresponding substance.