SOLID-STATE REACTIONS BETWEEN NI AND AL POWDERS INDUCED BY PLASTIC-DEFORMATION

The solid state reactions induced by high-energy ball milling of Al/Ni elemental mixtures at 25, 33, 50, 67 and 75 Ni at.% have been studied by X-ray diffraction and electron microscopy. The thermal stability o f the reaction products at different stages of milling has been invest igated by differential scanning calorimetry. The sequence of reactions observed in each sample depends even in its early steps on the initia l composition of the mixture. The end-product of the milling process i s, in some cases, a metastable phase. For long milling times, in fact, we have observed the formation of a metastable f.c.c. solid solution at the two Ni-rich compositions, of the AlNi phase at 50 and 33% Ni an d of the Al3Ni compound at 25% Ni. Thermal treatment of samples at an early stage of milling shows, contrary to the behaviour induced by pla stic deformation, that at all compositions the first thermally driven reaction is the formation of Al3Ni. The experimental observation of an enhancement of Ni diffusivity in Al during ball milling, suggests tha t this high diffusivity is the main cause of the observed differences.