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 of the reaction products at different stages of milling has been investigated by differential scanning calorimetry. The sequence of reactions observed in each sample depends even in its early steps on the initial composition of the mixture. The end-product of the milling process is, 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 and of the Al3Ni compound at 25% Ni. Thermal treatment of samples at an early stage of milling shows, contrary to the behaviour induced by plastic 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 that this high diffusivity is the main cause of the observed differences.