Structural and thermal characterizations of the solid-state reaction between Ni, Al, and Ti powders during mechanical alloying

The mixtures of Ni, Al, and Ti elemental powders with nominal compositions Ni-30 at.% Al-10 at.% Ti were mechanically alloyed in a planetary ball mill under argon atmosphere. The structural and morphological changes during high-energy mechanical alloying were investigated by X-ray diffraction and scanning electron microscopy. We have found differences in the kinetics of solid-state reactions which depend on the differences in the individual properties. The final products of the mechanical alloying process were nanocrystalline nickel-rich solid solutions with a mean crystallite sizes in the range of a few nanometers. As a result, the prolonged milling time allows the disordered Ni-3(Al,Ti) solid solution to dissociate to the L20-Ni(Al,Ti) phase and L12-Ni-3(Al,Ti) phase. Further, it was found that the formation of L20-Ni(Al,Ti) phase was favored when considering the total reaction of the finite quantities of Al and Ti in the presence of an excess of Ni. Calorimetric studies demonstrate the coexistence of overlapping exothermic peaks characteristic of crystallization processes associated with the identified phases.