EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE ORDER-DISORDER TRANSFORMATION IN NI3A1

The crystalline disordered phase obtained by mechanical alloying of el emental 75 at. % Ni and 25 at. % Al powders has been investigated. The stability of this phase with respect to the thermal reordering proces s leading to the L1(2) Structure has been analyzed by means of x-ray d iffractometry' scanning electron microscopy, and differential scanning calorimetry. Atomistic simulations on an Ni3Al model, reproduced via molecular dynamics using a many-body potential, have been used to inte rpret experimental data. The ordering transformation takes place in an extended range of temperature (from 320 to 600-degrees-C) and occurs simultaneously with the release of internal strain. Numerical simulati ons performed under different conditions show that the activation ener gy of the Ni-vacancy migration mechanism responsible for the ordering process depends on the local state of strain, thus suggesting an expla nation for the considerable lowering of this energy in samples obtaine d by ball milling.