THE EARLY STAGES OF SOLID-STATE REACTIONS IN NI AL MULTILAYER FILMS/

Ni/Al multilayer films with pair thicknesses of 10 and 20 nm and with overall compositions in the range 48-88 at. % Al were prepared by sput tering. For comparison, Ni-Al alloy films in the same concentration ra nge were prepared by co-deposition of the elements. The films were stu died by x-ray diffraction, electron diffraction, and differential scan ning calorimetry. It was found that the B2 NiAl phase with a metastabl e concentration of approximately 63 at. % Al was the first phase to gr ow upon annealing of the multilayer films. The growth of this phase co uld be described by Johnson-Mehl-Avrami kinetics with an activation en ergy of 0.8 eV and an Avrami exponent of 0.5. This low activation ener gy was consistent with the observation that the phase had formed durin g deposition and continued to grow upon annealing at low temperatures to thicknesses of a few nanometers. If the reactant phases were not fu lly consumed by the B2 phase growth, the subsequent reaction was the f ormation of NiAl3, previously thought to be the first product phase in the Ni-Al system. The reduction of driving force by the preceding B2 phase growth explains why the formation of NiAl3 takes place by a nucl eation-and-growth process, an observation that has been discussed cont roversially in the recent literature. The nucleation and growth of NiA l3 had an activation energy of 1.5 eV in agreement with previous studi es. (C) 1996 American Institute of Physics.