Al/Ni formation reactions: characterization of the metastable Al9Ni2 phase and analysis of its formation

The metastable phase Al9Ni2 was investigated in order to characterize the thermodynamics and kinetics of its formation. Here, Al9Ni2 was observed as the first phase to form in a series of sputter-deposited Al/Ni multilayer foils, but it did not form in foils with a very small bilayer period (12.5 nm). In these foils, the stable phase Al3Ni was the first phase observed. Using differential scanning calorimetry, the heat of formation and the Gibbs free energy for the formation of Al9Ni2 were both calculated to be −28 kJ/mole·atom. The average activation energy for Al9Ni2 was determined using a Kissinger analysis and was found to vary with bilayer thickness, with an average value of 1.58 ± 0.06 eV. In light of these experimental results, a simple nucleation model based on thermodynamics and diffusional intermixing is proposed to explain why Al9Ni2 forms before Al3Ni in most cases, except in foils with very small bilayers where it is absent.