Thermal and ion-induced, metastable-cubic Al3M phases in Al-Ti and Al-Hf thin films

Thermal and ion-induced reactions in codeposited, multilayer and bilayer Al-Ti and Al-Hf structures were studied by transmission electron microscopy and Rutherford backscattering. A metastable Al3 M (M being Ti or Hf) phase was found by both ion beam mixing and thermal annealing. The metastable phases have a cubic structure with lattice parameters a=3.98±0.01 Å and 4.08±0.01 Å for Al-Ti and Al-Hf, respectively, at a composition of Al3M. The metastable phase transforms to the tetragonal equilibrium compound at annealing temperatures approximately 400 °C. The effectiveness of ion beam mixing is demonstrated by its lower formation temperatures and faster reaction rates. The thermally induced metastable phase formation is ascribed to a lower nucleation barrier than that of the equilibrium phase.