Thermal and ion-induced, metastable-cubic Al sub(3)M 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 Al sub(3)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 plus or minus 0.01 angstrom and 4.08 plus or minus 0.01 angstrom for Al-Ti and Al-Hf, respectively, at a composition of Al sub(3)M. The metastable phase transforms to the tetragonal equilibrium compound at annealing temperatures approximately 400 degree 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.