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 A and 4.08 +- 0.01 A 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 0C. 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