Intermetallic phase formation in nanometric Ni/Al multilayer thin films

Ni/Al multilayer films were deposited by dc magnetron sputtering from pure nickel and aluminium targets. Films with periods between 5 and 140 nm were produced. All the multilayers were tailored in order to have an overall atomic composition close to 50Al:50Ni. The as-deposited films are constituted by alternate nickel and aluminium layers that upon thermal annealing react with each other to form intermetallic compounds. During deposition an amorphous phase is formed at the interfaces, which is more evident for low periods where the interfaces constitute a large portion of the multilayers. For equiatomic chemical compositions and periods up to 140 nm, it has been shown that after annealing at increasing temperatures the ultimate phase is B2-NiAl. However, the structural evolution towards equilibrium strongly depends on the multilayer period. The presence of Al-rich intermediate phases, namely NiAl3 and Ni2Al3, is observed for the higher periods. The intermediate periods seem to be the most promising for future use in joining applications.