Shape Memory Behavior of Ti-Ni Thin Films Annealed at Various Temperatures

Abstract

Ti-Ni thin films with three different types of compositions, titanium-rich Ti-Ni, near equiatomic TiNi and nickel-rich Ti-Ni were prepared by sputtering. The sputter-deposited films were annealed at various temperatures between 773 and 973K in order to crystallize. After the heat treatment, the shape memory behavior was examined with a thermomechanical tester. The shape memory behavior of the near equiatomic composition films was not affected by the heat treatment. On the other hand, the shape memory behavior of the other composition films strongly depended on the annealing temperature. As the annealing temperature increases, the martensitic and reverse martensitic transformation temperatures of the nickel-rich films decreased and those of the titanium-rich films increased. These opposite behaviors of the transformation temperatures can be explained by the opposite dependence of the precipitation of the second phase on the annealing temperature. All the nickel-rich films exhibited good shape memory effect over a wide range of stress, while all the titanium-rich films exhibited a small plastic strain only in the stress range where the shape changes associated with both the R-phase and the martensitic transformations were observed.