High strength Ti–Ni-based shape memory thin films

Low temperature heat treatments of TiNi-based sputter-deposited thin films, which were proved previously to be a new method for producing very good shape memory properties for Ti–48.2at.%Ni films, have been applied to TiNi alloys with Ti-richer contents and Ti–Ni–Cu alloys. These heat treatments are classified into two types: (a) heating at the crystallization temperature, Tc; and (b) heating at a somewhat lower temperature than Tc (typically, Tc−50 K), but in both cases the as-sputtered amorphous thin foils are heated directly at those temperatures. Extremely thin plate precipitates that are non-equilibrium phase (bct) but perfectly coherent with the matrix are generated randomly in the former case, while, in the latter case, nanocrystals that have the exactly same orientation throughout a grain of 1–2 μm are produced with these thin plate precipitates at their boundaries. Owing to these coherent precipitates, very good shape memory effects are obtained, especially with high shape recovery stresses.