Microstructure and shape memory behavior of annealed Ti44.5Ni55.5−XCuX (X = 15.3–32.8) thin films with low Ti content

Abstract

(Ni,Cu)-rich Ti–Ni–Cu amorphous films with a low Ti content of 44.5at% and a Cu content of 15.3–32.8at%, formed by sputtering, were annealed for 1h at 773, 873, and 973K, and their microstructure and shape memory behavior were investigated. Ti(NiCu)2 precipitates were observed in all the annealed films. In addition to these precipitates, a TiCu phase was detected in the Ti45.6Ni21.6Cu32.8 films. These precipitates were densely distributed within the B2 grains in the films annealed at 773K. As the annealing temperature increased, the precipitates grew in size within the B2 grains. Unlike Ti–Ni–Cu films with 48.5at% Ti or greater, a few precipitates exclusively grew along the {100} planes and divided the original grains for the films annealed at 873K. The grain size increased with increasing Cu content from 15.3 to 27.3at%, but decreased for the Ti45.6Ni21.6Cu32.8 film. The films annealed for 1h at 773K were found to be brittle because of serrated grain boundaries with intergranular precipitates. The critical stress for slip decreased with increasing annealing temperature, whereas the martensitic transformation start (Ms) temperatures and maximum recoverable strains of the films annealed at 873 and 973K were almost the same. As the Cu content increased, the critical stress and the Ms temperature increased, but the maximum recoverable strain decreased. The relationship between the microstructure and shape memory behavior of the films was discussed.