Shape memory behavior of sputter-deposited Ni46.2Ti51.1Co2.7 (at.%) thin films

Abstract

In order to analyze the relationship between the shape memory behavior of Ti-rich NiTiCo thin films and the microstructures obtained after different annealing treatments, a batch of Ni46.2Ti51.1Co2.7 (at.%) thin films were produced by magnetron co-sputtering and annealed at 773 K, 873 K and 973 K for 60 min. The microstructure of the thin films was studied by X-ray diffraction and transmission electron microscopy. Electrical resistivity measurements were performed to determine martensitic transformation temperatures. Additionally, a home-made device was specially designed for studying the mechanical behavior associated with the martensitic transformations of the thin films at different constant external stresses. The sample annealed at 773 K presented Guinier-Preston (GP) zones within the NiTiCo grains, while the samples annealed at 873 K and 973 K showed Ti2Ni precipitates inside the grains with semi-coherent interface with the matrix. The appearance of GP zones can explain why the sample annealed at 773 K reaches higher values of recoverable strain than the other samples, with negligible irreversible strain. The maximum recoverable strain of 5.7 % for this specimen represents a noticeable mechanical behavior, similar or even better than the one observed in the binary alloy with similar Ti concentration. The maximum recoverable strains were analyzed in terms of the Sachs bound, which represents a theoretical upper limit.