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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 Ni 46.2 Ti 51.1 Co 2.7 (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 Ti 2 Ni 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. [Display omitted] • Ti-rich NiTiCo thin films show an excellent mechanical behavior. • The addition of Co offers a controlled lowering of martensitic transformation temperatures. • Specimens annealed at 773 K present GP zones within the NiTiCo grains. • Specimens annealed at 773 K reach a recoverable strain of 5.7 % with negligible irreversible strain. • The recoverable strain of 5.7 % represents a promising response, in comparison with the Sachs bound. [ABSTRACT FROM AUTHOR]

The microstructure and internal friction of Ni-Ti alloys after hydrogen absorption have been investigated by means of optical microscopy, X-ray diffraction, differential scanning calorimetry and low frequency torsional internal friction apparatus. The results show that, after hydrogen absorption, the grains tend to be elongated, and the nucleation and growth of hydrides are mainly concentrated at the grain boundaries. In addition, a new phase ultimately identified as Ti2NiH0·5 phase forms after absorption of hydrogen. The hydrogen induced martensite promotes the emergence of a two-stage transformation. However, the growth of hydrides causes a reduction of the hydrogen induced martensite. The hydrides act as strong pinning points, resulting in a dramatic increase in the internal friction. In addition, the marked change of the internal stress, caused by the microscopic strain and the mismatching of the volumes, also improves the internal friction. [ABSTRACT FROM AUTHOR]

Four characteristic interface microstructures between habit plane variants (HPVs) in the self-accommodation morphologies of B19′ martensite in Ti–Ni alloys have been investigated by scanning transmission electron microscopy (STEM). The straight interface of a B19′ type I twin is present at interface I. The relaxation of the transformation strain at interface II is achieved by a volume reduction of the minor correspondence variants (CVs) in the relevant habit plane variants (HPVs). The relaxation of the transformation strain at interface III is mainly due to the formation of a B19′ type I twin between the two major CVs. Subsequently, local strain around the tips of the minor CVs perpendicular to the interface is released by the formation of micro-twins with the ⟨011⟩B19′ type II and/or B19′ type I relation. The major and minor CVs in each HPV are alternately connected through fine variants with the B19′ type I twin relation parallel to interface IV. The results are compared with macroscopic observations and the predictions of PTMC analysis. [ABSTRACT FROM PUBLISHER]

[Display omitted] From a systematic atom probe tomography (APT) characterization of the radiation-induced segregation (RIS) in dilute Fe-Ni and Ni-Ti model alloys, we highlight fluctuations of the solute local concentration up to the scale of the APT specimens. We deduce the RIS at dislocation loops from a solute diffusion equation, that is solved at steady state, within the Voronoi's volume occupied by a single loop. From a statistical sampling of the Voronoi's volume and the dislocation loop radius modeled after the characterization of the microstructure by transmission electron microscopy, we provide the full RIS distribution. The present statistical approach of RIS demonstrates that the fluctuation of local solute concentrations in Fe-Ni and Ni-Ti mainly results from the dispersion in size and density of the dislocation loop population. Besides, we highlight the impact of the post-treatment parameters used in the APT protocol on the extracted RIS profiles. In Ni-Ti alloys, the simulated Ti-depletion profiles are in very good agreement with the measured ones. Furthermore, the dispersion of the loop radius and density is shown to play a critical role on the fluctuations of the Ti local concentration. In Fe-Ni, the identification of discrepancies between the simulated Ni-enrichment profiles and the measured ones provides a signature of additional operating mechanisms of the solute redistribution, such as radiation-induced precipitation. [ABSTRACT FROM AUTHOR]