Microstructural characterization of Ti-Ta-based surface alloy fabricated on TiNi SMA by additive pulsed electron-beam melting of film/substrate system

TiNi shape memory alloys (SMAs) are unique metallic biomaterials due to the combination of super-elasticity and high corrosion resistance. Important limitations for biomedical applications of TiNi SMAs are the release of toxic Ni into adjacent tissues, as well as insufficient level of X-ray visibility. These limitations can be overcome by fabrication of a Ti-Ta-based surface alloy on the TiNi substrate, since Ti-Ta alloys being high-temperature SMAs are attractive biomaterials with potentially good mechanical compatibility with TiNi substrate. In the present work, this approach is realized for the first time through the multiple (N = 20) alternation of magnetron co-deposition of Ti70Ta30 (at.%) thin films and their liquid-phase mixing with TiNi substrate by microsecond low-energy, high current electron beam (similar to 2 mu s, similar to 15 keV, similar to 2J/cm(2)). Surface SEM/EDS, AES, XRD and cross-sectional HRTEM/EDS/SAED analyses were used for microstructural characterization of studied material. It was found that similar to 1 mu m-thick Ti-Ta-based surface alloy with a composition close to that of co-deposited films has been formed, and it consists of several sublayers with a depth-graded amorphous-nanocrystalline structure. Nanocrystalline sublayers consist essentially of randomly oriented grains of alpha ''(Ti-Ta)-martensite and beta(Ti-Ta)-austenite (bcc-disordered). Beneath the surface alloy, similar to 1 mu m thick intermediate zone has been formed. It has also a multilayer predominantly randomly oriented nanocrystalline structure and characterized by a monotonous depth replacement of Ta with Ni and a diffusion transition to TiNi substrate. The depth-graded structure of studied material is associated with the features of additive thin-film deposition/pulsed melting manufacturing process. (C) 2017 Elsevier B.V. All rights reserved.