Impact of electron beam surface modification on deformation behavior and fracture properties of TiNi shape memory alloy

The study deals with the impact of the pulse number at low-energy High-Current Pulsed Electron Beam (HCPEB) treatment at constant energy density ES upon the deformation behavior of TiNi alloy, its inelastic properties and fracture pattern under quasistatic uniaxial tension. It is shown that inelastic properties of the TiNi alloy under study can be kept at the initial (constant) level whereas ductility and ultimate strength can be increased when the following parameters of low-energy high-current pulsed electron beam treatment are used: pulse duration τ = 2–2.5 μs, maximum electron energy 25 keV, energy density ES = 3.8 ± 0.7 J/cm2 as well as the pulsed irradiation mode and optimal number of irradiation HCPEB pulses (n) are taken. The HCPEB modification of the TiNi surface layer under uniaxial static tension results in the increase of martensite yield plateau length ΔεM, which is 15–30% larger than one in the unirradiated TiNi samples. The reasons of different impact of the HCPEB irradiation on strength and elastoplastic properties of TiNi alloy (with regard to the n) are discussed. The main reason for the strength properties decrease of the HCPEB-modified TiNi alloy at n = 15, 32 is attributed to the formation of a columnar structure in the matrix B2-phase with a particular crystalline lattice orientation (B2) in the columnar B2 grains. Whereas after the HCPEB treatment at n = 5 these parameters are varied due to the change of the chemical composition, namely, the nickel depletion of the B2-phase in the surface layer. The mentioned variation of the chemical composition of the matrix B2-phase is responsible for the increase in the temperature of the martensite transformations. The latter results in a more complete realization of the mechanisms of inelastic strain accumulation induced by these transformations, as well as the accumulation of larger plastic strain in the ‘soft’ martensitic phase

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

The physical-mechanical properties of the Ti-Ta based surface alloy with thickness up to ∼2 μm fabricated through the multiple (up to 20 cycles) alternation of magnetron deposition of Ti70Ta30 (at.%) thin (50 nm) films and their liquid-phase mixing with the NiTi substrate by microsecond low-energy, high current pulsed electron beam (LEHCPEB: ≤15 keV, ∼2 J/cm2) are presented. Two types of NiTi substrates (differing in the methods of melting alloys) were pretreated with LEHCPEB to improve the adhesion of thin-film coating and to protect it from local delimitation because of the surface cratering under pulsed melting. The methods used in the research include nanoindentation, transmission electron microscopy, and depth profile analysis of nanohardness, Vickers hardness, elastic modulus, depth recovery ratio, and plasticity characteristic as a function of indentation depth. For comparison, similar measurements were carried out with NiTi substrates in the initial state and after LEHCPEB pretreatment, as well as on “Ti70Ta30(1 μm) coating/NiTi substrate” system. It was shown that the upper surface layer in both NiTi substrates is the same in properties after LEHCPEB pretreatment. Our data suggest that the type of multilayer surface structure correlates with its physical-mechanical properties. For NiTi with the Ti-Ta based surface alloy ∼1 μm thick, the highest elasticity falls on the upper submicrocrystalline layer measuring ∼0.2 μm and consisting of two Ti-Ta based phases: α′′ martensite (a = 0.475 nm, b = 0.323 nm, c = 0.464 nm) and β austenite (a = 0.327 nm). Beneath the upper layer there is an amorphous sublayer followed by underlayers with coarse (>20 nm) and fine (<20 nm) average grain sizes which provide a gradual transition of the mechanical parameters to the values of the NiTi substrate

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 superelasticity 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 (∼2 μs, ∼15 keV, ∼2 J/cm2). Surface SEM/EDS, AES, XRD and cross-sectional HRTEM/EDS/SAED analyses were used for microstructural characterization of studied material. It was found that ∼1 μm-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 α″(Ti-Ta)-martensite and β(Ti-Ta)-austenite (bcc-disordered). Beneath the surface alloy, ∼1 μm-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

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 superelasticity 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 (∼2 μs, ∼15 keV, ∼2 J/cm2). Surface SEM/EDS, AES, XRD and cross-sectional HRTEM/EDS/SAED analyses were used for microstructural characterization of studied material. It was found that ∼1 μm-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 α″(Ti-Ta)-martensite and β(Ti-Ta)-austenite (bcc-disordered). Beneath the surface alloy, ∼1 μm-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