A. Yu. Eroshenko

Nanostructured titanium: Structure, mechanical and electrochemical properties

The electrochemical behavior of coarse-grained and nanostructured titanium in various media is studied by the methods of potentiodynamic polarization and impedance spectroscopy and by the analysis of the etching curves. It is found that the dissolution rate of nanostructured titanium increases in a Ringer-Locke solution at 37°C in comparison with the coarse-grained state owing to a more defective structure of the natural oxide layer on the surface of nanostructured titanium and its lower polarization resistance. It is demonstrated that the etching of nanostructured titanium in a mixture of hydrofluoric and sulfuric acids occurs mostly via the mechanism of local destruction, whereas overall uniform etching occurs in the case of coarse-grained titanium, which is considerably displayed in a range from 40 to 75°C.

Nanostructural Titanium: Its Applications, Structure, and Properties

In most cases, medical implants are made from titanium alloys or stainless steel with good mechanical properties. However, these materials contain toxic ele� ments such as nickel, aluminum, and vanadium. For medical applications, it would be preferable to use titanium (1, 2), zirconium, niobium and their alloys, with distinctive physicomechanical and biological properties. The widespread use of titanium in implants is mainly hindered by its poor mechanical properties, including its durability under periodic and cyclic loads. This problem may be resolved by using nano� structural (ultrafinegrain) titanium. At present, we are able to produce large blanks of nanostructural tita� nium with good mechanical properties (3). The meth� ods employed are based on intense plastic deformation (3-7): equalchannel angular pressing (3, 4), abc pressing (5-7), and so on. By intense plastic deforma� tion, a nanostructural state may be established over the whole volume of the blank. That ensures mechanical properties matching those of moderately complex tita� nium alloys such as VT6 alloy. The creation of nanostructure permits fundamen� tal change in the mechanical properties of metals: the yield point and strength, the fatigue life, the wear resis� tance, the cyclic durability, etc. Note that at least two successive methods of intense plastic deformation must be used to obtain a nanostructural state of tita� nium (6, 7). Those methods may be combined in a sin� gle cycle (3). Twostage intense plastic deformation was pro� posed in (6, 7): abc pressing in a mold; and multipass rolling. In that approach, the initial upsetting in the mold involves successively changing the compression axis three or four times (analogously to multistage abc pressing (5)). In the first stage, the blank is deformed in a hydraulic press at 10 -3 -10 -1 s -1 . At specified tem� perature, each cycle includes onetime 40-50% upsetting, with subsequent change of the deformation axis by 90° rotation of the blank around the longitudi� nal axis. The temperature of the blank is reduced in stages in the range 700-400°C on passing to the next cycle. In the second stage, the blank is deformed by rolling at room temperature; the rollers may be grooved or smooth. The accumulated strain in rolling is 90%. Rolling produces blanks in the form of rods or plates. The final blanks are annealed in argon at 250 or 300 °C to remove the internal stress and increase the plasticity. As a result of twostage treatment and lowtemper� ature annealing, uniform grain-subgrain nanostruc� ture is formed in the blank (Fig. 1). The mean size of the elements (grains, subgrains, fragments) is less than 100 nm. This nanostructural state ensures plasticity of 6-10%, yield point of 1100 MPa, and strength of 1160 MPa.

Role of polycrystalline titanium grain size in the formation of the concentration profiles of implanted aluminum ions

The dependence of the depth of penetration of implanted aluminum atoms into polycrystalline titanium on the grain size of initial target samples is analyzed. The irradiation was carried out by a pulse-frequency ion beam of a Diana-2 source. The increase in the modified layer thickness to 250 nm with decreasing grain size in the initial material is revealed. In the interpretation of the observed regularities, we take into account the energetically inhomogeneous composition of a beam represented by three components and probable intense sputtering of the target surface by ions. In terms of the simulation, it is found that, in samples with relatively fine grains, a significant contribution to the formation of the depth profiles of implanted atoms comes from the radiation-induced diffusion; in samples with coarse grains, it comes from the diffusion along migrating extended defects, which appear and rearrange themselves in the process of ion implantation.

Fatigue failure stages of VT1-0 titanium in different structural states. Study by acoustic emission method

The paper studies the kinetics of fatigue damage accumulation in VT1-0 titanium by the acoustic emission (AE) method. Technical grade titanium VT1-0 in various structural states was tested under cyclic bending. Submicrocrystalline Ti-specimens (SMC, with subgrain size of 200–300 nm) were fabricated by equal channel angular pressing (ECAP) from polycrystalline titanium. Ingots with ultrafine grain structure (UFG, with structure element size of 1–2 µm) and coarse grain structure (CG, with structure element size of 20–30 µm) were prepared by annealing at different temperatures. Fatigue stages were identified by analyzing the AE signal parameters with their classification by the source type (dislocations, micro-and macrocracks). It was revealed that the specimens with a smaller grain size are of higher fatigue durability, while AE signals at the stages of yielding and microcracking are detected later because of their low energy.

Structure and properties of nanostructured, ultrafine grained and coarse grained titanium implanted with aluminium ions

The microstructure and mechanical properties of titanium are studied in the nanostructured, ultrafine- and coarse-grained structured states. The temperature dependence of the titanium grain size is analyzed, and the Hall-Petch coefficient is determined. A decrease in the grain size in the initial material leads to the penetration of the doping element to a considerable depth as a consequence of radiation-induced diffusion along grain boundaries, which constitute a separate phase for nanomaterials. This can provide a positive effect on the properties of titanium implanted with aluminum ions.

Acoustic emission analysis of fatigue damages of titanium alloys

The paper presents the results of studies of the kinetics of accumulation of fatigue damages in titanium VT1-0 and titanium alloy OT4 by acoustic emission method (AE). Identification of the sources of acoustic emission (dislocations, micro- and macro-cracks) is based on the methodology developed by the authors. According to the activity of various types of acoustic emission sources, the stages of fatigue are identified in conditions of flat cantilever bending. The data of the acoustic emission analysis were experimentally confirmed by the results of microstructural studies.The paper presents the results of studies of the kinetics of accumulation of fatigue damages in titanium VT1-0 and titanium alloy OT4 by acoustic emission method (AE). Identification of the sources of acoustic emission (dislocations, micro- and macro-cracks) is based on the methodology developed by the authors. According to the activity of various types of acoustic emission sources, the stages of fatigue are identified in conditions of flat cantilever bending. The data of the acoustic emission analysis were experimentally confirmed by the results of microstructural studies.

Features of the microstructure of Ti–Nb alloy obtained via selective laser melting

Ti–Nb alloy with 40 wt % of Nb is obtained from a composite Ti–Nb powder by means of selective laser melting. The Ti–Nb alloy has a two-phase microstructure. The main β-phase of the solid titanium–niobium solution forms grains ranging in size from ~2 to 20 μm. A nonequilibrium α″-phase is found in the forms of lamellar, globular, and packet martensite inside the grains of the β-phase and along their boundaries.

Features of the Ti-40Nb alloy prototype formation by 3D additive method

The structure of Ti-40Nb alloy prototype obtained by selective laser melting (SLM) on “VARISKAF 100MV” installation was considered by the methods of optical metallography, scanning and transmission electron microscopy. It was revealed that the most of the specimens’ surface is uniform flowed surface with typical banded structure formed by laying-on molten pools. The process of the individual layer formation was followed by drop formation. This leads to the porosity formation on the specimen’s surface. The structure of entire specimen is not homogeneous throughout the transverse section. The porosity of three kinds is observed. They are cavities of not full contact and melting of the layers, drawholes, gas pores. The porosity optimization requires more careful SLM modes selection. The alloy has a grain structure with anisotropy from small (2–8 µm) to medium (8–20 µm) grain size. The anisotropy of the specimen is formed in each layer and is retained during building of the specimen. The grains of microstruct...

Producing titanium-niobium alloy by high energy beam

The research is involved in producing a Ti-Nb alloy surface layer on titanium substrate by high energy beam method, as well as in examining their structures and mechanical properties. Applying electron-beam cladding it was possible to produce a Ti-Nb alloy surface layer of several millimeters, where the niobium concentration was up to 40% at. and the structure itself could be related to martensite quenching structure. At the same time, a significant microhardness increase of 3200-3400 MPa was observed, which, in its turn, is connected with the formation of martensite structure. Cladding material of Ti-Nb composition could be the source in producing alloys of homogeneous microhardness and desired concentration of alloying niobium element.

Production of ultrafine-grain bioinert alloys

The microstructure and mechanical properties of bioinert titanium, zirconium, and niobium alloys in the ultrafine-grain state are investigated. The ultrafine-grain structure is obtained by severe plastic deformation, including multicyclic abc pressing at specified temperatures, multipass rolling in shaped rollers at room temperature, and low-temperature non-recrystallizing annealing. Annealing increases the plasticity of the ultrafine-grain alloys, without changing the grain size. As a result of two-stage treatment—severe plastic deformation and annealing—ultrafine-grain structure with grains and subgrains of mean size 0.16–0.25 μm is formed. That considerably improves the mechanical properties (ultimate strength, yield point, and microhardness) of the alloys. At the same time, the formation of ultrafine-grain structure in the alloys does not change the elastic modulus, even with considerable increase in the ultimate strength and plasticity.