Kamilla Mukhtarova

This work describes the preparation and properties of oxide ceramic nano/microfibers composed of group IV (Ti, Zr, Hf) and group V (Ta, Nb) refractory transition metals. In this work, the utilization of reactive needleless electrospinning allowed simple preparation, in a single step, of the polyacrylonitrile polymer-based pre-ceramic composite PAN/MeOx fibers, where Me = Ti, Zr, Hf, Nb, Ta. These precursors were further transformed into porous ceramic nanofibers with an overall composition TiZrHfNbTaO11 via calcination at different temperatures. The heat treatment temperature here plays the key role and allows to prepare materials with precisely tuned phase composition and properties. Performed thermogravimetric studies described the thermal properties of the preceramic composite fibers and allowed defining the necessary heat treatment profile. It was found that calcination at 600-700 °C results in amorphous ceramic fibers, while higher heat treatment temperatures 800-1100 °C resulted in the gradual crystallization of the ceramics and formation of a mixed-phase structure at 1100 °C. The thermally induced morphological and phase evolution were analyzed via a combination of transmission and scanning electron microscopy, ex situ X-ray diffraction, and X-ray photoelectron spectroscopy. Changes in surface area were determined via nitrogen adsorption measurements. Photocatalytic hydrogen production via water splitting was systematically investigated to assess the effects of thermal calcination on the high-entropy oxide fibers, with particular emphasis on the co-catalytic function of surface-loaded palladium. Specifically, loading 1 wt.% Pd co-catalyst onto the crystalline 1100 °C matrix established a Schottky barrier that suppressed charge recombination, yielding a peak photocatalytic hydrogen production rate of ~470 µmol/g after 5 hours—significantly outperforming the amorphous HEO sample.

A low-elastic-modulus metastable β-Ti–Nb–Zr alloy was fabricated by Laser Powder Bed Fusion (LPBF) for biomedical applications. Post-heat treatment was applied to the as-built state, and the mechanical behaviors of both alloys were compared. The as-built alloy mainly consists of the β phase, with an ω-HCP phase. Synchrotron X-ray diffraction indicates that the ω phase is suppressed after heat treatment (550 °C for 1h), and a small amount of α-HCP precipitates form within the β matrix. Tensile testing at room temperature shows discontinuous yielding with upper and lower yield stresses of 733 MPa and 711 MPa, followed by hardening that delays fracture. In situ TEM revealed screw dislocation cross-slip, causing localized deformation and softening. Strain hardening resumes with dislocation multiplication, leading to high ductility. The heat-treated alloy reaches only 17% of the as-built's elongation and deforms mainly via {332}〈113〉 twinning. Despite limited ductility, its elastic modulus is about 39 GPa, compared to 51 GPa for the as-built.

In this study, the relationship between the extrusion ratio and the corrosion resistance of pure Mg deformed using extrusion with an oscillating die (KoBo) without preheating of the initial billet was investigated. The materials investigated in this study were extruded at high deformation ratios, R₁ 5:1, R₂ 7:1, and R₃ 10:1, resulting in significant grain refinement from the very coarse grains formed in the initial billet to a few µm in the KoBo-extruded samples at room temperature, which is not typical for hexagonal structures. Our research clearly shows that KoBo extrusion improves the corrosion performance of pure Mg, but there is no straightforward dependence between the extrusion ratios and corrosion resistance improvement. Although it was expected that the smallest grain size should provide the highest corrosion resistance, the dislocation density accumulated in the grain interiors during deformation at the highest extrusion ratio, R₃ 10:1, supports dissolution reactions. This, in turn, provides the answers for the greater grain size observed after deformation at R₂ 7:1, where dynamic recovery prevailed over dynamic recrystallization. This situation led to the annihilation of dislocation, leading to better corrosion resistance of the respective alloy. Therefore, the alloy with the greatest grain size has the best corrosion resistance.

Experiments were conducted to reveal the nanostructure evolution in additively manufactured (AMed) 316L stainless steel due to severe plastic deformation (SPD). SPD-processing was carried out using the high-pressure torsion (HPT) technique. HPT was performed on four different states of 316L: the as-built material and specimens heat-treated at 400, 800 and 1100 °C after AM-processing. The motivation for the extension of this research to the annealed states is that heat treatment is a usual step after 3D printing in order to reduce the internal stresses formed during AM-processing. The nanostructure was studied by X-ray line profile analysis (XLPA), which was completed by crystallographic texture measurements. It was found that the as-built 316L sample contained a considerable density of dislocations (10¹⁵ m⁻²), which decreased to about half the original density due to the heat treatments at 800 and 1100 °C. The hardness varied accordingly during annealing. Despite this difference caused by annealing, HPT processing led to a similar evolution of the microstructure by increasing the strain for the samples with and without annealing. The saturation values of the crystallite size, dislocation density and twin fault probability were about 20 nm, 3 × 10¹⁶ m⁻² and 3%, respectively, while the maximum achievable hardness was ~6000 MPa. The initial and textures for the as-built and the annealed samples were changed to due to HPT processing.