Gradient bandgap narrowing in severely deformed ZnO nanoparticles

Gradient nanostructured metallic materials with a gradual change of grain boundary and dislocation density display unprecedent mechanical properties. Herein, we uncover a gradient of point defects concentration and concomitant gradient bandgap (E g) narrowing in metal oxide nanoparticles processed by a combination of severe shearing and frictional sliding deformation. Using the valence electron-energy loss spectroscopy technique, we find a gradual decrease of E g from 2.93 eV in the interior to 2.43 eV at the edge of the high-pressure torsion processed ZnO flake-shaped particle. This work paves the way to strain engineering of gradient-structured metal oxide semiconductors for unique functional properties

Microstructure, microhardness and corrosion resistance of WE43 alloy based composites after high-pressure torsion

The structure and properties of a composite consisting of Mg-Y-Nd-Zr alloy (WE43) and various oxides are studied. The particles of the WE43 powder were coated by the nanocrystalline oxide layer by means of a wet chemical deposition process. After that the powder is compressed into solid samples and deformed using high-pressure torsion at room temperature. A second phase is present, both, in pure WE43 alloy and in the one with deposited oxides. We observed that the modification of the alloy by the oxide layer deposition and deformation by high-pressure torsion changes the phase composition and properties of the samples. The samples modified by TiO2 showed the best microhardness and corrosion resistanc

Omega phase formation in ti–3wt

It is well known that severe plastic deformation not only leads to strong grain refinement and material strengthening but also can drive phase transformations. A study of the fundamentals of α → ω phase transformations induced by high-pressure torsion (HPT) in Ti–Nb-based alloys is presented in the current work. Before HPT, a Ti–3wt.%Nb alloy was annealed at two different temperatures in order to obtain the α-phase state with different amounts of niobium. X-ray diffraction analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied for the characterisation of phase transitions and evolution of the microstructure. A small amount of the β-phase was found in the initial states, which completely transformed into the ω-phase during the HPT process. During HPT, strong grain refinement in the α-phase took place, as did partial transformation of the α- into the ω-phase. Therefore, two kinds of ω-phase, each with different chemical composition, were obtained after HPT. The first one was formed from the β-phase, enriched in Nb, and the second one from the α-phase. It was also found that the transformation of the α-phase into the ω-phase depended on the Nb concentration in the α-Ti phase. The less Nb there was in the α-phase, the more of the α-phase was transformed into the ω-phase

High-Pressure torsion-Induced Grain Growth in Electrodeposited Nanocrystalline Ni

Deformation-induced grain growth has been reported in nanocrystalline (nc) materials under indentation and severe cyclic loading, but not under any other deformation mode. This raises an issue on critical conditions for grain growth in nc materials. This study investigates deformation-induced grain growth in electrodeposited nc Ni during high-pressure torsion (HPT). Our results indicate that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters. Also, texture evolution suggests that grain-boundary-mediated mechanisms played a significant role in accommodating HPT strain

High pressure torsion induced structural transformations in Ti- and Zr-based amorphous alloys

The melt-spun (MS) Ti50Ni25Cu25 alloy and the Zr62Cu22A110Fe5Dy1 bulk metallic glass (BMG) were subjected to high pressure torsion (HPT). X-ray diffraction (XRD) measurements show a shift of the first diffraction halo to a low angle after HPT processing, which corresponds to an increase in the values of the radius of the first coordination sphere and the free volume. Direct density measurements confirmed an increase in free volume values. A special TEM procedure was used for a detailed study of the microstructure of both amorphous alloys after HPT processing. The study revealed the formation of a large density of shear bands (SBs) in both alloys. Nanocrystals are formed directly in shear bands as a result of strain-inducted nanocrystallization. Amorphous nanoclusters with a size of 20 nm are formed in an amorphous matrix surrounding the SBs in the HPT-processed MS alloy Ti50Ni25Cu25. The formation of nanoclusters was not observed in BMG Zr62Cu22A110Fe5Dy1 after HPT processing

Formation and thermal stability of ω-Ti(Fe) in α-phase-based Ti(Fe) alloys

In this work, the formation and thermal stability of the ω-Ti(Fe) phase that were produced by the high-pressure torsion (HPT) were studied in two-phase α-Ti + TiFe alloys containing 2 wt.%, 4 wt.% and 10 wt.% iron. The two-phase microstructure was achieved by annealing the alloys at 470 °C for 4000 h and then quenching them in water. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were utilized to characterize the samples. The thermal stability of the ω-Ti(Fe) phase was investigated using differential scanning calorimetry (DSC) and in situ high-temperature XRD. In the HPT process, the high-pressure ω-Ti(Fe) phase mainly formed from α-Ti. It started to decompose by a cascade of exothermic reactions already at temperatures of 130 °C. The decomposition was finished above ~320 °C. Upon further heating, the phase transformation proceeded via the formation of a supersaturated α-Ti(Fe) phase. Finally, the equilibrium phase assemblage was established at high temperatures. The eutectoid temperature and the phase transition temperatures measured in deformed and heat-treated samples are compared for the samples with different iron concentrations and for samples with different phase compositions prior to the HPT process. Thermodynamic calculations were carried out to predict stable and metastable phase assemblages after heat-treatments at low (α-Ti + TiFe) and high temperatures (α-Ti + β-(Ti,Fe), β-(Ti,Fe))

Microstrain and electrochemical performance of garnet solid electrolyte integrated in a hybrid battery cell

Garnet type solid electrolytes are promising candidates for replacing the flammable liquid electrolytes conventionally used in Li-ion batteries. Al-doped Li7La3Zr2O12 (LLZO) is synthesized using nebulized spray pyrolysis and field assisted sintering technology (FAST), a novel synthesis route ensuring the preparation of samples with a homogeneous elemental distribution and dense ceramic electrolytes. Ceramic preparation utilizing field assisted sintering, in particular the applied pressure, has significant influence on the material structure, i.e. microstrain, and thereby its electrochemical performance. The phenomenon of microstrain enhancement of electrochemical performance might open a new route towards improved garnet solid electrolytes. A detailed mechanism is proposed for the lattice distortion and resulting microstrain during sintering. The charge transfer resistance of Li-ions at the interface between LLZO and Li is characterized using AC impedance spectroscopy and is amongst the best reported values to date. Additionally, the solid electrolyte is integrated in a full hybrid cell, a practical approach combining all the advantages of the solid electrolyte, while maintaining good contact with the cathode material

Free volume measurement of severely deformed Zr62Cu22Al10Fe5Dy1 bulk metallic glass

The Zr62Cu22Al10Fe5Dy1 bulk metallic glass (BMG) was subjected to high pressure torsion (HPT) at room temperature and at 150 °C. XRD shows a shift of first amorphous halo towards a low angles, which corresponds to an increase in the first coordination sphere radius and an increase in free volume content approximately by 0.44 % and 0.74 % after HPT processing at temperatures of 20 and 150 °C, respectively. Direct density measurements revealed that HPT at 20 °C and 150 °C leads to a decrease in the density values by 2.1% and 1 %, respectively, in comparison with the initial state. Value of density decrease for state HPT 150 °C estimated by direct density measurements is close to value of free volume increase estimated by shift of first amorphous halo

Сплавы для медицинских применений на основе β-титана

Titanium alloys have been used for medical purposes for over 60 years. They are used in the manufacture of artificial heart valves, stents of blood vessels, endoprostheses of bones and joints (shoulder, knee, hip, elbow), for auricle reconstruction, in facial surgery, and also as dental implants. In first-generation materials (such as commercially pure titanium or VT6 alloys), the matrix consisted of the α-Ti phase or α-Ti and β-Ti mixture. Unfortunately, implants made of first-generation materials require replacement after 10–15 years of usage. This is due to the degradation of implants and loss of contact with the bone. Recently, these materials have been replaced by β-Ti alloys. These second- generation materials make it possible to exclude the harmful effect of aluminum and vanadium ions released during the gradual implant corrosion, and their elastic modulus is closer to the values for living bone than those for α and α+β alloys. Important areas in the development of β-Ti alloys include increasing their mechanical strength, fatigue strength, corrosion resistance and biocompatibility. New methods for the production and thermo-mechanical processing of titanium alloys arise and develop such as additive technologies or severe plastic deformation. Expensive alloying elements (such as tantalum, zirconium or niobium) are quite successfully replaced with cheaper ones (for example, chromium and manganese). As a result, the properties of titanium implants are gradually getting closer to that of the human bone, and their service life is steadily increasing. Therefore, this paper describes a comparative analysis conducted in relation to β-titanium-based alloys for medical applications.Титановые сплавы используют в медицинских целях уже более 60 лет: при изготовлении искусственных сердечных клапанов, стентов кровеносных сосудов, эндопротезов костей и суставов (плечевых, коленных, тазобедренных, локтевых), для реконструкции ушных раковин, в лицевой хирургии, а также в качестве зубных имплантатов. В материалах первого поколения (таких как технически чистый титан или сплавы типа ВТ6) матрица состояла из фазы α-Ti или смеси α-Ti и β-Ti. К сожалению, имплантаты из материалов первого поколения требуют замены уже через 10–15 лет эксплуатации. Это происходит из-за деградации имплантатов и потери контакта с костью. В последнее время на смену этим материалам пришли β-Ti-сплавы. Материалы второго поколения позволяют исключить вредное влияние ионов алюминия и ванадия, выделяющихся при постепенной коррозии имплантата, а их модуль упругости ближе к значениям для живой кости, чем у α- и α + β-сплавов. К важным направлениям развития β-Ti-сплавов относится повышение их механической прочности, усталостной прочности, коррозионной стойкости и биосовместимости. Возникают и развиваются новые методы получения и термомеханической обработки титановых сплавов, такие как аддитивные технологии или интенсивная пластическая деформация. Весьма успешно идет замена дорогих компонентов (таких как тантал, цирконий или ниобий) на более дешевые (например, хром и марганец). В результате характеристики титановых имплантатов постепенно все больше приближаются к свойствам человеческой кости, а срок их службы неуклонно возрастает. В связи с этим в настоящей работе проведен сравнительный анализ сплавов на основе β-титана для медицинских применений