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 Entropy Alloys for Energy Conversion and Storage: A Review of Grain Boundary Wetting Phenomena

This research was funded by the Russian Ministry of Science and Higher Education (contract no. 075-15-2021-945 grant no. 13.2251.21.0013). Support from the University of the Basque Country (project GIU19/019) and from the Basque Government (project IT1714-22) is also acknowledged.The multicomponent alloys with nearly equal concentration of components, also known as high entropy alloys (HEAs), were first proposed 22 years ago. The HEAs quickly became very important in materials science due to their unique properties. Nowadays, the HEAs are frequently used in energy conversion and storage applications. HEAs can consist of five, six or more components. Plasma cladding permits coating of the large surfaces of cheap substrates with (often expensive) HEAs and to enlarge, in such a way, their application area. The large-area coatings deposited by plasma cladding possess multiple advantages such as low thermal distortion, very high energy density, as well as low dilution of the substrate material. Plasma cladding ensures good metallurgical bonding between coating and substrate. The costs of operation and equipment are also very attractive. During plasma cladding, the mixed powders are blown by carrier gas into a plasma torch or are positioned on a substrate. This powder mixture is then melted in or under the plasma torch. The plasma torch, in turn, sequentially scans the substrate. After finalizing the crystallization process, the solid polycrystal appears which contains few residual melts. This remaining melt can completely or incompletely wet the grain boundaries (GBs) in solid phase of the polycrystal. These completely or incompletely wetted GBs can strongly influence the microstructure of HEA coatings and their morphology. In this review we analyze the GB wetting HEAs containing one phase in HEAs with two, three and more phases, as well as in HEAs reinforced with particles of carbides, nitrides, borides, or oxides. We also analyze the microstructure of the rather thick coatings after plasma cladding after additional laser remelting and observe how GB wetting changes over their thickness.--//-- Published under the CC BY 4.0 licence.Russian Ministry of Science and Higher Education (contract no. 075-15-2021-945 grant no. 13.2251.21.0013); University of the Basque Country (project GIU19/019); Basque Government (project IT1714-22); Institute of Solid State Physics, University of Latvia as the Center of Excellence acknowledges funding from the European Union’s Horizon 2020 Framework Programme H2020- WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

РОСТ ЗЕРНОГРАНИЧНОЙ ПРОСЛОЙКИ (αTi) В СПЛАВАХ Ti–Co

The paper studies the effect of temperature on the formation of (αTi) grain boundary layer in Ti–2wt.%Co and Ti–4wt.%Co alloys in the (αTi) + (βTi) two-phase region of the Ti–Co phase diagram within the temperature range of 690–810 °C. The paper studies kinetics of thickness growth (∆) of the (αTi) phase grain boundary layer in Ti–2wt.%Co alloy at a temperature of 750 °C. It demonstrates dependence of the thickness of grain boundary layer on the annealing time as ∆ ~ t 1/3. Analysis of experimental results suggests that increase in ∆ is a manifestation of the (αTi) coalescence process controlled by volume diffusion.Исследовалось влияние температуры на образование зернограничной прослойки (αTi) в сплавах Ti–2мас.%Co и Ti–4мас.%Co в двухфазной области (αTi) + (βTi) фазовой диаграммы Ti–Co в интервале температур 690–810 °С. Изучена кинетика роста толщины (∆) зернограничной прослойки фазы (αTi) в сплаве Ti–2мас.%Co при температуре 750 °С.Показана ее зависимость от времени отжига как ∆ ~ t 1/3. Анализ результатов экспериментальных наблюдений позволяет предположить, что увеличение ∆ является проявлением процесса коалесценции (αTi), который контролируется объемной диффузией

ЗАКОНОМЕРНОСТИ ОБРАЗОВАНИЯ ЗЕРНОГРАНИЧНЫХ ПРОСЛОЕК ФАЗЫ α-Ti В БИНАРНЫХ ТИТАНОВЫХ СПЛАВАХ

The microstructure of polycrystalline titanium alloys with added chromium (2, 4, and 5,5 wt.%), cobalt (2 and 4 wt.%) and copper (2 and 3 wt.%) was studied. A series of long isothermal annealing cycles (under vacuum) was performed for these materials within the temperature range from 600 to 850 °C. The annealing temperatures were in the α(Ti,Me) + β(Ti,Me) two-phase regions of the Ti–Cr, Ti–Co and Ti–Cu phase diagrams. The temperature dependence plots were built for the β(Ti,Me)/β(Ti,Me) portion of grain boundaries completely «wetted» by layers of the α(Ti, Me) second solid phase and the average contact angle. The results of microscopic investigation showed that the type and content of the second component in the alloy greatly affect formation of equilibrium grain boundary layers. We were the first to discover a non-monotonic temperature dependence of the portion of grain boundaries completely «wetted» by layers of the second solid phase in the absence of bulk «ferromagnetic–paramagnetic» phase transitions. Статья поступила в редакцию 28.07.14 г., доработана и подписана в печать 20.08.15 г. Исследована микроструктура поликристаллических сплавов титана с хромом (2, 4 и 5,5 мас.%), кобальтом (2 и 4 мас.%) и медью (2 и 3 мас.%). Проведены серии длительных изотермических отжигов этих материалов в интервале температур от 600 до 850 °С (в вакууме). Температуры отжигов лежали в двухфазных областях α(Ti,Me) + β(Ti,Me) фазовых диаграмм Ti–Cr, Ti–Co и Ti–Cu. Построены температурные зависимости доли границ зерен β(Ti,Me)/β(Ti,Me), полностью «смоченных» прослойками второй твердой фазы α(Ti,Me), и среднего контактного угла. Результаты микроструктурных исследований показали, что тип и концентрация второго компонента в сплаве сильно влияют на образование равновесных зернограничных прослоек. Впервые обнаружена немонотонная температурная зависимость доли границ зерен, полностью смоченных прослойками второй твердой фазы, в отсутствие фазовых превращений ферромагнетик–парамагнетик в объеме.

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

Diffusive and Displacive Phase Transformations in Nanocomposites under High Pressure Torsion

The high-pressure torsion (HPT) of Ti-Fe alloys with different iron content has been studied at 7 GPa, 5 anvil rotations and rotation speed of 1 rpm. The alloys have been annealed before HPT in such a way that they contained different amounts of α/α' and β phases. In turn, the β phase contained different concentration of iron. The 5 anvil rotations correspond to the HPT steady-state and to the dynamic equilibrium between formation and annihilation of microstructure defects. HPT leads to the transformation of initial α/α' and β-phases into mixture of α and high-pressure ω-phase. The α → ω and β → ω phase transformations are martensitic, and certain orientation relationships exist between α and ω as well as β and ω phases. However, the composition of ω-phase is the same in all samples after HPT and does not depend on the composition of β-phase (which is different in different initial samples). Therefore, the martensitic (diffusionless) transformations are combined with a certain HPT-driven mass-transfer. We observed also that the structure and properties of phases (namely, α-Ti and ω-Ti) in the Ti – 2.2 wt. % Fe and Ti – 4 wt. % Fe alloys after HPT are equifinal and do not depend on the structure and properties of initial α'-Ti and β-Ti before HPT