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A mixture of kaolin (92 wt.% of kaolinite) and laboratory-grade CaCO3 was used for anorthite preparation using the spark plasma sintering (SPS) technique. The powder was heated up to the maximum temperature (850 °C–1100 °C with a step of 50 °C, heating rate 50 °C/min) using the SPS device. The as-prepared samples were compared with a reference sample produced by dry pressing and conventional firing. Rietveld refinement performed on the X-ray diffraction data revealed that before anorthite formation, gehlenite and Ca-feldspar appeared, which gradually transformed into anorthite with increasing temperature. The sample prepared by SPS at 1100 °C contained 90 wt.% of anorthite, while the anorthite content in the reference sample reached only 47 wt.%. The porosity of the samples prepared by SPS reached significantly lower values compared to that of the reference sample. Therefore, SPS can be considered as a promising technique in the preparation of anorthite ceramics at lower temperatures.

Titanium dioxide (TiO2) was plasma sprayed using a high feed-rate spray system with hybrid water–argon stabilization torch (WSP-H). TiO2 powder agglomerated from nanometric particles was used as the feedstock. The produced coating had thickness over 4 mm, to be comparable with bulk ceramic bodies. The deposit was removed from the substrate and annealed in air to reoxidize the oxygen deficient as-sprayed titania and to obtain material with a proper dielectric behavior. Besides the microstructure studies, radiofrequency dielectric spectroscopy and microwave dielectric spectroscopy were in the focus of investigation. The approach to annealing of an extremely thick coating is studied: The significance of annealing of a large-area thick coating is discussed, as the role of grain interior and grain boundaries in the charge transport is influenced. Relative permittivity er 94 and the Q*f product 12,800 GHz were reached at frequency 4 GHz, whereas the frequency response under 1 MHz was mapped continuously and the values (i.e., low and stable loss tangent) confirmed that a sprayed and air-annealed TiO2 is able to approach or also overperform dielectric parameters typical for TiO2 sintered bulk.

Hybrid water-stabilized plasma (WSP-H) torches provide high-enthalpy plasma which may be utilized for high-throughput and yet economical spraying of coatings from powders, suspensions, and solutions. It was previously demonstrated that microstructures and functional properties of the WSP-H coatings may be tailored to a wide extent for new applications, namely those requiring high coating thickness and/or coating of large components. In this study, applicability potential of WSP-H technology for spraying of novel thermal barrier coatings (TBCs) is demonstrated. WSP-H technology was used for spraying of yttria-stabilized zirconia (YSZ) top-coats from powder, suspension, and solution. Yttria content in the top-coat feedstock was 7-8 wt.%. In addition, gadolinium zirconate (Gd2Zr2O7-GZO) was sprayed from suspension for comparison. NiCrAlY bond-coat was also deposited by WSP-H, and Hastelloy-X alloy was used as substrate material. Microstructure, phase composition, and endurance of the deposited coatings in thermal cycling fatigue (TCF) test and during high-temperature short-term annealing were evaluated. All coatings showed excellent high-temperature stability and TCF resistance withstanding more than 650 cycles, surpassing some of the currently commercially used TBCs. Lifetime of the TBC with columnar top-coat deposited from YSZ suspension exceeded even more than 900 cycles.

Solution precursor plasma spraying (SPPS) presents a modern route to deposit coatings directly from liquid feedstock. In this study, unique ability of SPPS to intermix different precursors on the atomic scale was employed to deposit pure and Cr-doped Al2O3 (technically, synthetic ruby) from several mixtures of aluminum and chromium nitrates. Highly porous coatings (> 60% porosity) were deposited by high-enthalpy WSP-H torch. Homogeneous distribution of Cr atoms in Al2O3 resulted in coatings color ranging from pink to grayish-green correlated with increasing Cr content and change of lattice parameters as observed by x-ray diffraction. High content of over 80 wt.% of α-alumina phase was reached for all coatings. For the Cr-doped Al2O3 coatings, distinct and fully reversible thermochromic behavior was observed by temperature-resolved colorimetry. Further increase in α-phase content (up to 95 wt.%) and coating densification were achieved by additional surface remelting by plasma torch.

The equiatomic CoCrFeMnNi alloy is currently one of the most studied high-entropy alloys. In our previous work, a new grade of CoCrFeMnNi high-entropy alloy with yttria-based particle dispersion was introduced showing improved mechanical properties, especially high-temperature strength and creep rate. In this study, a detailed analysis of room to high-temperature thermal properties was performed, as it is important for defining the future application of the alloy and opening the discussion on tailoring the thermal response. Thermal properties are not well known for the original CoCrFeMnNi alloy or for CoCrFeMnNi with yttria dispersion. As a result of the analysis, thermal properties of both alloys and the effect of the oxide dispersion are discussed. Most of the thermal properties remain unchanged. However, it was found that the presence of 0.3% yttria influences the oxidation rate of CoCrFeMnNi alloy at the initial and intermediate oxidation times. The onset of the oxidation is described in detail. Yttria nano-oxide dispersion and related grain refinement also reduce the microstructural degradation caused by the formation of voids due to outward Mn diffusion.

Biomedical Ti-6Al-7Nb alloy was prepared by a dedicated thermal treatment followed by equal-channel angular pressing (ECAP) and extrusion. Ultra-fine grained duplex microstructure consisting of deformed primary α-grains and fragmented α + β region was achieved. Microstructural changes during heating with the rate of 5 °C/min were studied by in-situ electrical resistance. Microstructure after deformation and also after subsequent heating was thoroughly characterized by scanning electron microscopy, X-ray diffraction, and positron annihilation spectroscopy (PAS). X-ray diffraction and positron annihilation spectroscopy proved a very high dislocation density and the presence of high concentration of vacancy clusters in deformed material. The ultra-fine grained microstructure of Ti-6Al-7Nb alloy is stable up to 440 °C, while upon heating to 550 °C and to 660 °C, the dislocation density decreases and vacancy clusters disappear. Enhanced microhardness can be achieved by ECAP followed by aging at 500 °C. Upon heating to 660 °C, the microhardness decreases due to ongoing recovery and recrystallization. Coincidence Doppler broadening (CDB), a special method of PAS, proved that dislocation cores are preferentially occupied by Al atoms that are known to cause substitutional solid solution strengthening.

Harnessing nuclear fusion is a challenging task, in particular because of the demands put on the used materials. In tokamaks, future energy sources, the inner-most chambers are to be coated with dense coatings of W, or W-Cr-based alloys. So far, the attempts for such coatings formation by other methods failed due to oxidation, high porosity, insufficient adhesion, high specific surface, or even insufficient thickness below 10 &mu, m. Cold spraying seems a promising technology for the task. In our study, we demonstrate the first successful fabrication of thick pure W coatings. W-Cr and W-Cr-Ti coatings were further prepared without oxidation of the metals. All coatings exhibited high hardness levels, good interface quality with three tested substrates and, importantly, a promising potential for formation of stable Cr 2 WO x phases.

Thanks to the low working temperature, less product size limitations and one order of magnitude higher deposition rates compared to the established additive manufacturing techniques, more attention has been brought to the potential of cold spraying for additive manufacturing . As a process dealing with deformation of solid particles possibly leaving non-bonded interfaces and causing work hardening, any optimization should (i) adjust spray parameters to obtain high performance as-sprayed parts and (ii) tune ductility and internal stresses by post-treatments. The present study first deals with strategies to optimize spray parameters for fabrication of high performance steel 316 L deposits. Next, the performances of deposits are further adjusted by various heat treatments. The structural strength of the freestanding samples before and after the heat treatments is evaluated under static and cyclic axial loading and supported by fatigue crack growth rate analysis. The results highlight the feasibility of obtaining high quality steel 316 L deposits using N2 as process gas, rather than the costly He that is commonly suggested. This study demonstrates the potential of cold spraying to be used for deposition of freeform structural components with a static strength comparable to that of bulk and laser-based additive manufactured materials and a fatigue strength similar to that of bulk cast material.

Alloying of Mg with rare-earth (RE) elements proved to be beneficial for their in-vitro and in-vivo performance. In this work, a novel WN43 (Mg-4 wt%Y-3 wt%Nd) alloy with a well-defined composition was prepared, where, unlike in the commercial WE43 alloy, the possibly harmful RE mischmetal was substituted by less toxic Nd. A modern spark plasma sintering (SPS) technique was used to effectively produce WN43 samples from atomized powders. Sintering temperatures of 400 °C – 550 °C and holding times of 3 or 10 min were used and well-compacted final materials were successfully prepared. It was shown that a superior combination of corrosion and mechanical properties was attained in the samples sintered at 500 °C and 550 °C, while the effect of sintering time was rather negligible. The performance of this material was exceptional within the group of Mg alloys prepared by powder metallurgy and comparable with conventionally prepared alloys. Moreover, it was shown that a great variety of mechanical and corrosion characteristics can be obtained by altering the SPS parameters so as to fulfill case-specific requirements typical of biomedical materials. Consequently, the novel WN43 alloy prepared by SPS seems to be a particularly suitable material for biomedical use.

Dynamical thermomechanical analysis of 5 illite-based clays from deposits in Slovakia, Estonia, Latvia, and Hungary is presented. The clays consist of illite (37&ndash, 80 mass%), quartz (12&ndash, 48 mass%), K-feldspar (4&ndash, 13 mass%), kaolinite (0&ndash, 18 mass%), and calcite (0&ndash, 3 mass%). Young&rsquo, s modulus is measured during the heating and cooling stages of firing (25 &deg, C &rarr, 1100 &deg, 25 &deg, C). The liberation of the physically bound water increases Young&rsquo, s modulus by &sim, 70% for all studied clays. By increasing the temperature, dehydroxylation and the &alpha, &rarr, &beta, transition of quartz take place without a significant effect on Young&rsquo, s modulus. Sintering, which starts at 800 &deg, C, leads to an intensive increase in Young&rsquo, s modulus up to the highest temperature (1100 &deg, C). The increase remains also in the early stage of cooling (1100 &deg, 800 &deg, C). This increase of Young&rsquo, s modulus is also the result of solidification of the glassy phase, which is finished at &sim, 750 &deg, C. A sharp minimum of Young&rsquo, s modulus is observed at around the &beta, &alpha, transition of quartz. Then, Young&rsquo, s modulus still decreases its value down to the room temperature. The physical processes observed during heating and cooling do not differ in nature for the studied clays. Values of Young&rsquo, s modulus vary at around 8 GPa, up to 800 &deg, C. During sintering, Young&rsquo, s modulus reaches values from 30 GPa to 70 GPa for the studied clays. The microstructure and composition given by the origin of the clay play a cardinal role for the Young&rsquo, s modulus of the final ceramic body.

High-enthalpy hybrid water/argon-stabilized plasma (WSP-H) torch may be used for efficient deposition of coatings from dry powders, suspensions, and solutions. WSP-H torch was used to deposit two complete thermal barrier coatings (TBCs) with multilayered top-coat. NiCrAlY was used as bond-coat and deposited on nickel-based superalloy substrates. Top-coat consisted of up to three sublayers: (i) yttria-stabilized zirconia (ZrO2-8 wt.%Y2O3-YSZ) deposited from solution, (ii) gadolinium zirconate (Gd2Zr2O7-GZO) deposited from suspension, and (iii) optional yttrium aluminum garnet (Y3Al5O12-YAG) overlayer deposited from suspension. Each of the sublayers was intended to provide different functionalities, namely improved fracture toughness, low thermal conductivity, and high erosion resistance, respectively. High-temperature performance and thermal shock resistance of the deposited coatings were tested by thermal cycling fatigue “TCF” test (maximum temperature 1100 °C, 1 h dwell per cycle) and “laser-rig” test (maximum temperature ~ 1530 °C, 5 min dwell per cycle) exposing samples to isothermal and gradient thermal conditions, respectively. In both tests, coatings endured around 800 test cycles which shows great potential for further development of these layers and their application in demanding thermal conditions. Analysis of the samples after the test showed microstructural changes and identified reason of ultimate coating failure.

In this work, CoCrNi, FeCoCrNi and CoCrFeMnNi concentrated alloys with a Y-Ti oxide particle dispersion were prepared by mechanical alloying and Spark Plasma Sintering. The alloy consists of an FCC Ni-based matrix with a Y-Ti oxide dispersion and additional phases of Cr23C6 and Cr2O3. The effect of Fe, Mn, and Y-Ti oxide particles on the formation of oxide scales and the composition of the adjacent CoCrNi and FeCoCrNi alloys was studied. It was found that alloys without Mn in their composition form a protective Cr2O3 scale. The Cr23C6 particles provide an alternative mechanism for balancing the chromium loss during the oxidation. Y and Ti from the oxide particles participate in the formation of the protective oxide scales. Fe promotes Y and especially Ti diffusion through the Cr2O3 scale, resulting in the formation of Ti-depleted regions in the alloy. The findings will serve for the further development of these new materials.

Refractory metal high entropy alloy HfNbTaTiZr with ultrafine grained structure and grain size of ≈80 nm was processed by high pressure torsion. The development of microstructure, lattice defects and mechanical properties with increasing strain was examined. Grain refinement of HfNbTaTiZr alloy deformed up to the equivalent strain e ≈ 50 resulted in a significant enhancement of strength while keeping sufficient ductility. However, further straining e > 100 led to a decrease of strength and the loss of ductility due to the decomposition of solid solution facilitated by vacancies introduced by severe plastic deformation.

Čistá suspenze hydroxidapatatu byla vyrobena pomocí chemické sysntézy. S použitím WSP hořáku byl vytvořen HA povlak na substrátu z SS304 a Ti6Al4V. Vlastnosti povlaku byly hodnoceny pomocí SEM, XRD a EDX. PO dopozici bylo dosaženo 4-10% podílu amorfní fáze a 75-82% krystalické HA fáze v tloušťcě cca 145 um. Tribologické chování bylo hodnoceno pomocí testu pin-on-disc. Přídavek AgNO3 do suspenze vedl k vzniku Ag disperze mezi splaty v 8% podílu Ag. Úspěšné přidání antibakteriálního Ag je pokrokem v oblasti výzkumu materiálů kloubních náhrad. Pure hydroxyapatite suspension was produced by wet chemical synthesis. Using a hybrid water-stabilized torch, a series of HA coatings were produced on SS304 and Ti6Al4V substrates and their properties were characterized by SEM, EDX and XRD techniques. After deposition, the amorphous phase content reached 6-10% and the coatings retained 75-82% of crystalline HA phase. Their thickness reached 145 lm. To understand the wear behavior of the coatings, pin-on-disc tribology evaluation was performed. Additionally, a set of HA coatings was prepared with pure metallic Ag content. This formed by in situ chemical decomposition of AgNO3 added into the HA suspension. The Ag was dispersed evenly within the coatings in the form of submicron-sized particles situated predominantly along the HA splats boundaries with a total Ag content of 8 wt.%. Given the antibacterial properties of Ag, such result presents a promising step forward in the hard tissue replacement research.

A homogeneous HfNbTaTiZr high-entropy alloy was successfully processed via powder metallurgy route. For the initial powder feedstock material fabrication, the electrode induction-melting gas atomization procedure was used, resulting in a spherical powder morphology and dual bcc phase composition distinguishable within the individual particles. Spark plasma sintering was then used for the powder compaction at sintering temperatures ranging from 800 to 1600 °C. By the characterization of the compact microstructures, lattice defects (microscopic porosity and vacancy-like misfit defects), and mechanical properties (hardness and three-point bending strength), the sintering conditions were optimized to obtain a fully dense, homogeneous, single-phase bcc material. It was found that such properties are achieved when sintering at 80 MPa pressure for 2 min at temperatures above 1200 °C, where the single bcc phase structure exhibited ductile behavior with considerable flexural strength and ductility at ambient temperature. Positron annihilation spectroscopy was used to characterize the evolution of atomic and mesoscale defects during optimization of the sintering process.

Barium titanate coatings were first time sprayed by a high feed-rate plasma torch with water stabilization. Two power levels of the torch were applied. Two substrate materials were used—stainless steel and carbon steel. Various substrate preheating levels from 125 to 450 °C were applied to change the cooling conditions. Microstructure, phase composition including crystallinity quantification, surface roughness and microhardness were evaluated. Dielectric measurements proved that coatings with medium-level relative permittivity and with stable loss factor course versus frequency of the AC electric field were fabricated by spraying. The cold substrates provided coatings with low reflectivity and visibly darker surface compare to the hot substrates. The coatings exhibited good dielectric properties—on the top of an expectable range for plasma-sprayed BaTiO3. This study is targeted to coatings prospective for electrical industry, namely via optimization of conditions at the high feed-rate spraying.

Rare-earth containing magnesium alloy AE42 was prepared by powder metallurgy technique of spark plasma sintering for the first time. The effect of sintering parameters on the microstructure, precipitation of secondary particles and mechanical properties was investigated. The gas-atomized AE42 alloy powder was sintered at three different temperatures of 450 °C, 500 °C and 550 °C for three minutes, which was sufficient to achieve almost full density. Despite the negligible effect of the sintering temperature on the grain size, sintering had a pronounced effect on the Al11RE3 secondary phase particles. Fine eutectic lamellar structure of Al11RE3 in gas-atomized powder transformed to almost globular particles during SPS and size of these secondary phase particles increased with increasing sintering temperature. As the result, SPS temperature had significant effect on the mechanical strength of the material via altering the size and morphology of Al11RE3 particles. Sintered material was compared with the gas-atomized powder subsequently annealed at 550 °C. It was found that the kinetics of the globularization of the Al11RE3 secondary phase particles is much faster during spark plasma sintering than during conventional annealing at the same temperature. This can be associated with increased diffusivity along the matrix/precipitate interface or possibly with locally increased temperature due to electron transport.

Tungsten alloys currently represent prospective candidates to replace tungsten in the first wall applications in future fusion facilities. Tungsten has many advantageous features; however, it is rather susceptible to oxidation at temperatures above around 500 °C. To mitigate/suppress this, various oxide-forming elements are being added to tungsten to induce self-passivation. The most common ones are chromium, titanium and silicon. The alloyed powder is frequently prepared by mechanical alloying and then consolidated by a sintering method. Most of the published results are related to alloys consolidated by HIP process that leads to formation of multiphase material. In the presented study, W-10Cr alloy with hafnium oxide particle dispersion was prepared by spark plasma sintering. Unique features of the microstructure are discussed and compared with other processing methods Phase and thermal stability of the alloy was evaluated at three fusion-relevant temperatures using detailed X-ray diffraction analysis, differential thermal analysis and microstructural observations in a scanning electron microscope. Impact of the heat treatment on crucial properties such as thermal conductivity and heat capacity was evaluated.

Axial-suspension-plasma spraying (ASPS) is a fairly recent thermal spray technology which enables production of ceramic top coats in TBCs, incorporating simultaneously the properties of both the co ...

A novel thermal barrier coating (TBC) material, strontium zirconate SrZrO3, was sprayed by a high feed-rate water-stabilized plasma torch WSP 500. Stainless steel coupons were used as substrates. Coatings with a thickness of about 1.2 mm were produced, whereas the substrates were preheated over 450 °C. The torch worked at 150 kW power and was able to spray SrZrO3 with a high spray rate over 10 kg per hour. Microstructure and microhardness, phase composition, adhesion, thermal conductivity and thermal expansion were evaluated. The coating has low thermal conductivity under 1 W/m K in the interval from room temperature up to 1200 °C. Its crystallite size is slightly over 400 nm and thermal expansion 12.3 µm K−1 in the similar temperature range.

Deposition of aluminium oxide (Al 2 O 3 ) coatings from liquid feedstocks using a high enthalpy hybrid water-stabilized plasma torch was investigated. The entire process included the optimization of spraying parameters using a commercial ready-to-spray suspension, optimization and deposition of custom-made suspensions and one solution and evaluation of mechanical properties of the most durable coatings. Acquired coating microstructures varied from porous with typical columnar morphology to well-sintered dense structures with low porosity. The microstructural features were clearly linked to the feedstock formulation. The most durable coating with the highest density microstructure prepared from ethanol-based suspension showed outstanding mechanical properties, e.g. high values of hardness (up to 1211 HV0.3), tensile adhesion strength (up to 51 MPa) and wear resistance (material removal rate as low as 1.94·10 − 5 mm 3 ·N − 1 ·m − 1 as measured by pin-on-disc test). Moreover, deposition from a water-based solution of aluminium nitrate resulted in deposition of highly porous coating composed predominantly of stable α-alumina phase.

Mg-Gd alloy containing 22 wt.% Gd was studied in the present work. As cast alloy contains Mg 46 Gd 9 phase located at pockets along grain boundaries. In addition the as-cast alloy contains a low concentration of rectangular GdH 2 particles. Solution treatment at 530° C was performed in order to dissolve Gd in the Mg matrix. The solution treatment in air enhanced the volume fraction of GdH 2 particles by one order of magnitude. The formation of GdH 2 particles is caused by hydrogen-induced decomposition of the Mg 46 Gd 9 intermetallic phase. The source of hydrogen is water vapor reacting with Mg surface. A Gd-rich oxide layer was formed during solution treatment in air. Building of the oxide layer requires diffusion of Gd solutes from bulk to the surface and leads to formation of a Gd-depleted sub-surface region. It was found that solution treatment in vacuum results in substantially lower volume fraction of GdH 2 particles. Alternative methods of formation of GdH 2 particles in Mg-Gd alloy were examined as well and are discussed in the paper.

In the present work positron annihilation spectroscopy was employed for investigation of defects created in titanium by hydrogen loading. Pure titanium samples were firstly annealed to remove dislocations introduced by cutting and polishing. Subsequently the samples were loaded with hydrogen up to various hydrogen concentrations. Ti samples with different microstructures were compared: (i) conventional coarse grained sample, (ii) ultra fine grained material with microstructure refined by severe plastic deformation. Hydrogen gas loading of coarse grained and ultra fine grained samples was performed at hydrogen gas pressure of 103 bar and temperature of 150 °C. This resulted in formation of δ-TiH x phase in Ti matrix. The hydrogen content absorbed in the samples was determined by thermogravimetric analysis. The phase composition of hydrogen-loaded samples was characterized by X-ray diffraction. Hydrogen loading introduced vacancies which agglomerated in the sample into small vacancy clusters. In addition to vacancies, dislocations were created by α-Ti → δ-TiH x phase transition. Differential thermal analysis revealed that hydrogen is trapped at several kinds of traps characterized by different binding energies. The release of hydrogen from these traps precedes the decomposition of the δ-TiH x phase.

Positron annihilation spectroscopy was employed for investigation of vacancy-like defects in plasma-sprayed barium titanate. Defect studies were combined with measurement of dielectric properties of barium titanate coatings. Samples prepared by gas-stabilized plasma spray (GSP) torch and by plasma torch with the hybrid water-argon stabilization (WSP-H) were studied. Processing parameters were selected so that GSP coatings were sprayed in reductive conditions, whereas WSP-H coatings were prepared in oxidizing environment. As-sprayed GSP coating is dark, whereas WSP-H one is light. The dielectric properties of WSP-H coating are superior to those for GSP one. Defect studies revealed that both GSP and WSP-H coatings contain titanium vacancies. However, GSP coating contains in addition a considerable concentration of oxygen vacancies. Some fraction of oxygen vacancies in GSP coating is coupled with titanium vacancies forming partial Schottky defect. The structure of WSP-H coating is less disordered and contains only a low concentration of oxygen vacancies. This is consistent with reductive and oxidizing conditions in GSP and WSP-H spraying, respectively. Annealing at elevated temperatures in air leads to removal of oxygen vacancies which are filled by oxygen diffusing into the samples.

The aim of this work was investigation of hydrogen interaction with defects in Ti. Well-annealed Ti samples were loaded with hydrogen either electrochemically or using H2 gas phase. The hydrogen content and the phase composition of hydrogen loaded samples was determined by thermogravimetric analysis and by X-ray diffraction (XRD) respectively. Positron lifetime (LT) spectroscopy was employed for investigation of defects created by hydrogen loading. High-temperature H2 gas loading led to complete transformation of the whole sample into δ-TiH2 while low-temperature H2 gas loading and the electrochemical loading resulted in a non-uniform structure with most of the hydrogen absorbed in a sub-surface layer. All hydrogen-loaded samples exhibit positron lifetime component of ≈ 170 ps corresponding to positrons trapped at dislocations. Vacancy clusters were observed in the electrochemically loaded sample and the H2 gas phase loaded sample at low temperature.

Tungsten-based materials are the prime candidate plasma-facing materials for future fusion reactors, such as DEMO. Spark plasma sintering is a prospective fabrication technology with several advantageous features. The concurrent application of electric current, temperature and pressure enhances the sintering process, allowing for lower temperatures and shorter sintering times than traditional powder metallurgy processes. This in turn helps to avoid excessive grain growth and phase segregation in W-alloys. This study is focused on several factors that may influence the homogeneity of the sintered compacts&mdash, namely the diffusion of carbon from the graphite die, purity of the powder and sintering conditions. The following characteristics of spark plasma-sintered tungsten compacts were studied: composition (especially carbon and oxygen content), porosity, mechanical properties (hardness and fracture strength), and thermal diffusivity. The effects of the abovementioned processing factors were quantified, and local variations of selected properties were assessed.

High entropy alloys represent a new type of materials with a unique combination of physical properties originating from the occurrence of single-phase solid solutions of numerous elements. The preparation of nanostructured or amorphous structure in a form of thin films promises increased effective surface and high intergranular diffusion of elements as well as a high affinity to oxidation. In this work, we studied HfNbTaTiZr thin films, deposited at room temperature by DC magnetron sputtering from a single bcc phase target. Films exhibit cellular structure (∼100 nm) with fine substructure (∼10 nm) made of round-shape amorphous clusters. The composition is close to equimolar with slight Ti enrichment and without any mutual segregation of elements. Oxidation at the ambient atmosphere leads to the formation of Ti, Zr, Nb, Hf, and Ta oxide clusters in the film up to the depth of 200–350 nm out of the total film thickness of 1650 nm. Oxygen absorption takes place preferentially in the large vacancy clusters located in between the amorphous cluster aggregates. The dominant type of defect are small open volumes with a size comparable with vacancy. The distribution of these defects is uniform with depth and is not influenced by the presence of oxygen in the film.

Sintering of magnesium powder is often affected by the oxide layer, which is present on the powder already after gas-atomization. Its negative consequence demonstrates itself usually by limited diffusion between the individual powder particles and formation of compact 3D net-like structures, which promote crack propagation during mechanical loading. This work shows that magnesium-rare earth metals alloys, in this case Mg-4Y-3Nd, can benefit from the utilization of sufficiently high sintering temperature and pressure during the spark plasma sintering. A thorough analysis of the powder boundaries morphology and chemical composition revealed that application of temperature near the eutectic point of Mg-Y/Nd system together with high pressure results in severe diffusion of both Y and Nd into the powder boundaries. Consequently, local change in the chemical composition led to partial melting and the oxide shells became unstable and shattered. As a result, the interparticle diffusion was intensified and the powder consolidation became successful.

Thick NiTi shape memory alloy coatings (300–500 µm) were produced on graphite and AISI 304 substrates by radio frequency inductively-coupled plasma spray technology (RF-ICP) from feedstock NiTi powders. Their microstructure as well as chemical and phase composition were characterized and a methodology for the characterization of functional shape memory properties of the thick coatings was developed. The coatings exhibited cubic to monoclinic martensitic transformation and shape memory effect. The presented results prove that NiTi coatings with functional thermomechanical properties can be easily produced on structural materials by RF-ICP. Further optimization will be needed to prepare NiTi coatings with better microstructural and chemical homogeneity.

The growing interest in preparing refractory high entropy alloys via powder metallurgy reveals significant sensitivity to impurities, influencing the phase composition and microstructure and, consequently, the mechanical properties. Mechanical alloying of commercial purity powders is a cost-effective way of material preparation leading to a very fine microstructure when spark plasma sintering is used. However, the atmosphere of the milling process plays a significant role. This study shows that the phase composition of the HfNbTaTiZr alloy may be tailored by milling in ambient atmosphere, in argon or in nitrogen. Formation of a new fcc-ZrHfTi phase as a complex nitrogen and oxygen compound was elucidated and its significant hardening effect was shown. Finally, exceptional hardness was achieved by the synergic effect of nano-grained structure and high volume fraction of fcc-ZrHfTi phase.

A series of samples was sintered from nanometric BaTiO 3 powder by spark plasma sintering (SPS). Porosity on ultra-fine size scales, hardly accessible for quantification by conventional techniques, was measured by small angle neutron scattering (SANS). Band gap was estimated from diffuse reflectance measurements. Sintered tablets were polished, sputtered by aluminum to create electrode system and tested in DC and AC electric fields. The results were correlated with the SPS processing parameters and are discussed in connection with other BaTiO 3 samples produced by SPS with or without additional heat treatment. Majority of our SPS fired samples had, even without subsequent thermal treatment, advantageous combination of high permittivity, DC resistivity and low loss factor, seldom reported for SPS technology.

High entropy alloys (HEAs) have attracted researchers&rsquo, interest in recent years. The aim of this work was to prepare the HfNbTaTiZr high entropy alloy via the powder metallurgy process and characterize its properties. The powder metallurgy process is a prospective solution for the synthesis of various alloys and has several advantages over arc melting (e.g., no dendritic structure, near net-shape, etc.). Cold isostatic pressing of blended elemental powders and subsequent sintering at 1400 &deg, C for various time periods up to 64 h was used. Certain residual porosity, as well as bcc2 (Nb- and Ta-rich) and hcp (Zr- and Hf-rich) phases, remained in the bcc microstructure after sintering. The bcc2 phase was completely eliminated during annealing (1200 &deg, C/1h) and subsequent water quenching. The hardness values of the sintered specimens ranged from 300 to 400 HV10. The grain coarsening during sintering was significantly limited and the maximum average grain diameter after 64 h of sintering was approximately 60 &mu, m. The compression strength at 800 &deg, C was 370 MPa and decreased to 47 MPa at 1200 &deg, C. Porosity can be removed during the hot deformation process, leading to an increase in hardness to ~450 HV10.

Ceramic Al2O3-ZrO2-SiO2 coatings with near eutectic composition were plasma sprayed using hybrid water stabilized plasma torch (WSP-H). The as-sprayed coatings possessed fully amorphous microstructure which can be transformed to nanocrystalline by further heat treatment. The amorphous/crystalline content ratio and the crystallite sizes can be controlled by a specific choice of heat treatment conditions, subsequently leading to significant changes in the microstructure and mechanical properties of the coatings, such as hardness or wear resistance. In this study, two advanced methods of surface heat treatment were realized by plasma jet or by high energy laser heating. As opposed to the traditional furnace treatments, inducing homogeneous changes throughout the material, both approaches lead to a formation of gradient microstructure within the coatings, from dominantly amorphous at the substrate&ndash, coating interface vicinity to fully nanocrystalline near its surface. The processes can also be applied for large-scale applications and do not induce detrimental changes to the underlying substrate materials. The respective mechanical response was evaluated by measuring coating hardness profile and wear resistance. For some of the heat treatment conditions, an increase in the coating microhardness by factor up to 1.8 was observed, as well as improvement of wear resistance behaviour up to 6.5 times. The phase composition changes were analysed by X-ray diffraction and the microstructure was investigated by scanning electron microscopy.

One of the prominent applications of magnesium alloys, thoroughly investigated in recent years, is medicine. The commercial WE43 (Mg-4wt.%Y-3wt.%mischmetal) alloy was reported to exhibit superior in vitro and in vivo performance, however, the presence and possible harmful effect of rare earth (RE) elements mischmetal in this alloy have been vastly debated. For this reason, the RE mischmetal was substituted in this study by pure neodymium, which exhibits rather low toxicity. In this way, a novel WN43 (Mg-4wt.%Y-3wt.%Nd) alloy was prepared, with well-defined composition. In order to attain a good control over the grain size and phase distribution, a modern spark plasma sintering (SPS) method was employed. The main objective of this study was to examine the effect of sintering parameters on the resulting microstructure (type and morphology of secondary phases, grain structure, and residual strain) and microhardness (Hv). The application of relatively high pressure (100 MPa) during consolidation leads to the production of practically fully compact final material. Increasing sintering temperature (from 400 up to 500 °C) stimulated homogenization and stabilization of the microstructure and reduction of the internal strain. On the other hand, the effect of sintering time (3 or 10 min) was rather negligible. Furthermore, the microhardness experiments revealed that the softening effect due to homogenization and decrease in the dislocation density at higher sintering temperatures was well-compensated by precipitation hardening as the hardness values were comparable in all the samples. The understanding of microstructure evolution as a function of sintering parameters can be of particular importance for subsequent mechanical, corrosion and in vivo degradation testing of this novel biomedical magnesium alloy.

Wet-milling of magnesium powder was performed at room temperature in ethanol covered by argon atmosphere. It is shown that the wet-milling in ethanol can produce highly deformed powder of flake-like morphology. Subsequently, bulk samples were consolidated by spark plasma sintering technique in the temperature range 400–550 °C. It is shown that milling caused severe microstructural refinement. The flake-like shape of the milled particles was transferred into the compacts and caused formation of a layer-like microstructure which was predominantly perpendicular to the external force applied during sintering. Grain growth was highly limited by powder particles’ interface and secondary phase particles present in the matrix. Formation of texture in the compacts was attributed to the combination of SPS thermo-mechanical effect and continuous static recrystallization acting during the sintering.

In order to improve mechanical properties and corrosion resistance of low carbon steel, various composite coatings are usually deposited on its surface using different methods. Laser cladding is used nowadays very often due to its industrial advantages. Laser cladding forms dense and protective coatings with higher bond strength and increased deposition efficiency then those prepared by other coatings technologies. The present research aims to study the influence of the pulse laser power and Ti addition on WC-Co/NiCrBSi cladded coatings obtained by preplacing the feedstock material on a S235JR steel substrate. The properties of the coatings in terms of microstructure and corrosion resistance were investigated. The microstructure characterization was done by scanning electron microscopy (SEM) and EDS analysis; the phases identification was performed by XRD technique. In order to evaluate the properties of the WC-Co/NiCrBSi coatings with and without the addition of Ti, hardness measurements and the electrochemical corrosion tests in NaCl 3.5% solution were carried out.

Permeation barriers represent one of the crucial fields in materials development for thermonuclear fusion. Primary objective of the barriers is to suppress the permeation of hydrogen isotopes (mainly tritium) from future thermonuclear fusion facilities. Secondary objective is to reduce hydrogen retention in structural materials, potentially also improving their corrosion resistance. Expected reactor conditions put high demands on the material, as well as on the final barrier quality. Key properties are tritium permeation reduction, absence of defects (especially cracks), high-temperature stability and corrosion resistance, and compatibility with structural materials (mostly ferritic-martensitic steels). Regarding industrial scale production, ability of the deposition method to coat large complex surfaces is desirable. Thin nitride layers, identified as promising permeation barriers, were prepared by diffusion-based nitridation and physical vapour deposition (PVD) and characterized.

A non-isothermal kinetic analysis of high-temperature processes occurring during thermal treatment of kaolinite is performed by the thermodilatometric analysis. The measurements are carried out using the horizontal push-rod dilatometer on compact samples with heating rates from 1 to 10 °C min–1 in dynamic N2 atmosphere. For the parameterization of thermodynamically stable phases such as mullite and cristobalite, the Kissinger method is used. The obtained value of apparent activation energy for sinter-crystallization of mullite is (726 ± 9) kJ mol–1 and the thickening of long mullite needles is determined as a mechanism of this process. The results also show that the apparent activation energy for sinter-crystallization of cristobalite is (565 ± 38) kJ mol–1.

Hydrogen interaction with defects and structural development of Pd films with various microstructures were investigated. Nanocrystalline, polycrystalline and epitaxial Pd films were prepared and electrochemically loaded with hydrogen. Structural changes of Pd films caused by absorbed hydrogen were studied by in-situ X-ray diffraction combined with acoustic emission and measurement of electromotorical force. Development of defects during hydrogen loading was investigated by positron annihilation spectroscopy. It was found that hydrogen firstly fills open volume defects existing already in the films and subsequently it occupies also interstitial sites in Pd lattice. Absorbed hydrogen causes volume expansion, which is strongly anisotropic in thin films. This introduces high stress into the films loaded with hydrogen. Acoustic emission measurements revealed that when hydrogen-induced stress achieves a certain critical level rearrangement of misfit dislocations takes place. The stress which grows with increasing hydrogen concentration can be further released by plastic deformation and also by detachment of the film from the substrate.

The development of the microstructure in nanocrystalline, polycrystalline and epitaxial Pd films loaded with hydrogen is investigated. Structural changes in Pd films loaded with hydrogen were characterized by positron annihilation spectroscopy combined with electron microscopy and X-ray diffraction. It was found that hydrogen charging causes plastic deformation which leads to an increase of the defect density in all Pd films studied. Moreover, the formation of buckles was observed in nanocrystalline and polycrystalline Pd films loaded above a certain critical hydrogen concentration. Buckling leads to detachment of the film from the substrate and this is accompanied with in-plane stress relaxation and plastic deformation of the film.

Zirconium di-oxide (ZrO2 , zirconia) receives nowadays a big attention because of a variety of advantageous properties which make zirconia-based materials useful in numerous fields of practice, in particular, in ceramic industry and other high-temperature applications. To make high-temperature phases of zirconia stabilised down to room temperature, doping of the host lattice by proper metal cations has to be usually performed. Nanopowders are currently focused on as starting substances for manufacturing ZrO2-based ceramics by sintering, because well-homogenised materials of a low porosity can be produced more easily. Nanometer-sized defects associated to grain boundaries (GBs) become then to play an enhanced role in nanopowders due to enlarged volume fraction of GBs. Positrons and positronium (Ps) atoms can serve as efficient probes of different structures encountered in particular stages of manufacturing ZrO2-based functional materials. In the present contribution, conventional positron and Ps lifetime measurements were carried out on a variety of zirconia-based nanopowders and ceramics obtained by sintering these nanopowders. Nanopowders studied were doped with various metal cations (Y3+, Cr3+, Ce4+, Mg2+) and differed also in thermal treatment prior sintering. Lifetime experiments were conducted in air or in vacuum and combined with Doppler broadening measurements using slow-positron beam and supplemented with X-ray diffraction (XRD) and mass-density (MD) measurements. In Figure, variability of the lifetime spectra observed is illustrated. In a range of lifetimes from a few ns to ≈ 70 ns, up to three individual lifetime components could be identified, see Figure, (a) and (b). Such observations unambiguously testified Ps formation with subsequent ortho-Ps annihilation. On the other hand, an absence of the ortho-Ps components was found in certain nanopowders giving thus an evidence of a strong Ps inhibition, see Figure, (c). Pore sizes were estimated using current models of correlation between observed ortho-Ps lifetime and pore size. Origins of pores will be discussed on the basis of the ortho-Ps data in combination with the results of slowpositron beam, XRD and MD measurements.

Hydrogen absorption and diffusivity in high quality ZnO crystals were investigated in this work by X-ray diffraction combined with slow positron implantation spectroscopy and electrical resistometry. ZnO crystals were covered by a thin Pd over-layer and electrochemically charged with hydrogen. It was found that absorbed hydrogen causes plastic deformation in a sub-surface region. The depth profile of hydrogen concentration introduced into the crystal was determined by nuclear reaction analysis. Enhanced hydrogen concentration was found in the sub-surface region due to excess hydrogen atoms trapped at defects introduced by plastic deformation. Hydrogen diffusion in ZnO crystals with various orientations was studied by in-situ electrical resistometry. It was found that hydrogen diffusion in the c-direction is faster than hydrogen diffusion in the a-direction most probably due to open channels existing in the wurtzite structure along the c-axis.

W alloys are currently widely studied materials for their potential application in future fusion reactors. In the presented study, we report on the preparation and properties of mechanically alloyed W-Ti powders compacted by pulsed electric current sintering. Four different powder compositions of W-(3%–7%)Ti with Hf or HfC were prepared. The alloys’ structure contains only high-melting-point phases, namely the W-Ti matrix, complex carbide (Ti,W,Hf)C and HfO2 particle dispersion; Ti in the form of a separate phase is not present. The bending strength of the alloys depends on the amount of Ti added. The addition of 3 wt. % Ti led to an increase whereas 7 wt. % Ti led to a major decrease in strength when compared to unalloyed tungsten sintered at similar conditions. The addition of Ti significantly lowered the room-temperature thermal conductivity of all prepared materials. However, unlike pure tungsten, the conductivity of the prepared alloys increased with the temperature. Thus, the thermal conductivity of the alloys at 1300 °C approached the value of the unalloyed tungsten.

Magnesium alloys are promising candidates for biodegradable medical implants which reduce the necessity of second surgery to remove the implants. Yttrium in solid solution is an attractive alloying element because it improves mechanical properties and exhibits suitable corrosion properties. Silver was shown to have an antibacterial effect and can also enhance the mechanical properties of magnesium alloys. Measurements of microhardness and electrical resistivity were used to study the response of Mg-4Y and Mg-4Y-1Ag alloys to isochronal or isothermal heat treatments. Hardening response and electrical resistivity annealing curves in these alloys were compared in order to investigate the effect of silver addition. Procedures for solid solution annealing and artificial aging of the Mg-4Y-1Ag alloy were developed. The corrosion rate of the as-cast and heat-treated Mg-4Y-1Ag alloy was measured by the mass loss method. It was found out that solid solution heat treatment, as well artificial aging to peak hardness, lead to substantial improvement in the corrosion properties of the Mg-4Y-1Ag alloy.

In the present work hydrothermally grown ZnO single crystals were electrochemically charged with hydrogen. The influence of hydrogen on ZnO microstructure was investigated by positron annihilation spectroscopy (PAS) combined with X-ray diffraction (XRD) using synchrotron radiation. Hydrogen concentration in the samples was determined by nuclear reaction analysis (NRA). It was found that a high concentration of hydrogen can be introduced into ZnO by electrochemical loading. At low concentrations, absorbed hydrogen causes elastic volume expansion of ZnO crystal. At higher concentration, hydrogen-induced stresses exceed the yield stress in ZnO and plastic deformation of the crystal takes place leading to formation of a defected subsurface layer in the crystals.

Hydrogen absorption in Pd causes a significant volume expansion. In free-standing bulk Pd, the hydrogen-induced volume expansion is isotropic. However, the situation becomes more complicated in thin Pd films. Contrary to bulk samples, thin films are clamped to an elastically stiff substrate, which prevents in-plane expansion. Hence, the volume expansion of a thin film is strongly anisotropic because it expands in the out-of-plane direction only. Internal stresses introduced by absorbed hydrogen may become so high that detachment of a film from the substrate is energetically favorable and buckles of various morphologies are formed. In the present work, we studied hydrogen-induced buckling in a nanocrystalline thin Pd film deposited on a sapphire substrate. Slow positron implantation spectroscopy (SPIS) was employed as a principal tool for the characterization of defects and investigation of defect interactions with hydrogen. SPIS studies were combined with X-ray diffraction and direct observations of buckling by light microscopy. It was found that buckling of thin Pd film occurs at hydrogen concentrations xH > 0.1 and is accompanied by a strong increase of dislocation density.

The thermodynamics of structural phase transformations in thin films depends on the mechanical stress that can be released by plastic deformation. For thin films below a critical film thickness, plastic deformation is energetically unfavourable: thus, the system stays coherent and stress remains. For PdHc films less than 22 nm thick, a new situation emerges: while the interfaces between matrix and hydride precipitates remain coherent throughout the complete phase transition, misfit dislocations form between the hydride phase and the substrate.

Cílem článku je zhodnotit a porovnat korozní odolnost povlaku Hastelloy C-276 s povlakem Stellite 6 a Cr3C2-NiCr, deponovaných technologií HP/HVOF. Koroze byla testována pomocí směsi agresivních solí 40%Na2SO4-60% V2O5, simulujících prostředí spalovacích kotlů. Bylo realizováno 50 cyklů při teplotě 750°C a následné schlazení na pokojovou teplotu po dobu 20 min.Zkorodované vzorky byly testovány pomocí elektronového mikroskopu, a EDS analýz. V porovnání s Cr3C2-NiCr, povlaky Hastelloy a Stellite poskytly lepší ochranu. Korozní mechanismus Hastelloy C-276 je založen na tvorbě ochranné oxidické vrstvy, která brání dalšímu prnikání korozního média. Povlak Stellite 6 korodoval pohranicích jednotlivých splatů, což vedlo k jejich postupnému odloupávání. V případě povlaku Cr3C2-NiCr byla korozně napadena matrice, což vedlo k postupnému uvolňování karbidů a destrukci povlaku. The aim of this paper is to evaluate and compare the corrosion resistance of Hastelloy C-276 coating with Stellite 6 and Cr3C2%-25% NiCr coatings. The coatings were deposited usingHP/HVOF(High Pressure/High Velocity Oxygen Fuel) thermal spraying technology. The corrosion test was conducted in the corrosive-aggressive environment of molten salts mixture in the composition of 40% Na2SO4–60%V2O5, simulating the corrosion environment of boilers for residual fuel oils. The corrosion test under cyclic conditions included 50 cycles at the temperature of 750 °Cand subsequent cooling at the room temperature for 20 min. The corroded specimens were analyzed using scanning electron microscope (SEM) and elemental analysis (EDS). In comparison with Cr3C2-25% NiCr coating, both Stellite 6 and Hastelloy C-276 coatings provided the substrate material with sufficient protection against high-temperature corrosion. The corrosion mechanism of Hastelloy C-276 coating was based on the formation of protective oxide layer which prevented further penetration of corrosive media. Stellite 6 coating exhibited the corrosion attack along splat boundaries resulting in spat delaminating into the build-up. The corrosion mechanism of Cr3C2-25% NiCr coating included the attack on NiCr matrix resulting in gradual separation of carbide phases into the corrosive environment.