The effect of the initial size of alumina particles on the density, microstructure, hardness, and fracture toughness of ceramics obtained by conventional and spark plasma sintering (SPS) has been studied. We studied ceramics obtained from commercial Al2O3 powders with an initial particle size of 40 – 50 nm, 0.2 mm, and 1 mm, and domestic fine powders with an initial particle size of 0.2 – 3 mm, and Al2O3 + 0.25 vol. % MgO and Al2O3 + 10 vol. % ZrO2. It is shown that the density of alumina ceramics nonmonotonically depends on the initial size of Al2O3 powder particles. It has been established that an increase in the grain size leads to a nonmonotonic change in the hardness of alumina ceramics. It has been established that the addition of 0.25 vol. % MgO accelerates the sintering of alumina. The addition of 10 vol. % ZrO2 makes it possible to provide an optimal combination of hardness and fracture toughness. It is shown that fine-grained ceramics obtained by the SPS method have a higher hardness. It has been suggested that SPS of submicron alumina powders with an amorphous layer on the surface, additionally stabilized by zirconia particles, is promising for further increasing the hardness of alumina ceramics.

A hot salt corrosion (HSC) test was performed on the fine-grained titanium alpha-alloy Ti-2.5Al-2.6Zr (Russian industrial alloy PT-7M). The ultrafine-grained (UFG) microstructure in the titanium alpha-alloy was formed via cold Rotary Swaging. The grain size and volume fraction of the recrystallized microstructure in the alloy were varied by choosing appropriate annealing temperatures and times. The microstructure and corrosion resistance of UFG alloys were studied after 30 min of annealing at 500-700C and after 1000 h of annealing at 250C. Metallographic studies were carried out to investigate the effects of annealing on the nature and extent of corrosive damage in the titanium alpha-alloy Ti-2.5Al-2.6Zr. After HSC tests, surface analyses of the titanium alpha-alloy samples were conducted using X-ray diffraction and electron microscopy. During the HSC testing of the titanium alpha-alloy Ti-2.5Al-2.6Zr, a competitive interaction between intergranular corrosion (IGC) and pitting corrosion was observed. To the best of our knowledge, it was shown for the first time that annealing affects the relationship among the IGC, pitting corrosion and uniform corrosion rates of the titanium alloy. Prolonged low-temperature annealing at 250C resulted in a more pronounced increase in the uniform corrosion rate than short-term high-temperature annealing for 30 min at 500-700C. An in-depth analysis of the effect of the structure and phase composition of the grain boundaries on the susceptibility of the alpha-alloy Ti-2.5Al-2.6Zr to HSC was conducted.

The conditions for obtaining composite W + Ni powders with the core (W)–shell (Ni) structure and alloys based on them were studied in the work. W + Ni powders (5, 10, 20 wt

Samples of ceramics based on Sr0.5Zr2(PO4)3 phosphate with the structure of the kosnarite mineral (NaZr2(PO4)3, NZP) were obtained by spark plasma sintering. Submicron phosphate powders with particle sizes less than 1 μm were produced by the sol–gel method. Powders and ceramics have a single-phase NZP structure. The relative density of the ceramics was 97.6

Abstract—The possibility of low-temperature in situ synthesis of (Ti, W)C using plasma-chemical WC nanopowders and industrial micron TiC powders is demonstrated. Sintering/synthesis of WC–(25, 50, and 75) wt

The chemical stability of Nd0.33Zr2(PO4)3 fine-grained ceramics, which can be used for immobilization of rare earth elements (REE) that are part of high-level waste was studied. Single-phase Nd0.33Zr2(PO4)3 submicron powders with the structure of the mineral kosnarite were prepared by colloid-chemical synthesis. Powders were prepared by successive annealing at 600, 800, and 900°C for 6 h at each stage. Nd0.33Zr2(PO4)3 ceramic was Nd0.33Zr2(PO4)3 ceramics was produced using spark plasma sintering method (SPS). The relative density of the ceramic was 89.9

The features of spark plasma sintering of submicron Al2O3 powders with different contents (0, 0.5, 1.5, 5 vol

Oxide Y2.5Nd0.5Al5O12 with the structure of the garnet mineral has been obtained by coprecipitation after annealing at 1000°C. Fine-grained ceramics has been produced by spark plasma sintering based on synthesized powder. The relative density of the ceramics was 99.1

The temperature and heating rate affecting the shrinkage kinetics are studied for cylindrical workpieces obtained from submicron and fine aluminum oxide powder. The studies involve the powder from three batches: (1) submicron ( 0.15 µm) α-Al2O3 powder, (2) submicron ( 0.2 µm) α-Al2O3 powder having an amorphous layer deposited on the particle surface, and (3) fine ( 1 µm) α-Al2O3 powder. It is established that the powder particles in all batches has a monocrystalline structure. The powder workpieces are sintered using the electric pulse (spark) plasma sintering (SPS) technique. The shrinkage curves are analyzed using the Young–Cutler and the Coble models. The kinetics of sintering workpieces is shown to depend on diffusion developing between the powder particles. The sintering kinetics of workpieces made from submicron powder depends on intensity of the grain-boundary diffusion. In the sintering workpieces made of finely dispersed powder, the kinetics is additionally dependent on simultaneously developing volumetric and grain-boundary diffusion. It is established that the presence of an amorphous layer on the surface of particulate α-Al2O3 having submicron size affects the rate of migration of grain interfaces and the parameters of the Coble equation at the final SPS stage. It is assumed that the accelerated growth of grains and an increase in the microhardness of samples obtained through sintering workpieces made from submicron powder with an amorphous layer on the particle surface is caused by a higher density of defects at the grain interfaces. The elevated density of defects at grain interfaces can result from crystallization of the amorphous layer.

The mechanisms of high-speed sintering of tungsten and W+5wt%Ni nanopowders obtained by high-energy ball milling (HEBM) have been studied. The phase composition, microstructure parameters, hardness and fracture resistance of the obtained samples were investigated. It is shown that the samples have high relative density, small grain size and increased hardness. It is established that the formation of strong intermetallic phases MexWyCz and MexWy, as well as MeO oxides occurs at SPS of mechanically activated nanopowders. The simultaneous increase in the content of intermetallic phases and reduction of the grain size leads to a non-monotone character of the dependence of the SPS activation energy on the HEBM time. It is shown that the main mechanism of SPS of tungsten W+5wt%Ni nanopowders is Coble creep.

α-Si3N4-based powder composites containing 3 wt

Samples of unalloyed titanium VT1-0 with high strength characteristics (ultimate tensile strength of 820 MPa), which exceed the values for this material manufactured using conventional technologies, were produced by selective laser melting. To solve the problem of substitution of titanium alloys with commercially pure titanium in medical applications, unalloyed titanium VT1-0 with record mechanical characteristics (ultimate tensile strength of 1350 MPa) was processed by selective laser melting and rotary swaging. This value exceeds the characteristics of the highstrength Ti – 6 % Al – 4 % V alloy. The fine-dispersed martensite formed as a result of high crystallization rates under optimal mode of selective laser melting is the reason for the strength characteristics increase of unalloyed titanium VT1-0.

Submicron-grade Y2.5Nd0.5Al5O12 oxide (YAG:Nd) powder with garnet structure was synthesised by the co-precipitation method. YAG:Nd ceramics with a relative density of similar to 99 % were obtained by Spark Plasma Sintering. The ceramics had a fine-grained microstructure and low YAlO3 impurity phase content. The radiation resistance of the ceramics was investigated with irradiation with accelerated Xe ions (E = 148 MeV) and Ar ions (E = 46 MeV), with the fluences ranging from 6 x 10(11) to 1 x 10(13) cm(-2). The phase composition, microstructure and microhardness of the irradiated ceramics were studied. X-ray diffraction (XRD) analysis in the symmetric mode and grazing incidence XRD were used to study the irradiated layer structure. A gradient-defect structure was shown to be formed in the YAG:Nd ceramics under irradiation: amorphous phase, deformed garnet phase and non-deformed garnet phase. The dependencies of the volume fraction of the amorphous phase and of the amorphisation structure on the fluence were analysed. The calculated value of the critical fluence was 1 x 10(13) cm(-2), corresponding to 0.044 displacements per atom (dpa). The microhardness Hv of the surface layer decreased with increasing ion fluence.

The mechanisms of high-speed sintering of tungsten and W+5wt

The study investigates the density, phase composition, microstructure and mechanical properties (microhardness, fracture toughness) of binderless WC + SiC and WC + SiC + C ceramics obtained by Spark Plasma Sintering (SPS). Nanopowders of a-WC produced by DC arc plasma chemical synthesis were used as raw materials. Powder compositions for sintering contained graphite (0.3, 0.5% wt.) or b-SiC (1, 3, 5% wt.) with 0.3% wt. graphite. It was shown that WC + 1% wt. SiC + 0.3% wt.C ceramics have a homogeneous fine-grained microstructure, high relative density, increased microhardness and Palmquist fracture toughness (Indentation Fracture Resistance). The kinetics of the initial sintering stage of WC + C and WC + C + SiC powder compositions was also analyzed using high-temperature dilatometry at the conventional pressureless sintering (CPS) conditions. The CPS and SPS activation energies of WC + SiC powder at the intensive shrinkage stage were determined using the Young-Cutler model. The CPS activation energies of WC, WC + C and WC + C + SiC powder compositions are close to the activation energy of diffusion of the carbon C along the a-WC grain boundaries. The SPS activation energies of WC + C and WC+ C + SiC powder compositions turn out to be lower than the activation energy of the C of a-WC grain boundary.

Dynamic compressive tests of alumina samples with different grain sizes obtained by spark plasma sintering (SPS) of submicron- and micron-sized α-Al2O3 powders have been performed. The effect of heating rate (Vh), sintering temperature (Ts), holding time (ts), and cooling rate (Vc) on the hardness, crack resistance, and dynamic ultimate strength (σY) of Al2O3 has been studied. An amorphous layer of a nanometer thickness was on the surface of submicron powders in the initial state. The transformation of an amorphous structure with an excess free volume into a crystalline phase occurs upon the SPS process with the formation of dislocation-type defects at the grain boundaries, which induce long-range internal stress fields. It has been shown that nanopores less than 50–100 nm in size are observed at the grain boundaries of ceramics. It has been shown that the nonmonotonic pattern of the dependence of σY on the temperature and time of the SPS is due to the simultaneous change in the density, the nonequilibrium state of grain boundaries, and the grain size of the ceramic. It has been shown that a decrease in the degree of nonequilibrium of the grain boundaries of alumina due to an increase in the SPS temperature or an increase in the holding time makes it possible to increase the dynamic strength of alumina. It has been established that an increase in the cooling rate leads to the formation of compressive residual stresses and a slight increase in σY of the ceramic. The maximum dynamic strength (σY = 1755 MPa) was reached for the alumina ceramic with an average grain size of 1.6–2 μm obtained by SPS at Vh = 50°С/min, Ts = 1520°С, and ts = 50 min.

The corrosion-fatigue strength in 3 % aqueous NaCl solution and the resistance against hot salt corrosion (HSC) of the fine-grained near-alpha alloy Ti-5Al-2V (Russian analogue of Grade 9 titanium alloy with increased aluminium content) have been studied. The properties of the Ti-5Al-2V alloy in the coarse-grained state, in the fine-grained state after cold Rotary Swaging (RS), in the partly recrystallized state and in the fully recrystallized state have been investigated. The mechanical properties of the alloy were characterised using compression tests and microhardness measurements. The effects of RS, the annealing temperature, the time on the characteristics of corrosion destruction of the surface and the composition of the products of the HSC were studied. RS was shown to increase the depth of the intergranular corrosion defects, whereas recrystallization annealing promoted an increase in the corrosion resistance of the Ti-5Al-2V titanium alloy. The parameters of the Basquin equation for the corrosion-fatigue curves for the near-alpha Ti-5Al-2V alloy in the coarse-grained state, in the severely strained state and after recrystallization annealing were determined for the first time. An effect of non-monotonous dependencies of the slopes of the corrosion-fatigue curves for the strained near-alpha Ti-5Al-2V alloy on the recrystallization annealing temperature has been observed.

Chemical stability of Nd0.33Zr2(PO4)3 fine-grained ceramics, which can be used for immobilization of REE that are part of the HLW was studied. Single-phase Nd0.33Zr2(PO4)3 submicron powders with the structure of the mineral kosnarite were obtained by colloid-chemical synthesis. Powders were obtained by successive annealing at 600, 800, and 900 °C for 6 h at each stage. Ceramics Nd0.33Zr2(PO4)3 has been obtained by Spark Plasma Sintering (SPS) method. The relative density of ceramics was 89.9 %; the average grain size was 5 – 20 μm. The chemical stability of the ceramics in the static mode at 90 °C in distilled water, mineral waters, as well as in acidic media (0.1 M HCl) and alkaline media (0.01 M NaOH) was studied. Ceramics have high hydrolytic stability. The influence of the contact medium on the rate leaching and mechanism of Nd leaching from the surface of Nd0.33Zr2(PO4)3 fine-grained ceramic samples has been studied. The de Groot-van der Sloot model was used to analyze the obtained time dependences of the leaching rate Ri. Nd leaching in acidic medium occurs due to dissociation of Nd from the ceramic surface, in alkaline medium and mineral water due to diffusion from the inner layers, in distilled water due to the dissolution of the ceramic surface layer.

Bimetallic samples based on unalloyed titanium and titanium alloy have been obtained. The main disadvantage of the most common material for medical implants Ti6Al4V alloy is the danger of toxic elements of aluminum and vanadium entering the human body during implant wear. To solve this problem, it is proposed to use bimetallic products designed in such a way that the part interacting with human body is made of unalloyed titanium, and the inner part is made of a high-strength titanium alloy providing high mechanical properties. The technology of selective laser melting allows to produce similar products in one technological cycle. The aim of the research is a comprehensive study of the physical and mechanical properties and structure of the bimetallic material of the system “unalloyed titanium – alloy Ti6Al4V”obtained by selective laser melting. The melting modes have been optimized to achieve high mechanical characteristics. It is shown that the strength characteristics with optimal conditions correspond to the characteristic value of unalloyed titanium – the ultimate tensile strength was 860 MPa. Embrittlement of the micro-welded bimetal compound is not observed: tensile fracture occurs with the least durable component – unalloyed titanium. Metallographic studies have shown the absence of micropores, microcracks and other defects in the interface zone of the two materials and adjacent areas. Thus, selective laser melting makes it possible to obtain a new class of bimetallic medical devices with high physical and mechanical properties, while ensuring that living tissues do not contact with toxic elements.

The sintering mechanisms of WC – Al2O3 nanopowder compositions with different contents of aluminum oxide particles (1, 3, 5 wt.%) were investigated. Samples of WC – Al2O3 ceramics were produced by Spark Plasma Sintering method (SPS) in vacuum, by heating to a temperature of 1450 °C at a rate of 50 °C/min under uniaxial stress 70 MPa. Plasma-chemical nanopowders of tungsten monocarbide and submicron powders of aluminum oxide were used to make the ceramics. The density, microstructure, phase composition, microhardness (Hv) and fracture toughness (KIC) of the ceramics were investigated. It was shown that the use of the SPS method makes it possible to obtain WC-Al2O3 ceramics with good relative density (95.4-98.1%) and a homogeneous microstructure with ultrafine grain size (0.1 – 0.2 μm). By the method of the X-ray phase analysis, it was established that in the process of SPS of WC-Al2O3 ceramics the formation of an undesirable W2C phase takes place, leading to decrease in the fracture toughness KIC. To reduce the intensity of W2C particle formation, colloidal graphite (0.1, 0.2, 0.3 wt.%) was added to WC – Al2O3 ceramics. Using the Young-Cutler model and the model of diffusion resorption of pores it was shown that the main mechanism of SPS of WC – Al2O3 ceramics is grain boundary diffusion. It is shown that the introduction of graphite leads to a decrease in the activation energy of SPS of WC – Al2O3 ceramics, which is probably due to a decrease in the W2C particle content to 0.5 wt.%.