Sergey Kalabukhov

Morphological effects on the thermoelectric properties of Ti0.3Zr0.35Hf0.35Ni1+δSn alloys following phase separation

Thermoelectrics are known as one of the emerging renewable power generation technologies. Half-Heusler based semiconducting intermetallic compounds show high potential as thermoelectric materials due to the abundance of their elements in nature, their high mechanical and chemical stability, and their favorable electronic properties. Their main limitation lies in their high lattice thermal conductivity, κl. In the current research, the potential of κl reduction due to generation of composites based on phase separation of off-stoichiometric Ti0.3Zr0.35Hf0.35Ni1+δSn alloys into half- and full-Heusler composites was investigated. Due to the strong metallic nature of the full-Heusler phase, its electronic effect on the thermoelectric transport properties under various morphological and compositional conditions was analyzed by the general effective media (GEM) approach. It was shown that although a major κl reduction of up to ∼37% was associated with phonon scattering by the embedded full Heusler phase, the electronic properties for the parallel-like alignment of this phase, following arc melting and spark plasma sintering, deteriorated with increase of the relative amount of this phase. Therefore a maximal ZT enhancement of ∼41%, compared to the uni-phase HH stoichiometric composition, was obtained for the minimal (δ = 0.01) deviation of the stoichiometry examined, corresponding to the minimal relative amount of the scattered FH phase inside the HH matrix.

Creep of Polycrystalline Magnesium Aluminate Spinel Studied by an SPS Apparatus

A spark plasma sintering (SPS) apparatus was used for the first time as an analytical testing tool for studying creep in ceramics at elevated temperatures. Compression creep experiments on a fine-grained (250 nm) polycrystalline magnesium aluminate spinel were successfully performed in the 1100–1200 °C temperature range, under an applied stress of 120–200 MPa. It was found that the stress exponent and activation energy depended on temperature and applied stress, respectively. The deformed samples were characterized by high resolution scanning electron microscope (HRSEM) and high resolution transmission electron microscope (HRTEM). The results indicate that the creep mechanism was related to grain boundary sliding, accommodated by dislocation slip and climb. The experimental results, extrapolated to higher temperatures and lower stresses, were in good agreement with data reported in the literature.

Dynamic compressive and tensile strengths of spark plasma sintered alumina

Fully dense submicron grain size alumina samples were manufactured from alumina nano-powder using Spark Plasma Sintering and tested in two kinds of VISAR-instrumented planar impact tests. In the first kind, samples were loaded by 1-mm tungsten impactors, accelerated to a velocity of about 1 km/s. These tests were aimed at studying the Hugoniot elastic limit (HEL) of Spark Plasma Sintering (SPS)-processed alumina and the decay, with propagation distance, of the elastic precursor wave. In the tests of the second kind, alumina samples of 3-mm thickness were loaded by 1-mm copper impactors accelerated to 100–1000 m/s. These tests were aimed at studying the dynamic tensile (spall) strength of the alumina specimens. The tensile fracture of the un-alloyed alumina shows a monotonic decline of the spall strength with the amplitude of the loading stress pulse. Analysis of the decay of the elastic precursor wave allowed determining the rate of the irreversible (inelastic) strains in the SPS-processed alumina at the ...

The Effect of Lithium Doping on the Sintering and Grain Growth of SPS-Processed, Non-Stoichiometric Magnesium Aluminate Spinel

The effects of lithium doping on the sintering and grain growth of non-stoichiometric nano-sized magnesium aluminate spinel were studied using a spark plasma sintering (SPS) apparatus. Li-doped nano-MgO·nAl2O3 spinel (n = 1.06 and 1.21) powders containing 0, 0.20, 0.50 or 1.00 at. % Li were synthesized by the solution combustion method and dense specimens were processed using a SPS apparatus at 1200 °C and under an applied pressure of 150 MPa. The SPS-processed samples showed mutual dependency on the lithium concentration and the alumina-to-magnesia ratio. For example, the density and hardness values of near-stoichiometry samples (n = 1.06) showed an incline up to 0.51 at. % Li, while in the alumina rich samples (n = 1.21), these values remained constant up to 0.53 at. % Li. Studying grain growth revealed that in the Li-MgO·nAl2O3 system, grain growth is limited by Zener pining. The activation energies of undoped, 0.2 and 0.53 at. % Li-MgO·1.21Al2O3 samples were 288 ± 40, 670 ± 45 and 543 ± 40 kJ·mol−1, respectively.

Bonding of TRIP-Steel/Al2O3-(3Y)-TZP Composites and (3Y)-TZP Ceramic by a Spark Plasma Sintering (SPS) Apparatus

A combination of the high damage tolerance of TRIP-steel and the extremely low thermal conductivity of partially stabilized zirconia (PSZ) can provide controlled thermal-mechanical properties to sandwich-shaped composite specimens comprising these materials. Sintering the (TRIP-steel-PSZ)/PSZ sandwich in a single step is very difficult due to differences in the sintering temperature and densification kinetics of the composite and the ceramic powders. In the present study, we successfully applied a two-step approach involving separate SPS consolidation of pure (3Y)-TZP and composites containing 20 vol % TRIP-steel, 40 vol % Al2O3 and 40 vol % (3Y)-TZP ceramic phase, and subsequent diffusion joining of both sintered components in an SPS apparatus. The microstructure and properties of the sintered and bonded specimens were characterized. No defects at the interface between the TZP and the composite after joining in the 1050–1150 °C temperature range were observed. Only limited grain growth occurred during joining, while crystallite size, hardness, shear strength and the fraction of the monoclinic phase in the TZP ceramic virtually did not change. The slight increase of the TZP layer’s fracture toughness with the joining temperature was attributed to the effect of grain size on transformation toughening.

Strength of ceramic–metal joints measured in planar impact experiments

SPS-processed alumina and reaction-bonded boron carbide ceramic composite (RBBC) were joined with Al10SiMg alloy by spark plasma sintering and tested in a series of planar impact experiments designed to measure dynamic tensile (spall) strength of the joints. The results of the impact testing, together with postmortem inspection of the fractured samples, confirmed the applicability of this approach for testing joint strength. The measurements show that in the case of an RBBC/metal joint, the dynamic tensile strength of the joint exceeds that of the ceramic part, and that fracture of the shock-loaded ceramic–metal pair occurred in the ceramic portion. The dynamic tensile strength of the interface between alumina and Al10SiMg alloy virtually coincides with that of the metal part, with the fracture occurring exactly at the interface. The coincidence may be explained based on the recently published results of atomistic calculations of the structure of an alumina–aluminum interface.

Transparent Polycrystalline Magnesium Aluminate Spinel Fabricated by Spark Plasma Sintering

Polycrystalline magnesium aluminate (MgAl2 O4 ) spinel (PMAS) exhibits a unique combination of physical, chemical, mechanical, and optical properties, which makes it useful for a wide range of applications, including UV lenses for lithography, electroinsulation, and structural windows for both VIS and IR region radiation and armor applications. Conventional two-stage processing of PMAS involves prolonged pressureless sintering followed by hot isostatic pressing. The costly processing of high-quality transparent PMAS ceramic is the main reason for the limited usage of this material in industry. Spark plasma sintering (SPS) is a relatively novel one-stage, rapid, and cost-effective sintering technique, which holds great potential for producing high-quality optical materials. Here, recent advances in the fabrication of transparent PMAS by the SPS approach, the influence of sintering parameters on microstructure evolution during densification, and their effects on the optical and mechanical properties of the material are reviewed.

Dynamic yield and tensile strengths of spark plasma sintered alumina

Fully dense alumina samples with 0.6 μm grain size were produced from alumina powder using Spark Plasma Sintering and tested in two types of VISAR-instrumented planar impact tests. In the tests of the first type the samples of 0.28 to 6-mm thickness were loaded by 1-mm tungsten impactors accelerated up to a velocity of about 1 km/s. These tests were aimed to study the Hugoniot elastic limit (HEL) of the SPS-processed alumina and the decay of the elastic precursor wave with propagation distance. In the second type of test the samples of ~3-mm thickness were loaded by 1-mm copper impactors accelerated up to velocities 100-1000 m/s. These tests were aimed to study the dynamic tensile (spall) strength of the alumina. The data on tensile fracture of the alumina demonstrate a monotonic decline of the spall strength with the amplitude of the loading stress pulse. The data on the decay of the elastic precursor wave allows for determining the rates of the irreversible (inelastic) strains in the SPS-processed alumina at the initial stages of shock-induced inelastic deformation and, thus, to derive some conclusions concerning the mechanisms responsible of the deformation.

On the Role of TiC in the Carbon Steel-Titanium Joining

TiC interfacial layer is formed during titanium alloy to carbon steel diffusion bonding and prevents the formation of FeTi and Fe2Ti intermetallic compounds, which have detrimental effect on the properties of joints. The TiC layer is extremely thermodynamically stable in contact with austenite and is formed at the steel/titanium interface even in the case when carbon content is about 0.1wt.%. The evolution of an initially formed interfacial titanium carbide layer includes its growth and simultaneously changing the composition of austenite (towards lower carbon content) and titanium (towards higher carbon content). The kinetics parameters of the interfacial layer growth (carbon diffusion coefficient and the activation energy) reflect the grain boundary mechanism of carbon diffusion through the interfacial layer with submicron grains microstructure.