Materials For High-temperature Applications Research Articles

Enhanced dielectrics, ferroelectric and optical properties of lithium niobate for high temperature applications using potassium oxide (K2O) additive

Dielectrics, Ferroelectric and Optical properties of Lithium Niobate has been examined using Potassium Oxide (K2O) as an additive on the samples prepared by solid state reaction method using high energy ball milling. The crystal structure analysis by Rietveld structure refinement confirms that the LiNbO3 (LN) with K2O additive also exhibit the rhombohedral structure with R3c space group. The SEM characterization of the microstructure reveals that the samples sintered at a temperature of 1050 °C, have regular and uniform grain morphology and better density. The average grain size decreases for the K2O added compositions in the beginning and increases later for higher concentration of additive. Thermal study with TGA suggests that as the activation energy is decreases, the polarization is enhanced and the LN composition with 2.72 wt% K2O additive shows the best thermal stability. The frequency and temperature dependent dielectric properties were investigated in a wide range of frequency and a substantial enhancement in the dielectric constant of LN is obtained by K2O addition. A useful enhancement in permittivity is obtained with increase of temperature. The band gap values for direct and indirect band gaps for the various compositions are also determined. The least direct band gap is discovered to be 2.36 eV for the composition with 2.72 wt% K2O additive. The composition-dependent photoluminescence (PL) emission studies reveal that the emission spectra for both the pure LiNbO3 and K2O added LiNbO3 compositions lies in the UV–visible region. All the investigated compositions exhibit distinct green, yellow, and red emission peaks. The LN with 2.72 wt% K2O additive shows the best dielectric, thermal stability and polarization response with maximum polarization Ps ∼1.16 μC/cm2. The present investigations suggest that properties of LN can be significantly improved by using K2O additive for sustainable, low cost, environmental friendly energy storage material for high temperature applications.

An In-Depth Exploration of Unconventional Machining Techniques for INCONEL® Alloys.

Build-up-edge (BUE), high-temperature machining and tool wear (TW) are some of the problems associated with difficult-to-machine materials for high-temperature applications, contributing significantly to high-cost manufacturing and poor tool life (TL) management. A detailed review of non-traditional machining processes that ease the machinability of INCONEL®, decrease manufacturing costs and suppress assembly complications is thus of paramount significance. Progress taken within the field of INCONEL® non-conventional processes from 2016 to 2023, the most recent solutions found in the industry, and the prospects from researchers have been analysed and presented. In ensuing research, it was quickly noticeable that some techniques are yet to be intensely exploited. Non-conventional INCONEL® machining processes have characteristics that can effectively increase the mechanical properties of the produced components without tool-workpiece contact, posing significant advantages over traditional manufacturing.

Oxidation behavior of Ti2AlC MAX phase-based coating on a γ-TiAl alloy TiAl48-2-2 produced by DC magnetron sputtering

The application of intermetallic γ-TiAl-based alloys is limited by the deterioration in strength and creep resistance as well as reduction in oxidation resistance above 750 °C. The deposition of protective coatings is a promising opportunity to enhance the oxidation resistance or the lifetime with regard to the sustainability by several orders of magnitude. Moreover, additive manufacturing processes could enable the production of components with more complex geometries in the future. Thus, designs of γ-TiAl turbine blades with internal cooling systems could be feasible in the near future. In this context, the deposition of thermal barrier coatings and, therefore, oxidation protective bond coatings became important. MAX phases are of increasing interest as coating materials for high temperature applications due to their unique combination of metallic and ceramic properties. Especially the alumina forming MAX phases such as Cr2AlC, Ti2AlC or Ti2AlN are promising coating materials.In the present work a Ti2AlC MAX-phase based coating was deposited by DC magnetron sputtering. Using three pure elemental target materials - Ti, Al and C and a two-fold rotation, a homogenous all-around coating was applied on the γ-TiAl alloy TiAl48-2-2 with a coating thickness of 10 μm. After the deposition process the stochiometric Ti2AlC coating was X-ray amorphous, therefore a post-heat treatment at 800 °C for 1 h was performed to achieve the desired hexagonal phase. Finally, the MAX-phase coated TiAl48-2-2 alloy was subjected to a cyclic oxidation test at 850 °C in laboratory air. The Ti2AlC MAX-phase coated TiAl48-2-2 alloy exhibits an excellent oxidation behavior, due to the formation of a thermally grown alumina top layer with the desired hexagonal Ti2AlC MAX-phase below. Extensive TEM analyses also showed the dependence of Al2O3 formation on post-heat treatment. θ-Al2O3 with a lower protective effect and the concomitant formation of TiN under the TGO was observed with post-heat treatment in Ar, while post-heat treatment in air lead to the formation of a protective α-Al2O3 layer.

Chromium diffusion coatings for improving the oxidation behavior of refractory metals at intermediate temperatures

Due to their very high melting points and high mechanical strength at elevated temperatures, refractory metals are promising structural materials for high temperature applications. However, their application is strongly restricted by their insufficient oxidation resistance. In this fundamental study, four different refractory metals (Mo, Nb, Ta, and W) were chromized by pack cementation to investigate the influence of chromium diffusion coatings on their oxidation resistance at intermediate temperatures. Thermodynamic calculations were used to find the most suitable conditions for the pack cementations. The formation of a homogeneous single-phase bcc Cr layer with a thickness below 10 μm on the sample surface was observed on all four refractory metals. In the case of Nb and Ta, intermetallic Cr2X-Laves phases (X = Nb, Ta) formed underneath the bcc Cr layer while an interdiffusion zone formed on Mo. The concentration profiles of the deposited Cr layers were used to determine the diffusion coefficients graphically via Boltzmann-Matano analysis. The Cr-coated samples were analyzed via thermogravimetric analysis at 700 °C and 900 °C for up to 100 h in dry synthetic air. Optical and electron microscopy, EPMA, EBSD and XRD were used to investigate the chromium diffusion layers and the oxide scales.

Preparation and properties of aluminosilicate aerogels from boehmite and waterglass

Aluminosilicate aerogels are promising thermal insulation materials for high temperature applications. In this study, aluminosilicate aerogels were effectively synthesised using affordable precursors: dispersible boehmite powder and waterglass solutions, with subsequent drying achieved by supercritical CO2 processing. According to the evaluation of their properties, the Al–Si aerogels exhibited low densities (0.11 g/cm3), low thermal conductivities (<0.021 W/m·K) and high specific surface areas (SSA) (516 m2/g for an Al:Si molar ratio of 3:1). When waterglass-based aerogel and aluminosilicate samples were treated at high temperature (1200 °C), the formation of mullite was demonstrated for the aluminosilicate aerogel with a molar ratio of Al:Si 3:1, which showed a higher value for SSA compared to the waterglass-based aerogel.

Feasibility study of wire arc additive manufactured P91 steel by cold metal transfer deposition

The objective of this study was to analyze and evaluate the feasibility of utilizing Wire Arc Additive Manufacturing (WAAM) technology for the constructing components made from P91 steel, which is widely used material for high-temperature applications. The utilization of the Cold Metal Transfer (CMT) method enables the deposition of thin steel walls by virtue of its low heat input. Samples collected from the deposited wall have been tested in two different microstructural conditions, in the as-built (AB) condition and in a normalized and tempered (NT) condition. The latter condition is the microstructural state commonly employed for producing components used in power plants. This post-deposition heat treatments (PDHT) comprise a normalization process conducted at 1050 °C for 40 min, followed by a tempering step at 760 °C for 2 h. The results of mechanical testing and microstructural characterization were then compared against the established standards for commercial P91 steel. SEM-EDAX, XRD, and EBSD analyses were employed to conduct a comprehensive microstructural examination of the samples. The hardness of the AB samples ranged from 384 to 439 HV, while the NT samples had a hardness range of 205 to 223 HV. In terms of tensile strength, the AB samples demonstrated a range of 1064 to 1266 MPa, whereas the NT samples exhibited a range of 635 to 626 MPa. These results were found to be consistent with the observed strong texture and grain orientations in the {001} and {111} planes, as revealed by the EBSD analysis. The NT components demonstrated significant strength, ductility, hardness, and toughness compared to the specified ASTM standards. Based on the test performance and feasibility criteria, this process has significant potential as an additive manufacturing method suitable for complex boiler components.

DFT based computational investigations of the physical properties of perovskites-structure SrXO3 (X = Si, Tb, Th) for optoelectronic and thermo-mechanical applications

Density Functional Theory (DFT) method is employed for investigating the structural, elastic, electronic, optical, and thermal properties of Perovskite structure SrXO3 (X = Si, Tb, and Th). The studied lattice parameters show good agreement with the previous theoretical and experimental results. The elastic stiffness constants, (Cij) of SrSiO3, SrTbO3, and SrThO3 satisfied the born stability criteria which confirm the mechanical stability of these phases. It is observed that when Tb or Th is replaced with Si, the brittleness behaviours of SrTbO3 or SrThO3 change to ductility, which improves its machinability for industrial applications. The investigation of anisotropic factors of SrSiO3, SrTbO3, and SrThO3 ensured that SrSiO3 is more anisotropic than SrTbO3 and SrThO3. By evaluating the electronic properties, it can be advantageous for applying the topological insulators (TI), solar cells, or PV (Photovoltaic) cell fabrication. Furthermore, the optical properties are studied and detail discussed with SrXO3 (X = Si, Tb, and Th). The reason of low Debye temperature (ΘD) and low minimum thermal conductivity (Kmin) has been perfectly explicated through the consideration of the mean atomic weight (M/n) of the compounds. Temperature-dependent of heat capacities (Cv, Cp) and linear thermal expansion coefficient (α) are also estimated using the quasi-harmonic Debye model and discussed. The SrTbO3 and SrThO3 compounds can be employed as potential thermal barrier coating (TBC) materials for high-temperature applications, according to the low values of Kmin, ΘD and α.

Physics-informed machine learning with high-throughput design module for evaluating rupture life and guiding design of oxide/oxide ceramic matrix composites

In this paper, a unified physics-informed machine learning (PIML) model is proposed for the accurate prediction of the creep rupture life of oxide/oxide ceramic matrix composites (CMCs) by incorporating prior physical information and microstructure features. The collected dataset undergoes missing value imputation and standardization methods for processing. Key feature parameters that influence rupture life are identified through global sensitivity analysis. Furthermore, a high-throughput design module is developed to explore optimal composition under specific creep conditions, providing design guidelines for new materials in oxide/oxide CMCs. This study demonstrates that the predicted results consistently fall within the ±3 error bands. Improved life expectancy is attained with fiber modulus (220–290 GPa) and matrix modulus (>310 GPa). The transformation from expensive creep testing to low-cost exploration of modulus and proportional limit is achieved, offering theoretical support and analytical methods for the design of CMC components and the development of new materials for high-temperature applications.

Formation of Form-Stable Polyamide/Reduced Graphene Oxide Phase Change Material for High Temperature Applications using Nitrile Butadiene Rubber Coating

Formation of Form-Stable Polyamide/Reduced Graphene Oxide Phase Change Material for High Temperature Applications using Nitrile Butadiene Rubber Coating

Titanium boride fibers prepared by chloride-assisted carbothermic method for high-performance electromagnetic wave absorption

The TiB2 fibers were prepared by a chloride-assisted carbothermic method accompanied with the vapor-liquid-solid (VLS) mechanism. The influences of TiO2, B2O3, C12H22O11, and calcination temperature on the yield of TiB2 fibers were investigated. In addition, the effects of TiB2 fibers content on the electromagnetic (EM) parameters and microwave absorption (MA) properties of TiB2 fibers/paraffin composite was studied in detail. The TiB2 fibers/paraffin composite showed excellent microwave absorption performance, with an optimized reflection loss (RL) of −46.63 dB at 14 GHz and a thickness of 1.5 mm. Moreover, it exhibited a broad effective absorption bandwidth (EAB) (RL < −10 dB) of 3.9 GHz, ranging from 12 to 15.9 GHz. TiB2 fibers exhibit promising potential as materials for high-temperature applications in the field of MA.

Effect of the MgO Addition on the Structure and Physical Properties of the High Entropy HfZrCeYO Fluorite Ceramics

One of the most promising applications of high entropy ceramics is their use as high temperature protective materials. Due to the additional entropic stabilization of the crystal structure, four- and five-element high entropy ceramics exhibit enhanced thermal and mechanical properties. For these applications, one of the most promising high entropy protective oxides are ZrO2- and HfO2-based protective HEOs. In this article, we study the HfO2–ZrO2–Y2O3–CeO2 equimolar system with the addition of MgO as a fifth component. We found that the HfZrCeY(Mg)O system maintains a single FCC crystalline structure up to the MgO concentration = 31.9 mol.%. Additionally, we determined that an addition of MgO at the close-to-equimolar HfZrCeY(Mg)O composition enhances the thermal properties of HEO, but reduces the mechanical properties such as hardness and resistance to crack formation. The minimum weight loss at the heating from RT up to 1450 °C was measured for the close-to-equimolar HfZrCeY(Mg)O composition at 18.4 mol.% MgO. The hardness of such composition was around 18 GPa. Due to the combination of these properties, the synthesized coating can be used as a protective material for high temperature applications, such as the protection of turbine parts.

Inhibiting the formation of planar defects in grown alumina scale on Ni-Al alloys

Twin boundaries and stacking faults (TBs&SFs) in thermally grown oxide scales can deteriorate the oxidation resistance of engineering alloys and it is challenging to inhibit them. Herein, we find that introducing a coherent NiO layer can significantly reduce the TBs&SFs density in γ-Al2O3 scales on Ni-Al alloys and thereby enhance the oxidation resistance. Importantly, such a NiO layer can be spontaneously formed on the surface of a nanograined Ni-7 wt%Al alloy, and its oxidation resistance is three times that of the β-NiAl intermetallics. This provides a novel strategy to develop advanced oxidation resistant materials for high temperature application.

Editorial

Editorial

Wear mechanism, subsurface structure and nanomechanical properties of additive manufactured Inconel nickel (IN718) alloy

Inconel nickel (IN718) superalloy is a cutting-edge material for high temperature applications owing to its superior mechanical properties in both cryogenic and high temperatures. Although the selective laser melting (SLM) processed IN718 superalloy has extraordinary properties, the tribological behavior of this new class of material is unclear. This study aims to explore the tribological behavior and subsurface mechanical properties of SLMed IN718 superalloy with the aid of nanomechanical and structural characterization techniques. The worn subsurface structure was revealed against the mechanical properties of the material using the ball-on-disk contact sliding test. Structural analysis confirmed that the SLMed IN718 superalloy consisted of Fe, Ni, Cr, Nb and Mo elements. EBSD analysis showed that a mixture of equiaxed and columnar grains emerged in <001>, <101> and <111> of SLMed IN718 superalloy. In-situ wear track investigations using 3D profilometer integrated tribometer revealed that its width and depth significantly increased with applied sliding load. Severe abrasive wear occurred at a low load. Under a high load, however, frictional heating brought about plastic deformation and led to increase wear in absence of any significant subsurface damages. The plastic deformation also generated shear-bands in the immediate subsurface and refined the grain structure, which significantly increased the nanohardness and elastic modulus of the deformed material layer.

Characterization of Iron Aluminide Diffusion Coatings Obtained after Friction Surfacing

Iron aluminides are considered as candidate materials for high temperature applications for their excellent high temperature corrosion and oxidation resistance. In the present work, iron-aluminide coatings were developed by friction surfacing (6351 aluminum alloy deposited on a low-carbon steel substrate) followed by a diffusion heat treatment. The initial coatings were found to be geometrically homogenous and adhered well to the steel substrate. The heat treatment process was carried out at 550 °C for 48, 72 and 96 h and the resulting coatings were characterized in terms of microstructure, chemical composition, hardness distribution and phase composition. After heat treatment, the coating/substrate interface morphology was modified and presented patterns typical of Fe-Al intermetallic formation, as well as a substantial increase in hardness (&gt;900 HV) relative to the initial as-deposited condition. With the diffusion treatment, initially Fe2Al5 was found to develop in the coatings, which was converted into FeAl2 after longer exposures.

CoFeMn0.5Al0.25: A high magnetisation CoFe based soft alloy with reduced Co content

The high cost of CoFe alloys has prevented their widespread use regardless of their promising properties such as high magnetisation, high Curie temperature, as soft magnetic materials for high temperature applications. Here, it is shown that AlMn can be used to create a CoFeMn0.5Alx alloy, which has reduced Co content but comparable soft magnetic properties as CoFe alloys, therefore providing a more economical alternative to pure CoFe soft magnets.

Tribological behavior of Cr/a-C multilayered coating against PEEK under dry sliding condition

PEEK is of increasing interest as self-lubricating material for high temperature applications due to its remarkable mechanical properties. However, PEEK is apt to be worn out under dry sliding condition. In this work, the amorphous carbon(a-C) coating interleaved with Cr layer was deposited by a DC magnetron sputtering technique. The friction behavior of the Cr/a-C multilayered coating against PEEK pair was performed in dry atmosphere using a pin-on-disk tribometer. The tribochemical reactions was discussed in terms of evolution of surface morphology and chemical composition of wear debris. Results showed that the tribological behavior of Cr/a-C coating against PEEK pair is dominated by a synergistic effect arisen from inherent graphitization of a-C layer and suppressed adhesive reaction of transferred Cr layer.

Microstructure and Mechanical Properties of High-Specific-Strength (TiVCrZr)100-xWx (x = 5, 10, 15 and 20) Refractory High-Entropy Alloys.

With the increasing demand for high-specific-strength materials for high-temperature applications, particularly in the aerospace field, novel (TiVCrZr)100-xWx (x = 5, 10, 15 and 20) refractory high-entropy alloys (RHEAs) were developed. The phase formation, microstructure, and mechanical properties were studied. The (TiVCrZr)100-xWx RHEAs exhibit a relatively high specific strength and low density compared with the W-containing RHEAs and most of the W-free RHEAs. In (TiVCrZr)100-xWx RHEAs, Laves, BCC and Ti-rich phases are formed, where the Laves phase is the major phase, and the volume fraction of the BCC phase increases with increasing W content. (TiVCrZr)100-xWx RHEAs exhibit dendrite structures, where W is enriched in the dendrite region, and increasing W-rich precipitations corresponding to the BCC phase are observed. The improvement of the strength and hardness of RHEAs is mainly attributed to the evolution of the microstructure and corresponding strengthening effect of W. The empirical parameters and calculated phase diagram were investigated, which further explain and verify the formation and variation of phases. The present findings give more insights into the formation of multi phases in (TiVCrZr)100-xWx RHEAs, and explore their application potential in the aerospace industry and nuclear reactors due to their high specific strength and low-activation constituent elements.

Evaluation of Termite Hill as Refractory Material for High Temperature Applications

Evaluation of Termite Hill as Refractory Material for High Temperature Applications

Investigation on Strain Sensitivity and Temperature Behaviour of Nitrogen Doped 3C-SiC Thin Films

Electrical resistance-strain behaviour of nitrogen doped 3C-SiC thin films was investigated to determine whether they could be used as strain gauges. Using a hot wall vertical low pressure chemical vapour deposition (LPCVD) reactor, these films were deposited on thermally oxidised Si (100) and alumina substrates at 2.5 mbar pressure and 1040°C temperature from methyltrichlorosilane (MTS) precursor. The nitrogen doping gas utilised was ammonia (NH3). Using four-point bending method, the gauge factor (GF) was determined for all thin films (0, 9, 17, and 30 atomic % nitrogen doped). Systematic annealing of films in a vacuum atmosphere was used to determine the TCR of nitrogen doped 3C-SiC (111). Film sheet resistance was evaluated using a four-probe approach, and it was observed to decrease as the temperature rises from 40 to 550°C. The resistivity, average TCR, and strain sensitivity of film doped with 17 atomic % nitrogen concentration were 0.14 cm, -103 ppm/°C, and -9.6, respectively, indicating that it can be employed as a strain gauge material in high temperature applications. However, film doped with 30 atomic % of nitrogen concentration showed an increase in the resistivity, TCR and strain sensitivity.