MAX phases Ti2AlC(s), Ti3AlC2(s) are considered promising candidates for high-temperature structural applications due to their unique combination of metallic and ceramic characteristics. These phases are widely studied as protective coatings for extreme environments (1173-1673K), forming a well-adhered alpha-Al2O3 layer that enhances oxidation resistance. Understanding their thermodynamic stability is crucial for predicting long-term performance. In this study, we employ High-Temperature Knudsen Effusion Mass Spectrometry (HT-KEMS) to investigate the vaporization behavior of Ti2AlC(s) and Ti3AlC2(s), identifying dominant vapor species and coexisting phases. Phase field analysis of partially decomposed Ti2AlC(s) and Ti3AlC2(s) residue samples were carried out employing X-ray diffraction technique in conjunction with X-ray photoelectron spectroscopy. The vapor pressures of aluminum bearing species are quantified as. ln(p(Al),Ti2AlC/Pa)=(46761 +/- 362)/T+(28.1 +/- 0.2) (1416

A machine learning (ML) framework for predicting the martensite start temperature (Ms) of a wide variety of steels is presented. The study used a large Ms dataset sourced from publicly available databases and literature. Data cleaning was performed using a novel approach based on empirical equations, traditionally used to calculate the Ms of steels, which resulted in a significant improvement in the predictive capability of the ML models. The final dataset consisted of 1596 entries, of which 80% was used for training and the remaining (unseen data) was used for testing. As a corollary, the reliability of 18 empirical equations was also evaluated. In addition to the steel composition covering 17 elements, the ML dataset included 20 synthetic features. Seven ML regressors (CatBoost, XGBoost, Random Forest, Extra Trees, LassoCV, SGD, and Support Vector Regressor) were optimized and compared. Among these, CatBoost exhibited the highest predictive capability. Feature ranking techniques that combine the Pearson correlation coefficient (PCC), the generalized maximal information coefficient (GMIC), and SHAP-based recursive elimination identified key features that influence Ms. Only those features were included in the CatBoost-based final model. When tested with the unseen data, the final model resulted in an MAE (+/- 13.91 K), RMSE (+/- 18.72 K), and R2 (0.973). To the best of our knowledge, the unseen dataset (320 entries) employed in this study is the largest reported to date for ML modelling of Ms. Besides, the performance metrics achieved in the present work rank high among the other works for Ms prediction. The Ms temperature of pure iron predicted by the final model (754 degrees C) is very close to the theoretical value (770 degrees C), which is remarkable compared to the other ML models reported so far. The results demonstrate that the robust data cleaning strategy combined with advanced ML techniques yields a robust and generalizable model for Ms prediction, outperforming traditional regression approaches. The framework offers a scalable path for data-driven alloy design and phase transformation modelling in steels.

Two-dimensional (2D) MXenes are nanometer-thick sheets of transition metal carbides, nitrides, or carbonitrides with high-temperature capabilities. MXenes can be used as nanofillers and functional additives in ceramic hybrids, enhancing sintering and mechanical and electrical properties. In this study, we systematically investigate the incorporation of titanium carbide (Ti3C2Tx) MXene into micron-sized ( 6 μm) boron carbide (B4C) using a one-step electrostatic self-assembly method. We tuned the zeta potential of B4C and Ti3C2Tx MXene solutions, gradually added B4C into the MXene solution, and prepared green bodies with 1 to 10 vol.

In this work, we report the thermodynamic modeling of the Ta–N–O system using the Calphad approach combined with ab initio calculations and experiments. The formation enthalpies of stable and certain metastable compounds of the Ta–N and Ta–N–O systems were computed using the density functional theory (DFT) to serve as inputs for the Gibbs energy modeling. The DFT study has confirmed as an equilibrium phase. Hence, this phase is included as a stable phase in the Gibbs energy modeling of the Ta–N system. Wherever feasible, we have performed phonon calculations to estimate the heat capacities of compounds. The heat capacities of and TaON were measured using a differential scanning calorimeter. It is observed that ‐TaN, a high‐pressure polymorph of ‐TaN, stabilizes at atmospheric pressure with oxygen substitution. The thermochemical data obtained in the present work are combined with carefully selected thermochemical and constitutional data from the literature to obtain the Gibbs energy descriptions of the Ta–N and Ta–N–O systems.

In classical elasticity, materials with cubic symmetry are subject to specific restrictions, and only a few exhibit elastic isotropy or near-isotropy. Recent DFT calculations have shown that the entropy-stabilized oxide [(MgNiCoCuZn)O] displays elastic isotropy, with a Zener ratio close to 1. However, no experimental evidence exists to confirm this. In this study, we present a robust micromechanical approach based on the Voigt-Reuss-Hill model to estimate Single Crystal Elastic Constants (SECs) from polycrystalline materials. This method requires only two diffraction elastic constants and isotropic elastic constants for materials with cubic symmetry. Validation using phase-pure nickel showed excellent agreement with literature values, with a maximum deviation of 8.8 % for C12. Applying the methodology to [(MgNiCoCuZn)O], the calculated SECs were: C11 = 219 GPa, C12 = 116 GPa and C44=51 GPa. A comparison with DFT-calculated literature values revealed significant discrepancies, with bulk and shear moduli obtained using the Voigt-Reuss-Hill average differing by 25 % to 59 %. To investigate this disparity, we performed additional DFT calculations and examined the underlying factors influencing these results. This study not only introduces a reliable and straightforward methodology for SEC estimation for cubic materials but also provides the first experimental SEC values for [(MgNiCoCuZn)O] entropy- stabilized oxides under ambient conditions. These findings are crucial for developing accurate interatomic potentials in future.

Grain boundary complexions in the Hastelloy C-22 are modified to unravel the structure–property interplay of grain boundary precipitation (carbides), segregation, and grain boundary diffusion rates. Annealing treatments are used to create distinct grain boundary complexions with specific segregation-precipitation-structure states. Two grain boundary states were compared: those induced by low- (at 873 K, mainly for segregation) and by high-temperature (at 1123 K, for carbide precipitation) annealing treatments. A correlative analytical microscopy–tracer diffusion analysis approach is followed to elucidate the influence of the carbide formation, element segregation, and structure evolution on grain boundary diffusion. The tracer measurements with the 51Cr and 63Ni radioisotopes reveal a strongly non-linear dependence of the grain boundary diffusion rates on the pre-annealing time. Via in-depth characterization of the grain boundary microstructure, the evolution of Mo segregation and of incoherent metallic carbides at the interfaces is quantitatively related to the captured monotonic grain boundary diffusion behavior in Ni–Cr–Mo system.

In this study, we investigated the effect of strain rate on the work hardening, strain-induced martensite transformation, strain partitioning, and crystallographic variant selection in a medium-Mn steel. Uniaxial compression tests were performed under quasi-static and dynamic loading conditions at the strain rates of 10−3, 1 and 103s−1. Besides, to determine the contribution of temperature rise due to adiabatic heating at a higher strain rate, compression tests were carried out at 409 K (estimated temperature rise) under quasi-static conditions. The work hardening rate was higher at high strain rates and it rapidly decreased at all strain rates up to a true plastic strain of ∼0.05. Beyond the true plastic strain of ∼0.05, the work hardening rate increased at a low strain rate up to a true plastic strain of ∼0.15 while at a high strain rate, it remained relatively constant and gradually decreased beyond a true plastic strain of ∼0.15. From the analysis of microstructures obtained from electron backscatter diffraction, it was observed that the strain-induced transformation of retained austenite to α′ martensite is significantly impeded at higher strain rates and in the sample tested at 409 K at low strain rates. The higher stability of retained austenite is attributed to the adiabatic heating which decreases the chemical driving force for retained austenite to martensite phase transformation and increases the stacking fault energy. Analysis of the misorientation axis and angle evolution revealed that while the Kurdjumov-Sachs relationship was followed under quasi-static conditions, it deviated at higher strain rates. Analysis of crystallographic variants related to strain-induced martensitic transformation at a true plastic strain of ∼0.15 revealed that the samples deformed at a low strain rate did not show any noticeable variant selection. However, in samples deformed at a high strain rate, a significant variant selection was observed. It is reasoned that the martensitic transformation nucleates initially on favorably oriented habit planes of retained austenite grains at all strain rates and other variants get activated with an increase in strain. As the transformation is impeded beyond a critical strain at a higher strain rate, the variants activated at lower strain remains predominant.

The search for sustainable energy solutions has led to extensive research on new electrocatalysts that can convert electrical energy into chemical energy and back. Tantalum nitrides stand out as an intriguing class of materials, showcasing exceptional properties such as high melting points, remarkable mechanical strength, and notable resistance to corrosion. These attributes position tantalum nitrides (Ta-N) and allied phases (Ta-N-X) as compelling candidates for diverse applications, notably in electrocatalysis. While traditionally studied for their photocatalytic and photoelectrocatalytic properties, this review ventures into largely uncharted territory, illuminating the untapped potential of tantalum nitrides as electrocatalysts. Electrocatalysis assumes a pivotal role in numerous renewable energy technologies, including fuel cells and water electrolysis, which demand materials adept at catalyzing reactions efficiently. The distinctive characteristics of Ta-N phases, particularly their electrical conductivity, chemical stability, and expansive surface area, mark them as promising contenders in this arena. This comprehensive review article aims to unveil the electrocatalytic prowess of Ta-N phases, examining their catalytic performance concerning the Hydrogen Evolution Reaction (HER), Oxygen Evolution Reaction (OER), and Oxygen Reduction Reaction (ORR). Delving into recent advancements over the past five years, the article scrutinizes strategies employed to counter surface oxidation—a prevailing degradation issue that hampers activity in Ta-N phases. It also describes methodologies to mitigate photocorrosion observed during photocatalytic/photoelectrochemical (PEC) water splitting of Ta-N phases, offering potential blueprints for efficient design of their electrocatalytic counterparts. The exploration encompasses a thorough investigation into the role of various correlative spectroscopy techniques, including X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, and Fourier-Transform Infrared Spectroscopy (FTIR), in unraveling the involvement of oxygen-related species within Ta-N systems. Furthermore, the presence of oxygen necessitates an intricate comprehension of the thermodynamic stability of different Ta-N phases, both in the presence and absence of oxygen.This article underscores the importance of an exhaustive phase diagram analysis for the Ta-N system in the context of water splitting, critically evaluating thermochemical and constitutional data. Despite extensive research efforts, the phase diagram of the Ta-N system remains incomplete, restraining our understanding of phase stability and overall performance. This account aims to enhance understanding of Ta-N phases and provide insights that support cohesive electrocatalyst design, focusing on the key issue of long-term stability in electrocatalysis.

AbstractTransition metal nitrides, especially tantalum nitrides, are pivotal for applications in extreme environments demanding excellent mechanical properties and thermodynamic stability. Among them, ‐TaN, a high‐pressure polymorph of tantalum nitride with its exceptional bulk modulus (362 GPa) and hardness (31.7 GPa) promises to have many technological uses. Another nitride, , has gained importance as a photocatalyst for water splitting using visible light. The Ta–N phase diagram indicates that the thermal decomposition of pure leads to the formation of ‐TaN. However, usually has some amount of oxygen as an impurity mainly due to its synthesis route. We found that the ‐TaN phase, which is usually observed at high pressures, is formed during the thermal decomposition of oxygen containing . The presence of ‐TaN is verified using several experimental techniques such as X‐ray diffraction, Raman spectra, high‐angle annular dark field scanning transmission electron microscopy (STEM‐HAADF), and electron energy loss spectroscopy (EELS). Elemental distribution analyzed through energy dispersion X‐ray spectroscopy (XEDS) in STEM reveals about 7 at.% of oxygen in ‐TaN. First‐principle calculations are performed to examine the thermodynamic stability of oxygen substituted ‐TaN and pure ‐TaN via formation enthalpies, elastic constants, and phonon dispersion calculations. The computational studies confirm that oxygen in ‐TaN enhances its thermodynamic stability. The calculated electron localization functions establish the bonding characteristics between Ta, N, and O, confirming the same.

Transition metal nitrides, especially tantalum nitrides, are pivotal for applications in extreme environments demanding excellent mechanical properties and thermodynamic stability. Among them, theta-TaN, a high-pressure polymorph of tantalum nitride with its exceptional bulk modulus (362 GPa) and hardness (31.7 GPa) promises to have many technological uses. Another nitride, Ta3N5, has gained importance as a photocatalyst for water splitting using visible light. The Ta-N phase diagram indicates that the thermal decomposition of pure Ta3N5 leads to the formation of epsilon-TaN. However, Ta3N5 usually has some amount of oxygen as an impurity mainly due to its synthesis route. We found that the theta-TaN phase, which is usually observed at high pressures, is formed during the thermal decomposition of oxygen containing Ta3N5. The presence of theta-TaN is verified using several experimental techniques such as X-ray diffraction, Raman spectra, high-angle annular dark field scanning transmission electron microscopy (STEM-HAADF), and electron energy loss spectroscopy (EELS). Elemental distribution analyzed through energy dispersion X-ray spectroscopy (XEDS) in STEM reveals about 7 at.% of oxygen in theta-TaN. First-principle calculations are performed to examine the thermodynamic stability of oxygen substituted theta-TaN and pure theta-TaN via formation enthalpies, elastic constants, and phonon dispersion calculations. The computational studies confirm that oxygen in theta-TaN enhances its thermodynamic stability. The calculated electron localization functions establish the bonding characteristics between Ta, N, and O, confirming the same.

A quantitative approach is presented here that studies the H trapped by the precipitates of microalloying elements (Nb, Ti, and V) in low C ferritic steels. Three model steels, each with one of the three microalloying elements, were prepared in the laboratory and subjected to aging treatment after solutionizing to achieve precipitates in ferrite. The precipitation reactions in the alloys were simulated using the TC-PRISMA module in Thermo-Calc software (databases: TCFE11 and MOBFE6) and subsequently validated using a small-angle neutron scattering (SANS) study. The H trapping abilities in the aged alloys were estimated after saturation H charging using electrochemical means and correlated with the volume fraction, number density, size, and interface character of the precipitates (all estimated using SANS). The relative H trapping ability of the precipitates was estimated as TiC > NbC > VC. The optimum aging conditions to achieve the best H trapping abilities were identified. Moreover, the maximum number of H atoms that can be trapped at the precipitate-ferrite interfaces were estimated based on the geometric considerations using SANS-estimated precipitate quantifications for each alloy. Subsequently, it was argued that the total quantity of trapped H by these precipitates can be explained if H remains trapped within the precipitate bodies in addition to those trapped at interfaces.

Directionally solidified (DS) GTD444 is a boron-modified nickel base superalloy which is used in later stage tubine blades. Boron is added as a grain boundary strengthener and it modifies the grain boundary mi-crochemistry. During service, the blades undergo thermal exposure that could lead to further changes in the grain boundary chemistry. This is known to influence the grain boundary strength and affects the overall mechanical response of the superalloys. In the present study high resolution characterization has been carried out to investigate the evolution of grain boundary precipitates in DS GTD444 alloy under thermal aging at 900 oC. In addition, in situ picoindentation and high-temperature tensile tests were performed to determine the effect of changes in microstructure on the mechanical properties. Atom probe tomography, in conjunction with electron microscopy, shows that nano-sized borides present along grain boundaries in the as-received DS blades completely transform into carbides after aging. These carbides are of two types: M23C6, and M6C. These carbides are present discretely along the grain boundary. The evolved microstructure after 320 h of aging shows higher transverse ductility as compared to the as-received DS blades, and the failure in all tested samples was transgranular. This suggests that the presence of discrete precipitates (borides or carbides) along the DS boundaries strengthens the grain boundaries.(c) 2023 Elsevier B.V. All rights reserved.

Palladium-containing silicon oxycarbide (SiPdOC) ceramics were synthesized using polymethylsilsesquioxane modified with palladium acetate as a single-source precursor. Thus, pyrolysis in argon at 1100 ? led to nano composites consisting of Pd2Si nanocrystallites dispersed in an amorphous SiOC matrix. Exposure of SiPdOC to higher temperatures resulted in the precipitation of PdSi in addition to Pd2Si. The temperature-dependent evolution of the phase composition and microstructure in SiPdOC were analyzed using XRD and TEM respectively and rationalized by a ThermoCalc-based thermodynamic assessment showing the feasibility of the possible reactions. The formation of PdSi was perceived because of the shift in the Pd-Si atomic composition towards the higher Si side, caused by the diffusion of Si present in the matrix into the Pd-Si melt, formed upon the heat treatment above the melting point (1390 C) of Pd2Si. Further, Raman spectroscopic investigation indicated that Pd catalytically enhanced the graphitization of the free carbon in SiPdOC ceramics.

In this work, titania-silicon oxycarbide nanocomposites synthesized via the polymer derived ceramics route have been sintered into crack-free monoliths using spark plasma sintering and their mechanical properties as well as their apparent density have been characterized. The x-ray diffractogram clearly showed the stabilization of anatase phase of size less than 10 nm size in an amorphous silicon oxycarbide matrix at 1200 degrees C. The hardness and elastic modulus determined using nanoindentation were found to be 9 GPa and 82 GPa, respectively, and the fracture toughness calculated using indentation crack length method was found to be 2.34 MPa m(1/2). In addition to this, ball-on-three ball technique was used to evaluate the biaxial flexural strength and fractographic studies were carried out to understand the fracture mechanisms.

In this work, a novel ultra-high temperature resistant precursor-derived ceramic containing Zr, La, B, and C was synthesized through precursor modification of phenol formaldehyde resin. The thermal stability and resistance to crystallization of the ceramic at a temperature of 1600 degrees C was investigated and was found to be profoundly influenced by the boron content in the starting precursors. The ceramics remained amorphous at 1600 degrees C for 2 h in argon and upon sustained heat-treatment for up to 16 h resulted in nano-crystalline ultra-high temperature phases such as ZrB2, ZrC, LaB6 and La2Zr2O7. Thermodynamic equilibrium phase calculations show that even longer durations of heat treatment may be required to achieve thermodynamic equilibrium. High-resolution transmission electron microscopy revealed encapsulation of nanocrystals (<5 nm) in an amorphous matrix surrounded by turbostratic layers of carbon inhibiting its growth. Spectrochemical techniques confirmed the presence of boron substituted carbon in the amorphous matrix of the ceramic. The unique nature of the amorphous matrix lends the ceramic resistance to crystallization and chemical degradation that can surpass the likes of classical silicon-based precursor-derived ceramics.

Mg-based blowing agents exhibit the potential to yield aluminum foams with better structure and properties than those achieved by using conventional blowing agents. However, all the studies to date used a high amount of such blowing agents (e.g., 15 wt pct Mg) for foaming aluminum. In this study, we investigate the minimum amount of Mg blowing agent required for foaming. Al-Si13-MgX (X = 2.5–15 wt pct) alloy foams were produced employing the powder metallurgy route where Mg acted as the blowing agent. The macro- and microstructure of the foams were analyzed using X-ray tomography, microscopy, and X-ray diffraction. The foams were subjected to hardness and compression tests to evaluate their mechanical properties. Deformation behavior was studied in situ by monitoring the foam surface during the compression test. For the first time to our knowledge, the present study established that 5 wt pct of Mg is sufficient to achieve foam expansion similar to that achieved by 15 wt pct Mg. Moreover, the structural and mechanical properties of the 5 wt pct Mg-containing foams were much superior to the 15 wt pct Mg-containing foams. However, the highest strength was obtained using 10 wt pct of Mg. Many cracks were observed at the early deformation stages of 10 and 15 wt pct Mg-containing foams. We correlate the Mg content with the structure, properties, and deformation behavior of the foams.

Ablation behaviour of poly(hydridomethylsiloxane) derived open and closed porous structured SiOC ceramic foams was evaluated using oxy-acetylene flame at 1500 °C for various time durations. X-ray diffraction and scanning electron microscopy analyses of ablated SiOC ceramic foams revealed the formation of a thin protective SiO2 layer inhibiting further oxidation. The closed porous structured SiOC ceramic foams exhibited very low mass ablation rate in contrast to open porous structured SiOC ceramic foams owing to the differences in thermal energy dissipation mechanism. The feasibility of the plausible foam reduction reactions pertaining to the ablation mechanism was further investigated by computing the Gibbs energy and HR-TEM analysis. The study corroborated the significance of tailoring the microporous structured SiOC ceramic foams as potential thermal protection material for high temperature applications.

It is well known that the phase diagrams calculated using bulk Gibbs energy alone is insufficient when the dimensions of alloy particles are less than 100 nm. This is due to the surface energy contribution to the Gibbs energy, which increases as surface-to-volume ratio of alloy particles increases. In this work, we have used the CALPHAD approach to calculate the phase diagram of the Ga-Sb system taking into account of the size and shape of alloy particles. To accomplish this, we have added size-dependent surface energy terms and optimized the Gibbs energy expressions of various phases in the Ga-Sb system using surface tension information. The surface tension of the liquid phase is calculated using the Butler equation. The surface energy contribution to the Gibbs energy of the terminal phases is approximated using the surface tension expressions of the elements. The surface energy of the line compound GaSb is computed using the slab model within the framework of Density Functional Theory (DFT). The calculations show that there is lowering of liquidus and eutectic temperatures when size decreases. Among the two shapes considered, the depression is more prominent in the nano-spheres than the nano-rods.