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Exploring the oxidation behaviors of the Ti-V-Cr-Mo high-entropy MAX at 800°C for its self-lubricity
MAX phase ceramics is a large family of nanolaminate carbides and nitrides, which integrates the advantages of both metals and ceramics, in general, the distinct chemical inertness of ceramics and excellent physical properties like metals. Meanwhile, the rich chemical and structural diversity of the MAXs endows them with broad space for property regulation. Especially, a much higher self‐lubricity, as well as wear resistance, than that of traditional alloys and ceramics, has been observed in MAXs at elevated temperatures in recent decades, which manifests a great application potential and sparks tremendous research interest. Aiming at establishing a correlation among structure, chemical composition, working conditions, and the tribological behaviors of MAXs, this work overviews the recent progress in their high‐temperature (HT) tribological properties, accompanied by advances in synthesis and structure analysis. HT tribological‐specific behaviors, including the stress responses and damage mechanism, oxidation mechanism, and wear mechanism, are discussed. Whereafter, the tribological behaviors along with factors related to the tribological working conditions are discussed. Accordingly, outlooks of MAX phase ceramics for future HT solid lubricants are given based on the optimization of present mechanical properties and processing technologies.
A mathematical formula of high physical interpretation, and accurate prediction and large generalization power is highly desirable for science, technology and engineering. In this study, we performed a domain knowledge-guided machine learning to discover high interpretive formula describing the high-temperature oxidation behavior of FeCrAlCoNi-based high entropy alloys (HEAs). The domain knowledge suggests that the exposure time dependent and thermally activated oxidation behavior can be described by the synergy formula of power law multiplying Arrhenius equation. The pre-factor, time exponent (m), and activation energy (Q) are dependent on the chemical compositions of eight elements in the FeCrAlCoNi-based HEAs. The Tree-Classifier for Linear Regression (TCLR) algorithm utilizes the two experimental features of exposure time (t) and temperature (T) to extract the spectrums of activation energy (Q) and time exponent (m) from the complex and high dimensional feature space, which automatically gives the spectrum of pre-factor. The three spectrums are assembled by using the element features, which leads to a general and interpretive formula with high prediction accuracy of the determination coefficient
=0.971. The role of each chemical element in the high-temperature oxidation behavior is analytically illustrated in the three spectrums, thereby the discovered interpretative formula provides a guidance to the inverse design of HEAs against high-temperature oxidation. The present work demonstrates the significance of domain knowledge in the development of materials informatics.
Entropy stabilization is an effective method to design and explore MAX phases with outstanding properties via tuning constituent elements and crystal structures, which have received considerable critical attention. Currently, some medium‐/high‐entropy (ME/HE) MAX phases, whose A layers are composed of Al, S, and magnetic elements, are reported, while few discussions about ME/HE‐MAX phases with other A elements (e.g., Sn) are conducted. Herein, fully dense ME/HE‐MAX phase bulks ((TiVNb)2SnC, (TiVNbZr)2SnC, and (TiVNbZrHf)2SnC) are designed and synthesized because of the chemical diversity of MAX phases. The results of Rietveld refinement of X‐ray diffraction, scanning electron microscopy, and high‐resolution scanning transmission electron microscopy‐affiliated energy‐dispersive spectrometer analysis comprehensively verify the crystal structure of ME/HE‐MAX phases. Both the electrical conductivity and the charge carrier mobility are significantly lower than the reported correlation ternary MAX phases, due to the electron scattering and structural defects in ME/HE‐MAX phase crystal structures. Similarly, the electron contribution of thermal conductivity is gradually declining; on the contrary, the phonon plays an increasingly dominant role as temperature increases. Owing to the richness in composition of MAX phases, herein, a composition design route for discovering new MAX phases and tuning their properties is indicated. Medium/high‐entropy (ME/HE)‐MAX phases are successfully prepared with multiple principal elements at M site. Due to the increase in entropy of mixing, distortions and defects are created largely. Compared with ternary MAX phase, the contribution of electron transport to thermal conductivity of ME/HE‐MAX phases decreases with increasing temperature. Instead, phonon gradually dominates the thermal conductivity as temperature increases.
Chemically complex MAX phase-based ceramics in the (Ti,Zr,Hf,V,Nb)-(Al,Sn)-C system were synthesised by reactive hot pressing for 30 min at 1250–1450°C under a load of 30 MPa for the first time in this work. The dense bulk ceramics contained chemically complex double solid solution MAX phases, each comprising five M-elements and two A-elements. The predominant 211 (Ti0.23,Zr0.18,Hf0.20,V0.11,Nb0.28)2(Al0.42,Sn0.58)C MAX phase characterised all ceramics, while the 312 (Ti0.23,Zr0.31,Hf0.31,V0.08,Nb0.08)3(Al0.36,Sn0.64)C2 and 211 (Ti0.26,Zr0.07,Hf0.07,V0.47,Nb0.13)2(Al0.66,Sn0.34)C MAX phases were only present in the ceramics sintered at 1350–1450°C. A limited amount (4–5 vol%) of parasitic phases (mainly, binary intermetallics) was found in the pseudo-binary carbide-free ceramics sintered at 1350–1450°C.
In recent years, the microstructure and physicochemical properties of high‐entropy ceramics have received much interest by the combination of multiple principal elements. Herein, (Ti0.2V0.2Cr0.2Nb0.2Ta0.2)2AlC–(Ti0.2V0.2Cr0.2Nb0.2Ta0.2)C high‐entropy ceramics (M2AlC‐MC HECs) were prepared by the spark plasma sintering (SPS) technique, attributing to the structural and chemical diversity of MAX phases. The microstructure of M2AlC‐MC HECs was characterized from micron to atomic scales, and the phase composition of M2AlC‐MC HECs was analyzed by a combination of Maud and Rietveld analysis. The results indicate the successful solid solution of Ti, V, Cr, Nb, and Ta atoms in the M‐site of the 211‐MAX configuration, and all the samples show a classic layered structure. The weight percentage of (Ti0.2V0.2Cr0.2Nb0.2Ta0.2)2AlC in the M2AlC‐MC HECs was more than 90%. Furthermore, the thermoelectric properties of M2AlC‐MC HECs were investigated for the first time in this study, and the electrical conductivity and thermal conductivity of HECs are 3278 S cm⁻¹ and 2.78 W m⁻¹ K⁻¹at 298 K, respectively.
High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Although in the infant stage, the emerging of this new family of materials has brought new opportunities for material design and property tailoring. Distinct from metals, the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering. Aside from strengthening, hardening, and low thermal conductivity that have already been found in high-entropy alloys, new properties like colossal dielectric constant, super ionic conductivity, severe anisotropic thermal expansion coefficient, strong electromagnetic wave absorption, etc., have been discovered in HECs. As a response to the rapid development in this nascent field, this article gives a comprehensive review on the structure features, theoretical methods for stability and property prediction, processing routes, novel properties, and prospective applications of HECs. The challenges on processing, characterization, and property predictions are also emphasized. Finally, future directions for new material exploration, novel processing, fundamental understanding, in-depth characterization, and database assessments are given.
The catalytic performance with high conversion and high selectivity of Ti-based oxide catalysts have been widely investigated. Besides, stability, which is an essential parameter in the industrial process, lacked fundamental understanding. In this work, we combined computational and experimental techniques to provide insight into the deactivation of P25 and TS-1 Ti-based oxide catalysts during the methyl oleate (MO) epoxidation. The considered deactivation mechanisms are fouling and surface oxygen vacancy (OV). The fouling causes temporary catalyst deactivation through active site blockage but can be removed via calcination in air at high temperature. However, in this work, the OV formation plays an important role in the overall performance of the spent catalyst as the deactivated catalyst after regeneration, cannot be restored to the initial activity. Also, the effects of OV in spent catalysts caused (i) the formation of more Ti3+ species on the surface as evident by XPS and Bader charge analysis, (ii) the activity modification of the active region on the catalyst surface as the reduction in energy gap (Eg) occurred from the formation of the interstates observed in the density of states profiles of spent catalyst modeled by the O-vacant P25 and TS-1 models. This reduction in Eg affects directly the strength of Ti–OOH active site and MO bonding, in which high binding energy contributes to a low conversion because the MO needed an O atom from Ti–OOH site to form the methyl-9,10-epoxy stearate. Hence, the deactivation of the Ti-based oxide catalysts is caused not only by the insoluble by-products blocking the active region but also mainly from the OV. Note that the design of reactive and stable Ti-based oxide catalysts for MO epoxidation needed strategies to prevent OV formation that permanently deactivated the active region. Thus, the interrelation and magnitude between fouling and OV formation on catalyst deactivation will be investigated in future works.
In this work we systematically explore a class of atomically laminated materials,
Mn+1AXn (MAX) phases upon alloying between two transition metals, M´ and M´´,
from Group III to VI (Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W). The materials
investigated focus on so called o-MAX phases with out-of-plane chemical ordering of
M´ and M´´, and their disordered counterparts, for A = Al and X = C. Through use of
predictive phase stability calculations, we confirm all experimentally known phases to
date, and also suggest a range of stable ordered and disordered hypothetical elemental
combinations. Ordered o-MAX is favoured when (i) M´ next to the Al-layer do not
form a corresponding binary rock-salt MC structure, (ii) the size difference between
M´ and M´´ is small, and (iii) the difference in electronegativity between M´ and Al is
large. Preference for chemical disorder is favoured when the size and electronegativity
of M´ and M´´ is similar, in combination with a minor difference in electronegativity
of M´ and Al. We also propose guidelines to use in the search for novel o-MAX; to
combine M´ from Group 6 (Cr, Mo, W) with M´´ from group 3 to 5 (Sc only for 312,
Ti, Zr, Hf, V, Nb, Ta). Correspondingly, we suggest formation of disordered MAX
phases by combing M´ and M´´ within group 3 to 5 (Sc, Ti, Zr, Hf, V, Nb, Ta). The
addition of novel elemental combinations in MAX phases, and in turn in their potential
two-dimensional MXene derivatives, allow for property tuning of functional materials.
Textured Ti2AlC lamellar composites have been successfully fabricated by a new method in the present work. The composites exhibit high compressive strength of ca 2 GPa, fracture toughness of 8.5 MPa m1/2 (//c-axis), flexural strength of 735 MPa (//c-axis) and high hardness of 7.9 GPa (//c-axis). The strengthening mechanisms were discussed. The sintering and densification process was investigated and crystal orientation and microstructure were studied by electron backscattered diffraction techniques. The synthesis temperature is reduced to 1200 °C by using high surface-to-volume ratio Ti2AlC nano flakes. The Lotgering orientation factor of Ti2AlC and Ti3AlC2 {00l} planes in the textured top surface reaches 0.74 and 0.49, respectively. This new route may shed light on resolving the difficulties encountered in large scale fabrication of textured MAX phases.
High-entropy materials have attracted considerable interest due to the combination of useful properties and promising applications. Predicting their formation remains the major hindrance to the discovery of new systems. Here we propose a descriptor—entropy forming ability—for addressing synthesizability from first principles. The formalism, based on the energy distribution spectrum of randomized calculations, captures the accessibility of equally-sampled states near the ground state and quantifies configurational disorder capable of stabilizing high-entropy homogeneous phases. The methodology is applied to disordered refractory 5-metal carbides—promising candidates for high-hardness applications. The descriptor correctly predicts the ease with which compositions can be experimentally synthesized as rock-salt high-entropy homogeneous phases, validating the ansatz, and in some cases, going beyond intuition. Several of these materials exhibit hardness up to 50% higher than rule of mixtures estimations. The entropy descriptor method has the potential to accelerate the search for high-entropy systems by rationally combining first principles with experimental synthesis and characterization.
We report quantitative characterization of the high temperature oxidation process by using electron tomography and energy-dispersive X-ray spectroscopy. As a proof of principle, we performed 3D imaging of the oxidation layer of a model system (Mo3Si) at nanoscale resolution with elemental specificity and probed the oxidation kinetics as a function of the oxidation time and the elevated temperature. Our tomographic reconstructions provide detailed 3D structural information of the surface oxidation layer of the Mo3Si system, revealing the evolution of oxidation behavior of Mo3Si from early stage to mature stage. Based on the relative rate of oxidation of Mo3Si, the volatilization rate of MoO3 and reactive molecular dynamics simulations, we propose a model to explain the mechanism of the formation of the porous silica structure during the oxidation process of Mo3Si. We expect that this 3D quantitative characterization method can be applied to other material systems to probe their structure-property relationships in different environments.
Al2(MoO4)3 could be synthesized using two different methods, the traditional solid-state reaction and the hydrothermal reaction with post-heating. For preparation by the solid-state reaction, the starting materials were Al(OH)3 and MoO3. TG-DTA results suggested that the reaction temperature of the solid-state reaction should be more than 973K for crystallization of Al2(MoO4)3. In contrast, the hydrothermal technique required crystallization at lower temperatures. SEM photographs indicate that while the former yields larger particles, the latter yields finer and well-shaped particles. It is hoped that Al2(MoO4)3 obtained by this method will find applications in the manufacture of ceramic devices.
The crystal structure of the newly synthesized quaternary MAX phase (Cr2/3Ti1/3)3AlC2 was systematically characterized by various techniques. The space group of (Cr2/3Ti1/3)3AlC2 is determined to be P63/mmc by a combination of selected-area and convergent-beam electron diffraction techniques. Rietveld refinements of the neutron diffraction and X-ray diffraction data show that in (Cr2/3Ti1/3)3AlC2, Ti and Cr are ordered with Ti in the 2a and Cr in the 4f Wyckoff sites of a M3AX2 lattice. It is interesting to find that when the order of the magnetic moment of Cr atoms is considered, the ferromagnetic configuration of (Cr2/3Ti1/3)3AlC2 becomes the ground state. Meanwhile, the Raman-active mode wavenumbers of (Cr2/3Ti1/3)3AlC2 were calculated, and the theoretical data are quite consistent with the experimental data, further proving the ordered crystal structure of this phase. The formation of (Cr2/3Ti1/3)3AlC2 with a unique crystal structure may be related to the distinctly different electronegativities and covalent radii of Cr and Ti atoms.
Of all the Mn+1AXn phases, the most resistant to oxidation in air in the 900–1,400°C temperature range are Ti2AlC, Ti3AlC2 and Cr2AlC. A literature review, however, shows that while many claim the oxidation kinetics to be parabolic, others claim them to be cubic. Whether the kinetics are parabolic or better is of vital practical importance. By carefully re-plotting the results of others and carrying out one oxidation run for ≈3,000 h at 1,200°C on a Ti2AlC sample, we conclude that the oxidation kinetics are better described by cubic kinetics and that even that conclusion is an approximation. Lastly, we present compelling evidence that the rate-limiting step during the oxidation of Ti2AlC is diffusion down the alumina scale grain boundaries.
Ti2AlC and Ti3AlC2 are the most light-weight and oxidation resistant layered ternary carbides belonging to the MAX phases. This review highlights recent achievements on the processing, microstructure, physical, mechanical and chemical properties of these two machinable and electrically conductive carbides. Ti2AlC and Ti3AlC2 display superior properties such as fracture toughness, electrical and thermal conductivities, and oxidation resistance over their binary counterpart. This paper provides a comprehensive overview of the processing-microstructure-property correlations of these two carbides. Potential fields of applications for Ti2AlC and Ti3AlC2 are surveyed. In addition, we point out methods for further improving their properties in some specific applications through appropriate structural design and modification.
Insights into high temperature oxidation of Al2O3-forming Ti3AlC2 are made to understand the sub-parabolic oxidation rate and the absence of Al-depletion zone beneath the protective Al2O3 scale. The scale results from selective oxidation of Al that migrated from Ti3AlC2 according to Ti3AlC2 + 3/2y O2 → Ti3Al1−yC2 + 1/2y Al2O3, leaving Al vacancy in the substrate. Inward diffusion of oxygen via grain boundaries of the Al2O3 scale predominates for the scale growth. The Al2O3 grains grow with time, yielding sub-parabolic oxidation kinetics. The Al diffuses in a fast manner, accounting for the absence of Al-depletion zone.
The newly developed GSAS-II software is intended as a general purpose package for data reduction, structure solution and structure refinement that can be used with both single-crystal and powder diffraction data from both neutron and x-ray sources, including laboratory and synchrotron sources collected on both 2-D and 1-D detectors, to eventually replace both the GSAS and EXPGUI packages, as well as many other utilities. GSAS-II is open-source and is written largely in in object-oriented Python, but offers speeds comparable to compiled codes due to reliance on the Python NumPy and SciPy packages for computation. It runs on all common computer platforms. Graphics, both for a user interface and for interpretation of parameters, is highly integrated. The package can be applied to all stages of crystallographic analysis for constant-wavelength x-ray and neutron data. Plans for considerable additional development are discussed.
Fe-based self-lubricating composites, in which SiC particles act as precursors to in situ generated 2D turbostratic graphite as a lubricating phase, were produced via powder injection moulding (PIM). In order to study the precursor influence on the solid lubricant structure and properties, hexagonal (α) and cubic (β) SiC polytypes were used. The composites were tribologically tested in dry and low-viscosity oil-lubricated conditions. Distinct precursors did not lead to significant differences between composites. However, due to the highly disordered graphite-rich tribolayer formation, they do provide lubricity, low friction coefficient, and high wear resistance, even in oil-lubricated conditions.
A novel 312-type MAX-phase solid solution series in the Zr-Ti-Si-C system has been synthesized by the vacuum carbosilicothermic reduction method using mixtures of TiO2, ZrO2, SiC, and Si powders as starting materials. The upper limit for Zr content in metal sublattice of the synthesized (Zr,Ti)3SiC2 MAX phase solid solutions was found to be as much as approximately 66 at%, closely corresponding to a hypothetical quaternary Zr2TiSiC2 MAX phase. A wide miscibility gap inside the interval of Zr content in metal sublattice ranging between 22 at% and 55 at% was found. Crystal structure of the synthesized MAX-phase solid solutions was studied by HR-STEM/HAADF and XRD Rietveld analyses. The lattice constants were determined to be linearly correlated with Zr content as predicted by Vegard's law. A significant inhomogeneity in distribution of metal atoms similar to that of out-of-plane ordered quaternary MAX phases has been established for both Ti-rich and Zr-rich MAX-phase solid solutions.
In this work, tribological properties of Ti3AlC2, Cr2TiAlC2, and Mo2TiAlC2 MAX are evaluated in a wide temperature range of 25-800 ℃. By replacing the 4 f Wyckoff site Ti atoms of Ti3AlC2 with Cr or Mo, the MAXs deliver lower coefficient of friction (COF) than Ti3AlC2. Interestingly, as confirmed by the morphological, structural, and chemical analyses, the two MAXs are applicable in different temperatures. For Cr2TiAlC2, the lowest wear rate is 2.30 × 10⁻⁷ mm³ N⁻¹ m⁻¹ at 800 ℃, while the lowest wear rate of Mo2TiAlC2 is achieved at room temperature of 3.83 × 10⁻⁷ mm³ N⁻¹ m⁻¹. These distinct tribological behaviors are ascribed to the altered M-A and M-C bindings, as well as the properties of the produced oxides.
In this work, the Cr-Ti-Mo ternary o-MAX ceramics based on the Cr2TiAlC2 phase are synthesized, and their tribological performance at elevated temperatures up to 800 ºC in the air is evaluated. With Si3N4 as a tribocouple, an unexpected ultra-low wear rate reaching 6.1 × 10⁻⁸ mm³ N⁻¹ m⁻¹ is observed in Cr1.9Ti0.9Mo0.2AlC2 at 800 ºC, accompanied by a stable coefficient of friction (COF) around 0.33 in a wide temperature range. X-ray photoelectron spectroscopy (XPS) depth profiling confirms a tribofilm with gradient composition, which simultaneously offers fluid lubricating at elevated temperatures and self-healing after cooling. Particularly, confirmed by the theoretical simulations, the doping of Mo improves the interlayer binding as well as alters the oxidation behaviors of Cr2TiAlC2. With an optimal interlayer binding strength and oxidation rate, the Cr1.9Ti0.9Mo0.2AlC2 can generate a tribofilm possessing ideal composition, which simultaneously promotes lubrication and anti-wear performance at elevated temperatures.
Herein, a novel kind of high-entropy MAX phases, (Mo0.25Cr0.25Ti0.25V0.25)3AlC2 powders were successfully synthesized by a newly proposed two-step solid state reaction process. The oxidation experiments demonstrate that the oxidation products of Al2Mo3O12 and rutile TiO2 are formed at about 600 and 800 ℃, respectively. Besides, the dielectric and electromagnetic (EM) wave absorption properties of (Mo0.25Cr0.25Ti0.25V0.25)3AlC2 powders and those after oxidation at different temperatures were also examined. The results show that the as-synthesized (Mo0.25Cr0.25Ti0.25V0.25)3AlC2 powders possess excellent EM wave absorption performances with the minimum reflection loss (RL) of -45.80 dB (at 1.7 mm thickness) and the maximum effective absorption bandwidth (EAB) of 3.6 GHz (at 1.5 mm thickness). After oxidation at 400-800 ℃, due to the coupling of conductivity loss and polarization loss, (Mo0.25Cr0.25Ti0.25V0.25)3AlC2 powders can retain good EM wave absorption properties in a certain frequency range. In this paper, the effects of oxidation on EM wave absorption properties of high-entropy MAX phases were systematically investigated for the first time. This work manifests that high-entropy MAX phases are promising EM wave absorbing candidates and can maintain good EM wave absorption performances after oxidation.
Medium- or high-entropy materials have great potential for applications due to their diverse compositions and unexpected physicochemical properties. Herein, a novel medium-entropy (TiVNb)2AlC was synthesized via hot pressing at 1400 °C from three individual M2AlC (M=Ti, V, Nb) MAX phases. The microstructure of (TiVNb)2AlC was characterized from the microscale to the atomic scale by scanning electron microscope microscopy (SEM), scanning transmission electron microscopy (STEM), and energy dispersive spectroscopy (EDS). The results showed that Ti, V, and Nb atoms were fully solid-soluble in the M-sites of the M2AlC MAX phase. Compared with three individual MAX phases, the thermal conductivity of (TiVNb)2AlC was reduced greatly in the temperature range of 293–1473 K, and its mechanical properties (including Young's modulus, Vickers hardness, and bending strength) were all increased due to the solid solution strengthening and electronic mechanism.
A cost-effective HEA based self-lubricating composite was fabricated using CrFeNiAl0.3Ti0.3 as matrix and Ag as solid lubricant by hot pressing sintering (HPS), with Ag particles uniformly dispersed along grain boundaries of HEA matrix. Tribological tests revealed a remarkable impact of Ag tribo-film in reducing coefficient of friction and wear of the composites, from RT to 600 °C. 750 °C was a turning point for the change in wear mechanism from abrasive to oxidative wear. The formation of a subsurface tribo-layer and a protective oxide layer on the worn surface of CrFeNiAl0.3Ti0.3 HEA led to low wear. At 900 °C, associated with the absence of subsurface tribo-layer, Ag is not found in the oxide layer, causing an increase in the wear of composite.
Composition modification was introduced to improve the oxidation resistance by varying Al and excluding Co content from the Al-Co-Cr-Fe-Ni system. Since adjusting the composition shifted the valence electron concentration (VEC) of the alloys, the dual-phase structure of the alloys is expected to be more stable. At low temperatures (T<1273 K), the alloys formed mixed oxide products. As oxidation temperature increased, only Cr2O3 or Al2O3 dominated the alloy's surface. Compared to equiatomic AlCoCrFeNi (5-Equi), non-equiatomic AlCoCrFeNi (5-B 40) and four-component AlCrFeNi (4-B 2013) had better oxidation resistance due to monocrystalline-Al2O3 formation. Besides the role of oxide formation, maintaining BCC and B2 phases within the alloys is also beneficial to supporting the stable Cr2O3 or Al2O3.
VAlTiCrMo-based high-entropy alloy coatings with BCC structure were successfully prepared by magnetron sputtering, and the tribological properties were improved by changing Mo content. It is revealed that friction force changed the composition of lubricating phase in the oxide scale of VAlTiCrMo1.6 coating, that is, a complex oxide Al2(MoO4)3 as dominant lubricating phase was formed on the surface of oxide scale through tribochemical reaction between simple oxides. And the phase separation in the layered oxide scale caused by thermal diffusion, which is favorable to form continuous lubricating film during friction, was the prerequisite for VAlTiCrMo1.6 coating to acquire low friction and wear. In this process, the addition of Mo played a key role in uphill diffusion and segregation of elements.
Medium-entropy alloys (MEAs) and high-entropy alloys (HEAs) are good candidates for high-temperature applications, taking advantage of their exceptional thermal stability. However, most of them contain high-priced Co elements, which limited further promotion and application. Therefore, it is imperative to elaborate HEAs/MEAs with superb oxidation resistance and relatively low cost. An economically Fe-Cr-Ni MEA, which exhibits an outstanding high-temperature oxidation resistance, was developed by Ce-adulterated. The Ce-containing Fe-Cr-Ni MEA possessed a lower oxidation rate and a higher oxidative activation energy (461.4 kJ·mol⁻¹, 14 % increment compared with the reference MEA). The splendid high-temperature oxidation resistance primarily benefited from the protective oxide scales formed on the surface of MEA, which were denser, slow-growing, and well adherently. This research sheds light on exploiting low-cost MEAs with excellent resistance to oxidation for high-temperature applications.
Tungsten containing refractory high entropy alloys (HEAs) are potential candidate for high temperature applications owing to their remarkable mechanical properties. However, the oxidation resistance of W containing HEAs at high temperatures is problematic, and it received less attention. In this work, several equiatomic W containing WTaTiCr, WMoTaTiCr, WTaAlCr, and WMoTaAlCr refectory HEAs were fabricated by arc melting technique and their oxidation behavior at 1000 °C for 10 h was compared with MoTaTiCr medium entropy alloy (MEA). The as cast microstructure of the MoTaTiCr, WTaTiCr, WMoTaTiCr, and WMoTaAlCr alloys exhibited a single BCC solid solution, while the as cast WTaAlCr MEA showed some extent of intermetallic compounds along with the BCC phase. The best oxidation resistance was obtained for MoTaTiCr MEA, which was protected by a continuous CrTaO4 scale layer. The formation of tungsten oxides (mainly WO3) prohibited the evolution of a protective oxide layer of AlTaO4, CrTaO4 or Al2O3 on the surface of W containing alloys. WMoTaAlCr HEA exhibited the best oxidation resistance among all the W containing alloys, which was protected by a nearly continuous rutile type (Al,Cr)TaO4 oxide layer.
Nanolaminated V2AlC MAX phase has been synthesized through pressureless sintering by varying aluminum content. The oxidation stability of the V2AlC is studied under non-isothermal conditions through a TGA/DTA technique at multiple heating rates in the air atmosphere. It is observed that the oxidation of V2AlC occurred in two different stages. Thermodynamic calculations are also performed to predict the oxidation reaction pathway of V2AlC MAX phase. The kinetic triplets (activation energy, pre-exponential factor and reaction mechanism) are determined for both stages of oxidation. The Kissinger-Akahira-Sunose (KAS) and the Flynn-Wall-Ozawa (FWO) iso-conversional kinetic methods are used to calculate the activation energy. The reaction mechanism is identified by employing the integral master plot method. The results indicated that the nucleation mechanism dominated the oxidation process in the V2AlC MAX phase. P4 and A4 nucleation reaction mechanisms are identified in stage I and stage II of oxidation, respectively.
Nearly monolithic MAX phase containing 95 wt.% Ti3AlC2 and 5 wt.% TiCx was synthesized by spark plasma sintering under vacuum sintering conditions. Corrosion behaviour of Ti3AlC2 was investigated in molten LiCl–KCl salt at 600 °C under a dry Ar atmosphere. Evolution of microstructure and surface chemistry of the exposed sample was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and glancing angle X-ray diffraction (GAXRD). Results showed that Al dissolution led to delamination of the layered structure which favoured the ingress of chlorine and its subsequent intercalation into the Al–site plane to form a Ti3C2Cl2 exfoliation layer. De-twinning of the Ti3C2 layers possibly due to Cl- anions substitution by O results in non-stoichiometric TiC0.67 formation.
Mo-Si-B alloy is a promising candidate for high temperature environment, while its high temperature dry sliding wear behavior is not yet well investigated. In the present work, the Mo-12Si-8.5B alloy and its composite reinforced with 10 wt% MoAlB ceramic were prepared by spark plasma sintering. Effects of MoAlB ceramic addition on the microstructure, mechanical properties and dry wear properties at RT–1000 °C were investigated. The result indicates that the composite shows an enhanced strength and hardness, and a remarkably improved wear resistance. The excellent wear resistance is mainly attributed to the improved hardness and thermal softening resistance, as well as the formation of Al2(MoO4)3 layer at 800–1000 °C that results from the dispersive distribution of MoAlB particles.
Disordered multicomponent systems, occupying the mostly uncharted centres of phase diagrams, were proposed in 2004 as innovative materials with promising applications. The idea was to maximize the configurational entropy to stabilize (near) equimolar mixtures and achieve more robust systems, which became known as high-entropy materials. Initial research focused mainly on metal alloys and nitride films. In 2015, entropy stabilization was demonstrated in a mixture of oxides. Other high-entropy disordered ceramics rapidly followed, stimulating the addition of more components to obtain materials expressing a blend of properties, often highly enhanced. The systems were soon proven to be useful in wide-ranging technologies, including thermal barrier coatings, thermoelectrics, catalysts, batteries and wear-resistant and corrosion-resistant coatings. In this Review, we discuss the current state of the disordered ceramics field by examining the applications and the high-entropy features fuelling them, covering both theoretical predictions and experimental results. The influence of entropy is unavoidable and can no longer be ignored. In the space of ceramics, it leads to new materials that, both as bulk and thin films, will play important roles in technology in the decades to come.
Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multidimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behavior has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking.
The Mn+1AXn, or MAX, phases are nanolayered, hexagonal, machinable, early transition-metal carbides and nitrides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element (mostly groups 13 and 14), and X is C and/or N. These phases are characterized by a unique combination of both metallic and ceramic properties. The fact that these phases are precursors for
MXenes and the dramatic increase in interest in the latter for a large host of applications render the former even more valuable. Herein we describe the structure of most, if not all, MAX phases known to date. This review covers ~155 MAX compositions. Currently, 16 A elements and 14 M elements have been incorporated in these phases. The recent discovery of both quaternary in-and out-of-plane ordered MAX phases opens the door to the discovery of many more. The chemical diversity of the MAX phases holds the key to eventually optimizing properties for prospective applications. Since many of the newer
quaternary (and higher) phases have yet to be characterized, much work remains to be done.
A series of high-purity Ti3(Al1−xSix)C2 solid solutions with 0<x<1 were reaction sintered from Ti, Si, Al and TiC powders using Pulsed Electric Current Sintering (PECS).¹ The a-lattice parameter of all sintered solid solutions remains constant at approximately 0.307 nm, while the c-lattice parameter decreases from 1.858 nm to 1.763 nm with increasing amount of Si. The specific heat capacity of Ti3Al0.6Si0.4C2 was found to be comparable to that of end-member MAX phases, namely Ti3AlC2 and Ti3SiC2, while its coefficient of thermal expansion (CTE) was lower than that of the end members. Both Young's and shear moduli increase with increasing amount of Si. Vickers hardness (Hv) measurements demonstrate significant hardening effect in Ti3(Al1−xSix)C2 solid solutions regardless of the grain size, i.e. it changes from 4.1±0.14 GPa of Ti3AlC2 and 4.2±0.37 GPa of Ti3SiC2, up to 5.6±0.2 GPa for Ti3(Al0.4Si0.6)C2. At room temperature, the strengthening effect was found to be marginal in the fine grained structure (grain size approximately 7 x 3 μm), as the compressive strength of Ti3Al0.6Si0.4C2 and Ti3Al0.4Si0.6C2 is higher for only 7.6% when compared to that of the end members. However, significant strengthening effect was observed in coarse grained structures (grain size approximately 25 x 8 μm) as the room temperature compressive strength of solid solutions exceed those of two end-members for more than 30%. Nevertheless, above brittle-to-plastic transition temperature, the solid solution strengthening effect diminishes and the strength of Ti3SiC2 is significantly higher than that of Ti3AlC2 and solid solutions. Finally, it was found that Ti3Al0.6Si0.4C2 forms protective alumina oxide layer at 1200 °C, rather than silica that is commonly found on oxidized surfaces of Ti3SiC2.
Herein we report on the synthesis of a new layered ternary carbide, Mo2TiAlC2, that was synthesized by heating an elemental mixture at 1600 °C for 4 h under an Ar flow. Its hexagonal, a and c lattice parameters were calculated via Rietveld analysis of powder X-ray diffraction patterns to be, respectively, 2.997 Å and 18.661 Å. High-resolution scanning transmission electron microscopy showed that this phase is ordered, with Ti layers sandwiched between two Mo layers in a M3AX2 type ternary carbide structure.
Cr2AlC compounds were synthesized via a powder metallurgical route and their long-term oxidation behavior studied. Oxidation at temperatures between 700 and 1,000 °C for up to 360 days in air resulted in formation of a thin, adherent Al2O3 surface layer and a narrow Cr7C3 sublayer, accompanied by evaporation of carbon from the Cr2AlC. Preferential oxidation of Al on the surface suppressed oxidation of the less mobile Cr in Cr2AlC. In the Al2O3 layer, (0.7–8.3) at.% Cr was incorporated. In the Cr7C3 sublayer, Al was either absent or incorporated. When Cr2AlC oxidized at 850 and 1,000 °C for 30–360 days, metastable θ-Al2O3 blades formed on the α-Al2O3 layer. However, such blades were scarcely visible when the oxidation was carried out above 1,100 °C, because of the fast θ → α-transition. Moreover, the θ-Al2O3 were not noticeable during oxidation at 700 °C for 30–360 days, due to a small extent of oxidation.
An aluminum doped titanium silicon carbide with a composition of Ti3Si0.9Al0.1C2 was studied by neutron powder diffraction in comparison with the standard Ti3SiC2 and Ti3AlC2 compounds. The refinements of the diffraction data revealed the included impurity phases and the detailed structural information of these compounds. The analysis confirmed that the impurity phase TiC in Ti3SiC2 could be eliminated by the substitution of Si with a small amount of Al dopant, forming a solid solution of Ti3(Si1−xAlx)C2, with a unit cell slightly larger than that of Ti3SiC2, but smaller than that of Ti3AlC2. However, compared with the anisotropic structure change in Ti3AlC2, the structure change in Ti3Si0.9Al0.1C2 is basically isotropic, which is consistent with the structural optimization of the first principles calculation.
By now, it is fairly well established that the layered hexagonal MAX phases are thermodynamically stable nanolaminates displaying unusual and sometimes unique properties. These phases are so-called because they possess a Mn+1AXn chemistry, where n is 1, 2, or 3, M is an early transition metal element, A is an A-group element and X is C or N. They are highly damage tolerant, thermal shock resistant, readily machinable, and with Vickers hardness values of 2–8GPa, are anomalously soft for transition metal carbides and nitrides. Some of them display a ductile–brittle transition at temperatures>1000°C, while retaining decent mechanical properties at these elevated temperatures. Moreover, their layered nature suggests they may have excellent promise as solid lubricant materials. Recently, first generation MAX Phase based composites shafts were successfully tested against Ni-based superalloy at 50,000rpm from RT till 550°C during thermal cycling in a foil bearing rig. This study further demonstrates the potential of MAX Phases and their composites in different tribological applications. The main objective of this review is to present recent progress, and consequently develop a comprehensive understanding about the tribological behavior of MAX Phases and their composites. We are also proposing a way of classifying the different tribofilms to understand the complex tribological behavior of these solids over a wide range of different experimental conditions.
Studies on the mechanism of reaction leading to AlVMoO7 have been made using X-ray diffraction and infrared spectroscopy. The XRD study of the process was carried out in a high-temperature X-ray attachment. The AlVMoO7 phase can be synthesized by two methods. The investigations lead to the conclusion that both reactions run through a stage in which Al2(MoO4)3 is involved. The IR spectra of the AlVMoO7 and of samples after selected stages of the synthesis reaction are given. The diffraction pattern of AlVMoO7 was indexed. The phase crystallizes in an orthorhombic system with the unit cell parameters: a = 0.53826(11) nm, b = 0.81771(11) nm, c = 1.27493(20) nm.
In this paper we report on the synthesis and oxidation behavior of the layered ternary carbides V 2 AlC V2AlC and ( Ti 0.5 , V 0.5 ) 2 AlC . (Ti0.5,V0.5)2AlC. The oxidation kinetics were studied thermogravimetically in air in the 500 to 700°C temperature range. The oxidation of V 2 AlC V2AlC is governed by the inward migration of O, and possibly the outward migration of V; the Al atoms are essentially immobile. At 500 and 600°C, the oxide layers formed were layered and protective up to at least 24 h. The outermost layers were VO 2 VO2 at 500°C, and V 2 O 5 V2O5 at 600°C; the inner layer compositions were myriad solid solutions of V, Al, and O in varying ratios. The average oxidation state of the V ions decreased from +4 or +5 at the air/oxide interface to +3 at the substrate/oxide interface. The oxides that form on ( Ti 0.5 , V 0.5 ) 2 AlC (Ti0.5,V0.5)2AlC are amorphous or nanocrystalline, and are not protective, which implies the maximum-use temperature in air of the solid solution will have to be <500°C. The oxidation occurred exclusively by the inward migration of O; the cations were oxidized in situ. For both ternaries, some of the oxides formed at 700°C were molten which resulted in poor oxidation resistance. © 2004 The Electrochemical Society. All rights reserved.
In this, Part I of a two-part study, a generalized model for the oxidation of the ternary compounds, Ti n+1 AlX n . where n = 1-3 and X is carbon and/or nitrogen, is proposed. In all cases, the oxidation products in the 800-1100°C temperature range are rutile TiO 2 , in which some Al is dissolved, i.e., (Ti 1 -y Al y )O 2-y/2 , where y < 0.05 and Al 2 O 3 . The oxidation occurs by the inward diffusion of oxygen and the outward diffusion of Al 3+ and Ti 4+ ions through the (Ti 1-y Al y )O 2-y/2 layer. The C and N atoms are presumed to diffuse through the reaction layers and oxidize. The basic premises of the model are that the subjection of the (Ti 1 yAly)O2 y/2 layer to an oxygen chemical potential gradient results in its demixing, with the Al 3+ dissolving into the rutile at the low oxygen partial pressure and its precipitation as Al 2 O 3 at the high partial pressure side. If extensive, the demixing results in the formation of layers of porosity, through which the Al 3+ ions cannot diffuse but the O 2- ions can. The resulting microstructures can be highly striated where three layers; an Al 2 O 3 -rich layer, an (Ti 1 -y Al y )O 2-y/2 -rich layer, and a porous layer repeat numerous times. Comparison with previously published results on the oxidation of Ti 3 SiC 2 leaves little doubt that dissolution of the Al in the reaction layer enhances the oxidation kinetics. This is most probably accomplished by an increase in the oxygen vacancy concentration. The fact that the oxide scales are not fully dense is also believed to play an important role in enhancing the oxidation kinetics.
Weight change and oxygen consumption measurements were used to study the oxidation of molybdenum from 550° to 1704°C for pressures of 5 to 76 Torr. For temperatures of 550°–700°C two processes occurred simultaneously, oxide scale formation and molybdenum trioxide volatility. Above 800°C at pressures up to 76 Torr molybdenum trioxide volatilized as fast as it formed. At 900°C and 76 Torr using 1.2 cm² samples the primary chemical reaction gave a rate of about 10¹⁸ at. molybdenum/cm²/sec. Above this temperature for 1.2 cm² specimens the reaction was limited by gaseous diffusion of oxygen. Little change was found in the rate of oxidation to 1615°C. Pressure had only a small effect on the rate of reaction for these reaction conditions. However, in the chemically controlled region pressure had an important effect on the rate of oxidation. To extend the temperature region where the primary chemical reaction was rate controlling, samples of small area were used. A sample having a total area of 0.12 cm² gave a reaction rate of . For these very fast reactions, appreciable temperature rises occurred, and the actual sample temperature had to be estimated. A log K vs. 1/T plot of the primary chemical reaction data gave an energy of activation of 19.7 kcal/mole. Reaction conditions where gaseous diffusion processes are rate controlling were determined. All of the earlier studies were made for these reaction conditions. The activated state theory of surface reactions was applied to the primary chemical reaction in the oxidation of molybdenum. A mechanism of mobile adsorption was found to be the primary chemical reaction. This adsorption process probably occurred on a surface already covered with a layer of adsorbed oxygen atoms since was volatilized.