MAX Phase Compounds Research Articles

Hot corrosion behavior of Cr2AlC MAX phase and CoNiCrAlY compounds at 950 °C in presence of Na2SO4+V2O5 molten salts

In this research, a high-purity Cr2AlC MAX phase sample was prepared via spark plasma sintering (SPS) method with the hot corrosion behavior investigated in the presence of Na2SO4+V2O5 molten salts at 950 °C. Also, the hot corrosion resistance of this MAX phase was compared with a hot corrosion-resistant SPS-processed CoNiCrAlY sample. The results of the hot corrosion test after 30 h revealed that the MAX phase sample has better hot corrosion resistance compared to CoNiCrAlY sample. According to the results, corrosion kinetics of Cr2AlC sample followed near-cubic law with diffusion occurring along the grain boundaries. On the other hand, CoNiCrAlY sample followed parabolic kinetics where the diffusion of reactants occurred through the oxide scale. The results indicated that in the Cr2AlC sample, upon exposure time prolongation, a dense and uniform Cr-rich alumina layer was formed in the surface and Cr7C3 phase was created as a sub-layer, while in the CoNiCrAlY sample the oxide layer contained Al2O3 and porous spinel oxide phases. In the CoNiCrAlY sample, a considerable volume change and stress occurred during the non-uniform growth of spinel oxide causing the formation of defects such as microcracks which deteriorate its hot corrosion resistance.

Theoretical prediction, synthesis, and crystal structure determination of new MAX phase compound V2SnC

Guided by the theoretical prediction, a new MAX phase V2SnC was synthesized experimentally for the first time by reaction of V, Sn, and C mixtures at 1000 °C. The chemical composition and crystal structure of this new compound were identified by the cross-check combination of first-principles calculations, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and high resolution scanning transmission electron microscopy (HR-STEM). The stacking sequence of V2C and Sn layers results in a crystal structure of space group P63/mmc. The a- and c-lattice parameters, which were determined by the Rietveld analysis of powder XRD pattern, are 0.2981(0) nm and 1.3470(6) nm, respectively. The atomic positions are V at 4f (1/3, 2/3, 0.0776(5)), Sn at 2d (2/3, 1/3, 1/4), and C at 2a (0, 0, 0). A new set of XRD data of V2SnC was also obtained. Theoretical calculations suggest that this new compound is stable with negative formation energy and formation enthalpy, satisfied Born-Huang criteria of mechanical stability, and positive phonon branches over the Brillouin zone. It also has low shear deformation resistance c44 (second-order elastic constant, cij) and shear modulus (G), positive Cauchy pressure, and low Pugh’s ratio (G/B = 0.500 < 0.571), which is regarded as a quasi-ductile MAX phase. The mechanism underpinning the quasi-ductility is associated with the presence of a metallic bond.

Study of the structural, mechanical and thermodynamic properties of the new MAX phase compounds (Zr1-xTix)3AlC2

The structural, electronic, mechanical and the thermodynamic properties of the MAX phases (Zr1-xTix)3AlC2 compounds have been investigated by using the full-potential plane-wave FP-LAPW method as implemented in the Wien2k code. The exchange-correlation (XC) energy of electrons was treated using the Perdew–Burke–Ernzerhof parametrization. The ground-state properties for the studied compounds were calculated and compared with available experimental and theoretical data. The calculated lattice parameters are reasonably comparable with experimental and theoretical results. The formation energy has been evaluated in order to determinate the stability of our compounds. The calculation of the electronic structure was predicted for the first time for the present MAX phase compounds. These results indicate that, all our compounds exhibit metallic behavior and this metallicity is due to the p-d hybridization. The elastic constants have also evaluated by the Hex-elastic package. The mechanical stability reveal that, all our compounds are stable mechanically. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and Cauchy pressure were calculated and discussed in detail. Furthermore, the temperature and pressure effect on: Bulk modulus, Debye temperature and heat capacity at constant volume and constant pressure CV and CP, respectively have been investigated by the quasi-harmonic Debye model.

Distinguishing electronic contributions of surface and sub-surface transition metal atoms in Ti-based MXenes

MXenes are a rapidly-expanding family of 2D transition metal carbides and nitrides that have attracted attention due to their excellent performance in applications ranging from energy storage to electromagnetic interference shielding. Numerous other electronic and magnetic properties have been computationally predicted, but not yet realized due to the experimental difficulty in obtaining uniform surface terminations (Tx), necessitating new design approaches for MXenes that are independent of surface terminations. In this study, we distinguished the contributions of surface and sub-surface Ti atoms to the electronic structure of four Ti-containing MXenes (Ti2CTx, Ti3C2Tx, Cr2TiC2Tx, and Mo2TiC2Tx) using soft x-ray absorption spectroscopy. For MXenes with no Ti atoms on the surface transition metal layers, such as Mo2TiC2Tx and Cr2TiC2Tx, our results show minimal changes in the spectral features between the parent MAX phase and its MXene. In contrast, for MXenes with surface Ti atoms, here Ti3C2Tx and Ti2CTx, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds. First principles calculations provide similar trends in the partial density of states derived from surface and sub-surface Ti atoms, corroborating the spectroscopic measurements. These results reveal that electronic states derived from sub-surface M-site layers are largely unperturbed by the surface terminations, indicating a relatively short length scale over which the Tx terminations alter the nominal electron count associated with Ti atoms and suggesting that desired band features should be hosted by sub-surface M-sites that are electronically more robust than their surface M-site counterparts.

Ta-based 413 and 211 MAX phase solid solutions with Hf and Nb

New bulk MAX phase-based ceramics were synthesized in the Ta–Hf–Al–C and Ta–Nb–Al–C systems. Specifically, (Ta1-x,Hfx)4AlC3 and (Ta1-x,Nbx)4AlC3 stoichiometries with x = 0.05, 0.1, 0.15, 0.2, 0.25 were targeted by reactive hot pressing of Ta2H, HfH2, NbH0.89, Al and C powder mixtures at 1550 °C in vacuum. The produced ceramics were characterized in terms of phase composition and microstructure by X-ray diffraction, scanning electron microscopy, electron probe microanalysis and scanning transmission electron microscopy. The investigation confirmed the existence of such M-site solid solutions with low solute concentrations, as predicted by first-principles calculations. These calculations also predicted a linear trend in lattice parameter evolution with increasing Hf concentration, in agreement with the experimental results. In order to increase the low phase purity of the produced ceramics, Sn was added to form (Ta1-x,Hfx)4(Al0.5,Sn0.5)C3 and (Ta1-x,Nbx)4(Al0.5,Sn0.5)C3 double solid solutions, thus resulting in a higher content of the 413 MAX phase compounds in the produced ceramics.

The stability of irradiation-induced defects in Zr3AlC2, Nb4AlC3 and (Zr0.5,Ti0.5)3AlC2 MAX phase-based ceramics

This work is a first assessment of the radiation tolerance of the nanolayered ternary carbides (MAX phases), Zr3AlC2, Nb4AlC3 and (Zr0.5,Ti0.5)3AlC2, using proton irradiation followed by post-irradiation examination based primarily on x-ray diffraction analysis. These specific MAX phase compounds are being evaluated as candidate coating materials for fuel cladding applications in advanced nuclear reactor systems. The aim of using a MAX phase coating is to protect the substrate fuel cladding material from corrosion damage during its exposure to the primary coolant. Proton irradiation was used in this study as a surrogate for neutron irradiation in order to introduce radiation damage into these ceramics at reactor-relevant temperatures. The post-irradiation examination of these materials revealed that the Zr-based 312-MAX phases, Zr3AlC2 and (Zr0.5,Ti0.5)3AlC2 have a superior ability for defect-recovery above 400 °C, whilst the Nb4AlC3 does not demonstrate any appreciable defect recovery below 600 °C. Density functional theory calculations have demonstrated that the structural differences between the 312 and 413-MAX phase structures govern the variation of the irradiation tolerance of these materials.

Structural, elastic, thermal and lattice dynamic properties of new 321 MAX phases

A new series of the MAX family designated as 321 phases are recently reported with Nb3As2C, V3As2C, Nb3P2C and Ta3P2C. Most of physical properties of these new MAX phase compounds are unexplored and the present study aims to investigate their structural, elastic, thermal and lattice dynamic properties. Though all the phases are elastically anisotropic and brittle in nature, Nb3As2C is the most anisotropic and Ta3P2C is the most brittle. V3As2C compared to Nb3As2C and Ta3P2C compared to Nb3P2C should exhibit superior mechanical properties, as they possess larger elastic constants and moduli. Shear strength, bond covalency as well as the average bond strength and materials’ brittleness are predicted to follow: Ta3P2C > Nb3P2C > V3As2C > Nb3As2C. The estimated Debye temperature and lattice thermal conductivity are highest for Nb3As2C. Lattice dynamical features are investigated in detail and the infrared and Raman active modes are identified. The high melting temperatures of these compounds are beneficial for their potential applications at elevated temperatures.

Ti2AlC MAX phase for resistance against CMAS attack to thermal barrier coatings

To protect thermal barrier coatings (TBCs) from calcium-magnesium-alumina-silicate (CMAS) attack, depositing a protective layer has been a promising approach. In this study, a MAX phase compound, Ti2AlC, is found to highly resist to CMAS attack, which could be a candidate material for the protective layer. The interaction between Ti2AlC pellets and molten CMAS at 1250 °C for 0.5 h, 2 h and 8 h was investigated. Exposed to CMAS melt for 0.5 h, a dense Al2O3 film formed on the surface, accompanied by Al and Ti migrating to the melt. The accumulation of Al and Ti in CMAS triggered its crystallization, forming anorthite and melilite phases. The Al2O3 film and the crystallization of the melt significantly suppressed the CMAS penetration. The film and crystals grew with the heat time. By comparison, some pre-oxidized Ti2AlC pellets were also exposed to CMAS attack at 1250 °C. The CMAS on the pre-oxidized pellets only precipitated CaAl2Si2O8 anorthite crystals, which exhibited much lower sizes than those in the non-oxidized samples. Also, the pre-oxidized pellets were highly resistant to CMAS penetration.

Difference in physical properties of MAX-phase compounds Cr2GaC and Cr2GaN induced by an anomalous structure change in Cr2GaN

MAX-phase compounds with layered structure have been investigated extensively due to their excellent metallic and ceramic properties. Among them, Cr2GaC and Cr2GaN possess the same crystal structure and similar electronic structure, whereas Cr2GaN undergoes a spin-density-wave transition around 170 K but Cr2GaC is Pauli paramagnetism without any magnetic transition. To understand what make these happen, we studied the crystal structure between 300 and 35 K for both Cr2GaC and Cr2GaN. As the temperature decreases, the lattice constants (both a and c) decline monotonically in Cr2GaC, while an abnormal negative thermal expansion of c axis was observed below 170 K in Cr2GaN. The increase of c/a induced by abnormal increase of c leads to the enhancement of localization of Cr2N plane and causes more Cr-3d electrons confinement in Cr2N plane of Cr2GaN. This further facilitates the reconstruction of density of states near the Fermi surface and even forms the Fermi surface nesting, resulting in the spin-density-wave transition as well as the abrupt changes of resistivity, specific heat, and density of carriers around 170 K in Cr2GaN.

Soft MAX phases with boron substitution: A computational prediction

With a goal to improve upon the mechanical properties of the MAX phase, materials of high technological interest, we explore boron substitution in these compounds. Employing first-principles density functional theory (DFT) calculations, combined with continuum modeling to access the core structure of dislocations, we investigate the effect of boron-substitution on plastic deformation properties of a typical MAX phase compound, ${\mathrm{V}}_{2}\mathrm{AlC}$. Our $T=0$ K results show that, due to the differential nature of chemical bonding between V and B compared to that between V and C, both V-Al and V-B basal slip planes get activated in boron-substituted compounds, compared to only V-Al basal slip in the parent compound. This, in turn, makes the boron compounds significantly more ductile compared to their carbide counterpart ${\mathrm{V}}_{2}\mathrm{AlC}$. The computation of temperature-dependent free energies of stable and unstable stacking faults further reveals the important and interesting role of thermal fluctuations in the deformation behavior of the boride compounds at elevated temperature. This suggests a change in the microscopic mechanism of plastic deformation upon varying temperature condition in the proposed boron compounds. Our study should motivate future exploration of boron-based MAX compounds.

Anisotropic plastic deformation of single crystals of the MAX phase compound Ti3SiC2 investigated by micropillar compression

The anisotropic deformation behavior of single crystals of Ti3SiC2 has been investigated at room temperature as a function of loading axis orientation and specimen size by micropillar compression tests. Basal slip is found to be only the operative slip system at room temperature. The a/3[12¯10] basal dislocation responsible for basal slip dissociates into two partial dislocations of the Shockley-type and glides between Ti1 and Si atomic layers, as confirmed both experimentally and theoretically. The CRSS value for basal slip increases with the decrease in the specimen size, following an inverse power-law relationship with a very large power-law exponent of about 1.0, which is almost the upper limit of those reported for FCC metals. When the loading axis is parallel to the basal plane, kink band formation occurs only after the occurrence of basal-plane delamination and the subsequent activation of a-dislocations gliding on the basal plane. Kink bands consists of a/3[12¯10] basal edge dislocations of the same sign, each of which had glided between Ti1 and Si atomic layers. These results indicates that the kink formation can simply be interpreted by the classical dislocation-based kink-formation model.

Type II Hot Corrosion Screening Tests of a Cr2AlC MAX Phase Compound

Low-temperature hot corrosion tests were performed on bulk Cr2AlC MAX phase compounds for the first time. This material is a known alumina-former with good oxidation and Type I high-temperature hot corrosion resistance. Unlike traditional (Ni,Co)CrAl alumina formers, it contains no Ni or Co that may react with Na2SO4 salt deposits needed to form corrosive mixed (Ni,Co)SO4–Na2SO4 eutectic salts active in Type II hot corrosion. Cr2AlC samples coated with 20K2SO4–80Na2SO4 salt were exposed to 300 ppm SO2 at 700 °C for times up to 500 h. Weight change, recession, and cross-sectional microstructures identified some reactivity, but much reduced (< 1/10) compared to a Ni(Co) superalloy baseline material. Layered Al2O3/Cr2O3 scales were indicated, either separated by or intermixed with some retained salt. However, there was no conclusive indication of salt melting. Accelerated oxidation was proposed to explain the results, and coarse Cr7C3 impurities appeared to play a negative role. In contrast, the superalloy exhibited outer Ni(Co) oxide and inner Cr2O3 scales, with Cr–S layers at the interfaces. Massive spallation of the corrosion layers occurred repeatedly for the superalloy, but not at all for Cr2AlC. This indicates some potential for Cr2AlC as LTHC-resistant coatings for superalloys.

Electronic and lattice dynamical properties of Ti2SiB MAX phase

The structural, electronic, mechanic, vibrational and thermodynamic properties of Ti2SiB which is a hypothetical MAX phase compound, have been investigated using density functional theory calculations. The structural optimization of Ti2SiB has been performed and the results have been compared with Ti2SiC, Ti2SiN, and Ti2AlB that are studied in the literature. Then the band structure and corresponding partial density of states are computed. In addition, charge density and Bader charge analysis have been performed. The elastic constants have been obtained, then the secondary results such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and Vickers Hardness of polycrystalline aggregates have been derived, and the relevant mechanical properties have been discussed. Moreover, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young’s and Shear moduli. Furthermore, the phonon dispersion curves as well as corresponding phonon PDOS, and thermodynamical properties such as free energy, entropy and heat capacity have been computed and the obtained results have been discussed in detail. This study provides the first considerations of Ti2SiB that could have a potential application in nuclear industry.

Molten salt synthesis of MAX phases in the Ti-Al-C system

The molten salt method was used to synthesise the MAX phase compounds Ti2AlC and Ti3AlC2 from elemental powders. Between 900–1000 °C, Ti2AlC was formed alongside ancillary phases TiC and TiAl, which decreased in abundance with increasing synthesis temperature. Changing the stoichiometry and increasing the synthesis temperature to 1300 °C resulted in formation of Ti3AlC2 alongside Ti2AlC and TiC. The type of salt flux used had little effect on the product formation. The reaction pathway for Ti2AlC was determined to be the initial formation of TiC1-x templating on the graphite and titanium aluminides.

Synthesis and sintering of Ti3GeC2 MAX phase powders

Ti3GeC2 is one of the potential MAX phase compounds, for which, production and processing methods are yet to be optimized. There are very few reports on the synthesis of this compound, which are mostly in bulk form and contain large impurities. Present investigation attempts to synthesize Ti3GeC2 powder with improved phase purity and to understand phase evolution sequences. Synthesis was characterized as a liquid-solid reaction process; initiated after melting of Ge particles. At low temperature (~1200°C), yield was found to be very low, and the product had several intermetallics and carbides. At relatively higher temperature (at 1500°C), the conversion was found to be very high and particles were mostly finer than 10µm. Phase purity was further increased (over 96wt%) during sintering stage, as observed through x-ray diffraction analysis where impurity peaks were very small. Ti3GeC2 powder was found to exhibit poor densification behavior during pressureless sintering; however, it showed slow grain growth characteristics too, a highly desired feature for structural applications.

Electron correlation in Pauli paramagnetic Cr2AlC, Cr2GaC and Cr2GeC

Cr2AlC, Cr2GaC, and Cr2GeC are classical MAX phase compounds, for which successful synthesis of bulk equilibrium phases has been reported in an early stage. Although it has been established that they are Pauli paramagnetic down to the lowest temperature, the extent of the exchange enhancement depends on the A element (A = Al, Ga, and Ge). We discuss the nature of electron correlation by analyzing low-temperature resistivity, specific heat, and susceptibility in terms of the Kadowaki-Woods and Wilson ratios.

Density functional theory insights into ternary layered boride MoAlB

Density functional theory is used to provide theoretical insights into the ternary nanolaminated and layered transition metal boride (MAB phase) of MoAlB, with calculations of crystal structure, electronic structure, lattice dynamics and elastic properties, including a corresponding hypothetical MAX phase compound Mo2AlC for comparison. The calculated atomic configuration matches well with experiment. The metal-like electronic structure contributes to the physical origin of the high electrical conductivity of MoAlB. Strong covalent bonding is present between the B atoms, as well as between the Mo and B atoms, and significantly the much weaker AlAl bonds are consistent with the high fracture toughness and damage tolerance seen in MoAlB. With increasing pressure, the shrinkage is highest along the b axis, and lowest along the c axis. From the calculated second-order elastic constants, the bulk moduli B, shear moduli G, Young's moduli E and Poisson ratio μ are 207 GPa, 137 GPa, 336 GPa and 0.23, respectively. The G/B ratio of 0.66—similar in magnitude to values in MAX phases –demonstrate similarities in properties between MAB and MAX phases. Lattice dynamics are examined in detail, with 9 Raman-active modes and 6 infrared-active modes identified and analyzed in terms of their atomic motion and wavenumbers.

Electronic properties and applications of MXenes: a theoretical review

The recent chemical exfoliation of layered MAX phase compounds to novel two-dimensional transition metal carbides and nitrides, the so-called MXenes, has brought a new opportunity to materials science and technology.

Theoretical studies of electronic, mechanical and thermal properties of Ti3(SnxAl1-x)C2 solid solutions

Available experimental and theoretical studies demonstrate that Ti3AlC2 and Ti3SnC2 compounds exhibit excellent mechanical properties at high temperatures,and thus are rendered a promising candidate of high-temperature structural materials.However,these compounds each have a relatively low hardness,Young's modulus,and poor oxidation resistance compared with other MAX phases.In order to overcome these limits,solid solutions on the M,A and/or X sites of the MAX phase compound are considered as a promising strategy to further improve the mechanical properties. Very recently,the solid solutions of Ti3(SnxAl1-x) C2 have been synthesized.However,no theoretical work has focused on the Ti3(SnxAl1-x) C2 solid solutions so far.Therefore,in this work,we perform first-principles calculation to study the microstructures,phase stabilities,electronic,mechanical and thermal properties of Ti3(SnxAl1-x) C2 solid solutions. Particularly,the effects of Sn concentration (x) on the properties are discussed for the Ti3(SnxAl1-x) C2 solid solutions by varying x from 0 to 1.0 in steps of 0.25.All the present ab initio calculations are carried out based on density-functional theory method as implemented in the Cambridge Serial Total Energy Package (CASTEP) code.The electron-ion interaction is described by Vanderbilt-type ultrasoft pseudo-potential with an exchange-correlation function in the generalized gradient approximation (GGA-PW91).The equilibrium crystal structure is fully optimized by independently modifying lattice parameters and internal atomic coordinates,and we employ the Broyden-Fletcher-Goldfarb-Shanno minimization scheme to minimize the total energy and inter-atomic forces.For the reciprocal-space integration,a Monkhorst-Pack grid of 16164 is used to sample the Brillouin-zones for Ti3AlC2 and Ti3SnC2 compound,and 882 for 221 supercell Ti3(SnxAl1-x) C2(x=0.25-0.75) compounds.The present calculated results of the enthalpy formation energy and mechanical stability criteria indicate that all the Ti3(SnxAl1-x) C2(x=0-1.0) solid solutions are thermodynamic and elastically stable.Moreover,mechanical properties (including bulk modulus B and shear modulus G),the ductile and brittle behavior and the anisotropic factors of Ti3(SnxAl1-x) C2 solid solutions are investigated,and the results indicate that all these compounds are identified as brittle materials and isotropic in nature.On the other hand,the MAX phases are good thermal materials due to their high thermal conductivities varying from 12 to 60 W/(mK) at room temperature.As for the thermal conductivity,it has become one of the most fundamental and important physical properties of the MAX phase material,especially for applications at elevated temperatures.Therefore,the lattice thermal conductivities,the minimum thermal conductivities and temperature dependences of the lattice thermal conductivity of Ti3(SnxAl1-x) C2 solid solutions are studied.Furthermore,Debye temperatures and melting points of the Ti3(SnxAl1-x) C2 compounds are also reported.Present results predict that each of all Ti3(SnxAl1-x) C2 compounds has a relative high Debye temperature and melting point,indicating that each of all Ti3(SnxAl1-x) C2 compounds possesses a rather stiff lattice and good thermal conductivity.

Oxidative durability of TBCs on Ti2AlC MAX phase substrates

Air plasma spray (APS) and plasma-spray-physical vapor deposition (PS-PVD) yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC), ~ 80–100 μm thick, were produced on a commercial Ti2AlC MAX phase compound. They were oxidized in interrupted furnace tests for 500 h each, at five successive temperatures from 1100°–1300 °C. The APS coating survived 2400 accumulated hours, failing catastrophically after 500 h at 1300 °C. Porosity, large cracks, sintering, and high monoclinic YSZ phase contents were seen as primary degradation factors. The PS-PVD coating remained completely intact over 2500 total hours (65 cycles) including 500 h at 1300 °C, exhibiting only fine porosity and microcracking, with less monoclinic. These Ti2AlC systems achieved a minimum α-Al2O3 scale thickness of 29 and 35 μm, respectively, as compared to ~ 6 ± 2 μm on average at failure for conventional bond coats on superalloys. Accordingly, times predicted from thermogravimetric analyses (TGA) of oxidation kinetics project an improvement factor of ~ 25–50 × for the time to achieve these scale thicknesses at a given temperature. Extreme oxidative TBC durability is achieved because the thermal expansion coefficient of Ti2AlC is only slightly different than those for α-Al2O3 and YSZ. The strain energy term driving scale and TBC failure is therefore believed to be fundamentally diminished from the large compressive stress produced by higher expansion superalloys.