Accommodation,Accumulation, and Migration of Defects in Ti3SiC2 and Ti3AlC2 MAX Phases

We have determined the energetics of defect formation and migration in M_n+1AX_n phases with M = Ti, A = Si or Al, X = C, and n = 3 using density functional theory calculations. In the Ti₃SiC₂ structure, the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular configuration (C–Si = 1.889 Å) and to two apical Ti atoms (C–Ti = 2.057 Å). This carbon–metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti–Si, 17.3 eV for Ti–C, and 6.1 eV for Si–C. Broadly similar behavior was found for Frenkel and antisite defect energies in the Ti₃AlC₂ structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti₃AlC₂ is different and the energy is lower compared with the Ti₃SiC₂ structure. This effect, together with the observation of a stable C interstitial defect coordinated by three silicon species and two titanium species in Ti₃SiC₂, will have important implications for radiation damage response in these materials.     

Elastic constants of polycrystalline Ti3AlC2 and Ti3SiC2 measured using coherent inelastic neutron scattering

The magnitude of the single‐crystal elastic constant c₄₄ in the MAX phase Ti₃SiC₂ is under debate. In this paper, estimates for the magnitude of c₄₄ for MAX phases Ti₃AlC₂ and Ti₃SiC₂ are determined from a partially oriented polycrystalline sample via coherent inelastic neutron scattering. The largely quasi‐isotropic nature of these M_n+1AX_n phase elastic constants as previously predicted by density functional theory calculations is confirmed experimentally for Ti₃AlC₂ to be c₄₄=115.3 ± 30.7 GPa. In contrast, Ti₃SiC₂ is confirmed to be shear stiff with c₄₄=402.7 ± 78.3 GPa supporting results obtained by earlier elastic neutron diffraction experiments.     

Anisotropic corrosion of Ti2AlC and Ti3AlC2 in supercritical water at 500 ºC

Textured and untextured M_n+1AX_n compounds, Ti₂AlC and Ti₃AlC₂, namely MAX phases have been synthesized and examined with respect to their corrosion resistance in static supercritical water at 500 °C. The textured ceramics were obtained by hot forging process at high temperatures. Both X-ray diffraction and SEM analysis revealed well alignment of c-plane of MAX phases parallel to the hot-forging surface. Better corrosion resistance on the surface perpendicular to the hot-forged direction was verified by SEM. On the other hand, the side surfaces of the samples showed thick oxidation layers and abundant cracks. The (00l) faces consist of strongly bonded Ti₃C₂ and Ti₂C layers in Ti₃AlC₂ and Ti₂AlC, respectively, hence exhibit higher resistance to water corrosion. On the contrary, the side surfaces where most of weakly bonded interlayers of these hexagonal phases were exposed tend to be easily corroded especially through Al-layers. The corrosion process involved a phase transition of oxidized product, i.e. TiO₂ from anatase to rutile phase, which gave rise to the formation of cracks due to accompanied volume changes.     

Oxidation and hot corrosion behaviors of MAX-phase Ti3SiC2, Ti2AlC,Cr2AlC

Oxidation and hot corrosion behaviours of Ti₃SiC₂, Ti₂AlC and Cr₂AlC at 750 °C were investigated in this work. Ti₃SiC₂ and Ti₂AlC showed a linear increase in mass gain and a relatively poor oxidation resistance. This might be attributed to the porous TiO₂ scale. A dense α-Al₂O₃ layer was formed during the oxidation test. Cr₂AlC exhibited the best oxidation resistance. This dense oxide scale can effectively isolate the substrate from contact with oxygen leading to excellent oxidation resistance. In contrast to the oxidation test, Ti₃SiC₂ and Ti₂AlC showed relatively better resistance to hot corrosion, while Cr₂AlC showed inferior resistance to NaCl introduced hot corrosion. The hot corrosion mechanism of the MAX phases was analyzed. Due to the formation of Na₂TiO₃, Ti containing MAX phases showed a continuous increase in the mass gain. The corrosion products of Cr₂AlC were Al₂O₃, Cr₂O₃ and Na₂CrO₄. However, due to the volatilization of Na₂CrO₄, Cr₂AlC showed a mass loss during the hot corrosion test. The chemical reaction process of the MAX phase was also analyzed.     

Molten salt synthesis and formation mechanism of Ti3AlC2: A new path from Ti2AlC to Ti3AlC2

Fine, pure Ti₃AlC₂ powder is prepared in a very mild condition via Ti₃Al alloy and carbon black with the assistance of molten salts. X-ray diffraction, scanning electron microscopy, TG-DSC, and transmission electron microscopy (TEM) characterizations show that the high purity, nanosized Ti₃AlC₂ can be obtained at 900°C with the 1:1 salt-to-material ratio. The formation mechanism of Ti₃AlC₂ through this strategy of alloy raw material is fully studied under further TEM investigations, showing that the reaction process can basically be described as Ti₃Al and C → TiAl and TiC → Ti₂AlC and TiC → ψ and TiC → Ti₅Al₂C₃ and TiC → Ti₃AlC₂, where the key ψ, a modulated Ti₂AlC structure, is determined for the first time containing alternate-displacement Al layers along (0 0 0 2) of Ti₂AlC phase with a distinct selected area electron diffraction pattern. Such alternant displacement is considered a precondition of forming Ti₅Al₂C₃ through topotactic transition, followed by Ti₅Al₂C₃ converting into Ti₃AlC₂ by the diffusion of Ti, C atoms in the outside TiC. Several parallel orientations can be observed through the phase transition process: Ti₂AlC (0 0 0 2)//ψ (0 0 0 1), ψ (0 0 0 1)//Ti₅Al₂C₃ (0 0 0 3), Ti₅Al₂C₃ (0 0 0 3)//Ti₃AlC₂ (0 0 0 2). Such parallel orientations among these phases apply an ideal condition for the topotactic reaction. The distinct path of the phase transition brings a significant change of heat effect compared with the traditional method, leading to a fast reaction rate and a mild reaction condition.     

Tailoring Magnetic Properties of MAX Phases,a Theoretical Investigation of (Cr2Ti)AlC2 and Cr2AlC

(Cr₂Ti)AlC₂ is a newly discovered MAX phase with ordered occupations of Ti and Cr atoms on M sites. The Cr‐containing MAX phase is expected showing magnetic property, which provides functional applications in spintronics and as self‐monitoring smart coating. The magnetic states of (Cr₂Ti)AlC₂ are predicted by GGA and GGA + U methods and compared to those of Cr₂AlC. The ground states are predicted as FM or AFM‐XX configurations depending on the calculation methods. Analysis of the electronic structure shows that the magnetic moments mainly originate from the net spins of Cr 3d valence electrons, whereas the contribution of other atoms is negligible. The calculated magnetic moments of Cr atoms in (Cr₂Ti)AlC₂ are higher than those in Cr₂AlC due to the larger distance between the out‐plane Cr atoms separated by the intercalated nonmagnetic Ti–C slab. This work provides an insight on tailoring magnetic properties of MAX phases by modifying the crystal structure.     

Compressive Behavior of Ti3AlC2 and Ti3Al0.8Sn0.2C2 MAX Phases at Room Temperature

In this study, we report on the compressive behavior of Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂ MAX phases at room temperature. We found that these two phases could be classified as Kinking Nonlinear Elastic (KNE) solids. The cyclic compressive stress–strain loops for Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂ are typical hysteretic and fully reversible. At failure, both compositions fracture in shear with maximum stresses of 545 MPa for Ti₃AlC₂ and 839 MPa for Ti₃Al_0.8Sn_0.2C₂. Consequently, the macroshear stresses for failure, τ_c, are 185 MPa and 242 MPa for Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂, respectively. In addition to the grain size effects, the presence of a ductile Ti_xAl_y intermetallic distributed in the grain boundaries plays an important role in the enhancement of the ultimate compressive and macroshear stresses for Ti₃Al_0.8Sn_0.2C₂. SEM observations reveal that these two MAX phases exhibit crack deflections, intragranular fractures, kink band formation and delaminations, grain push‐in and pull‐out.     

Effect of thermal stability of Mn+1AXn phases on microstructure and mechanical properties of Mn+1AXn/ZA27 composites

Special layered structure endows ternary M_n+1AX_n phase ceramics with good electrical and thermal conductivity, excellent abrasive resistance, and perfect thermal shock resistance. In this work, three kinds of M_n+1AX_n phase ceramics (Ti₃SiC₂, Ti₃AlC₂, and Ti₂SnC) were chosen to reinforce the ZA27 alloys, respectively. By employing “two-step sintering” technology which is pressureless sintered at 870°C for 1 h firstly and then hot pressed at 500°C for 1 h, M_n+1AX_n/ZA27 composites were successfully fabricated. The effects of thermal stability of the above M_n+1AX_n on microstructure, mechanical properties, and friction performance of the three M_n+1AX_n/ZA27 composites were investigated. The different reaction degrees between the three M_n+1AX_n reinforcements and the ZA27 matrix were ascribed to the differences of chemical bond energy. The results demonstrated that at the sintering temperature of 870°C, Ti₂SnC was completely reacted in Ti₂SnC/ZA27 composite, and Ti₃AlC₂ partially reacted in ZA27 matrix, while no reaction happened between Ti₃SiC₂ and ZA27 matrix. Hence, the order of thermal stability for the three M_n+1AX_n phases in ZA27 matrix is Ti₃SiC₂ > Ti₃AlC₂ > Ti₂SnC. Besides, Ti₃AlC₂/ZA27 composites possess the best mechanical properties and wear resistance, which was attributed to interfacial reaction improved the boding between matrix and reinforcement.     

Phase stability and weak metallic bonding within ternary-layered borides CrAlB,Cr2AlB2, Cr3AlB4, and Cr4AlB6

To explore the potential for use of the Cr–Al–B borides, Cr₂AlB₂, Cr₃AlB₄, and Cr₄AlB₆ as well as hypothetical CrAlB are investigated using density functional theory. In the CrAl(CrB₂)_n series strong covalent bonding is present between the B and Cr atoms with, significantly, much weaker metallic Cr–Al and B–Al bonds, suggesting similar unusual properties to the MAX phases. The relative stiffness of the weakest and strongest bonds hint at similar unusual properties to the MAX phases with superior damage tolerance expected for hypothetical CrAlB, as evidenced in the lowest Al1–Al2 bond stiffness. The layered nature and metallic bonding are expected to result in high fracture toughness and damage tolerance. Anisotropic compression is demonstrated, with the stiffest axes along the direction of the B–B zigzag-/hexagonal-chains and the softest axes determined by an interplay between the soft metallic interlayers and the rigid covalent bonds. In general the elastic moduli in CrAl(CrB₂)_n increase as a function of n, however, without the price of an increase in density.     

Bonding and oxidation protection of Ti2AlC and Cr2AlC for a Ni-based superalloy

Alumina forming, oxidation and thermal shock resistant MAX phases are of a high interest for high temperature applications. Herein we report, on bonding and resulting interactions between a Ni-based superalloy, NSA, and two alumina forming MAX phases. The diffusion couples Cr₂AlC/Inconel-718/Ti₂AlC were assembled and heated to 1000 or 1100 °C in a vacuum hot press under loads corresponding to stresses of either 2 MPa or 20 MPa. The resulting interfaces were examined using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Good bonding between Cr₂AlC and NSA was achieved after hot pressing at 1000 °C and a contact pressure of only 2 MPa; in the case of Ti₂AlC a higher temperature (1100 °C) and pressure (20 MPa) were needed. In both cases, a diffusion bond, in which mainly Ni and Cr out diffused from the NSA into the MAX phase and a concomittant out diffusion of Al from the latter, was realized with no evidence of interfacial damage or cracking after cooling to room temperature. The reactions paths were determined to be: Cr₂AlC/Cr₇C₃/Cr₇C₃,β-NiAl/α-Cr(Mo)/NSA and Ti₂AlC/Ti₂AlC,Ti₃NiAl₂C/β-NiAl/α-Cr(Mo)/NSA. Twenty thermal cycles from room temperature to 1000 °C showed that Ti₂AlC is a poor oxidation barrier for Inconel-718. However, in the case of Cr₂AlC no cracks, delamination nor surface degradation was observed, suggesting that this material could be used to protect Inconel-718 from oxidation.     

Reaction synthesis of layered ternary Ti2AlC ceramic

Reactive sintering of 8Ti:Al₄C₃:C powder mixtures to form the ternary carbide Ti₂AlC is studied in the temperature range 570–1400 °C. After sintering at 1400 °C for 1 h, only the MAX phase Ti₂AlC and some TiC are produced. A series of intermediate phases, such as TiC, Ti₃Al, Ti₃AlC are detected during the reactive sintering process. From X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterizations, a reaction path is proposed for the intermediate phases and Ti₂AlC formation. Results show that reaction kinetics may play an important role in the understanding of the reaction mechanisms.     

Self-healing capacity of Mullite-Yb2SiO5 environmental barrier coating material with embedded Ti2AlC MAX phase particles

Repetitive heating and cooling cycles inevitably cause crack damage of hot gas components of gas turbine engines, such as blades and vanes. In this study the self-healing capacity is investigated of mullite + ytterbium monosilicate (Yb₂SiO₅) as EBC material with Ti₂AlC MAX phase particles embedded as a crack-healing agent. The effect of Ti₂AlC in the EBC was compared with the self-healing ability of the mullite + Yb₂SiO₅ material. After introducing cracks by Vickers indentation on the surface of each sample, crack healing was realized by controlling the temperature and time during the post-heat-treatment process. For the mullite + Yb₂SiO₅ composite with Ti₂AlC particles, crack healing occurred at 1000 °C, while in the case of the mullite + Yb₂SiO₅ composite without Ti₂AlC, a sustained temperature of 1300 °C or higher was required. Compared with the healing of the mullite + Yb₂SiO₅ composite by the formation of a eutectic phase, the addition of Ti₂AlC promoted healing via the oxidation of Ti and Al. Notably, the surface formation of a ternary oxide of Ti–Yb–O was confirmed, which completely covered the damage area. Consequently, the addition of a Ti₂AlC MAX phase to the EBC composite resulted in a complete strength recovery, while the mullite + Yb₂SiO₅ composite without Ti₂AlC showed a strength recovery of about 80%. Furthermore, by analyzing the indentation load–displacement curve to indicate the role of Ti₂AlC, the addition of Ti₂AlC improved both the hardness and stiffness of the composite.     

Low-temperature synthesis of high-purity Ti3AlC2 by MA-SPS technique

Ternary carbide Ti₃AlC₂ was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS) from elemental powder mixtures of Ti, Al and C, and the effect of Al content on formation of Ti₃AlC₂ during both processes was investigated. The results showed that adding proper Al content in the staring materials significantly increased the phase purity of Ti₃AlC₂ in the synthesized samples. Dense and high-purity Ti₃AlC₂ with <1 wt.% TiC could be successfully obtained by spark plasma sintering of powders mechanically alloyed for 9.5 h from a starting powder mixtures of 3Ti/1.1Al/2C at a lower sintering temperature of 1050 °C for 10–20 min.     

Oxidation and creep behavior of textured Ti2AlC and Ti3AlC2

The oxidation and creep behaviors of textured Ti₂AlC and Ti₃AlC₂ ceramics were characterized. The oxidation behavior of the two materials, which was studied in air at temperatures ranging from1000 to 1300 °C, was observed to be anisotropic and the materials exhibited a better oxidation resistance along a direction transverse to the c-axis. The correlation between the overall parabolic rate constant and oxidation temperature of both textured materials was characterized, providing new insights into the oxidation kinetics. The results indicate that the texturing has a negligible influence on the creep behavior in the assessed temperature range of 1000?1200 °C in air, for the applied stresses ranging from 40 to 80 MPa. In this stress regime, the creep behavior of textured Ti₂AlC and Ti₃AlC₂ appears to be controlled by grain boundary sliding. This behavior can be rationalized based on a model for superplastic deformation, indicating pure-shear motion under stationary conditions accommodated by lattice or grain-boundary diffusion.     

Structural Transitions Induced by Ion Irradiation in V2AlC and Cr2AlC

Nanolayered structural metallic ceramics, MAX phases, possess unique and highly attractive properties, including excellent radiation tolerance for some of them, whereas little is known about the detailed process of irradiation‐induced structural transitions. In this study, the microstructural transformations and the stabilities of V₂AlC and Cr₂AlC induced by 1 MeV Au⁺ ions irradiation over a wide range of fluences were investigated by grazing incidence X‐ray diffraction (GIXRD) and transmission electron microscopy (TEM). GIXRD analyses show different processes of phase transitions and amorphization tolerance under irradiation between these two MAX phases, which are consistent with the selected area electron diffraction (SAED) results and the high‐resolution observations. TEM observations reveal that the nanolamellar structures are disturbed and respective phase transitions occur at relatively low fluences, with the formation of stacking faults. As the fluence increases, Cr₂AlC becomes completely amorphous, while V₂AlC are gradually transformed into face‐centered cubic (fcc) structure from the original hexagonal close‐packed (hcp) structure without amorphization, indicating that V₂AlC is more tolerant of irradiation than Cr₂AlC. Based on the phase contrast images and the electron‐diffraction pattern (EDP) simulation of the microstructures, mechanisms of the phase transitions of V₂AlC and Cr₂AlC are proposed and the difference of the irradiation tolerance between them is discussed.     

Microstructure and High‐temperature Oxidation Behavior of Ti3AlC2/W Composites

Using spark plasma sintering, Ti₃AlC₂/W composites were prepared at 1300°C. They contained “core‐shell” microstructures in which a Ti_xW_1?x “shell” surrounded a W “core”, in a Ti₃AlC₂ matrix. The composite hardness increased with W addition, and the hardening effect is likely achieved by the Ti_xW_1?x interfacial layer providing strong bonding between Ti₃AlC₂ and W, and by the presence of hard W. Microstructural development during high‐temperature oxidation of Ti₃AlC₂/W composites involves α‐Al₂O₃ and rutile (TiO₂) formation ≥1000°C and Al₂TiO₅ formation at ~1400°C while tungsten oxides appear to have volatilized above 800°C. Likely due to exaggerated, secondary grain growth of TiO₂‐doped alumina and the effect of W addition, fine (<1 μm) Al₂O₃ grains formed dense, anisomorphic laths on Ti₃AlC₂/5 wt%W surfaces ≥1200°C and coarsened to large (>5 μm), dense, TiO₂‐doped Al₂O₃ clusters on Ti₃AlC₂/10 wt%W surfaces ≥1400°C. W potentially affects the oxidation behavior of Ti₃AlC₂/W composites beneficially by causing formation of Ti_xW_1?x thus altering the defect structure of Ti₃AlC₂, resulting in Al having a higher activity and by changing the scale morphology by forming dense Al₂O₃ laths in a thinner oxide coating, and detrimentally through release of volatile tungsten oxides generating cavities in the oxide scale. For Ti₃AlC₂/5 wt%W oxidation, the former beneficial effects appear to dominate over the latter detrimental effect.     

Phase formation of Nb2AlC investigated by combinatorial thin film synthesis and ab initio calculations

The phase formation of Nb₂AlC was studied by combinatorial thin film synthesis and ab initio calculations. Thin films with lateral chemical composition gradients were synthesized by DC magnetron sputtering at substrate temperatures of 710–870 °C. The lowest formation temperature for Nb₂AlC is between 710 and 750 °C. A predominantly single phase Nb₂AlC region where 99% of the X-ray diffraction intensity originate from Nb₂AlC was identified. Furthermore, selected area electron diffraction analysis reveals the local formation of single phase Nb₂AlC. The limited Al solubility in Nb₂AlC compared with Cr₂AlC can be understood by comparing the defect formation energy of Al substituting Nb and Cr in Nb₂AlC and Cr₂AlC, respectively. This methodology may serve as indicator for the magnitude of the A-element homogeneity range in M_n+1AX_n phases. The structural and elastic properties of Nb₂AlC determined experimentally are in very good agreement with the ab initio calculated data.     

Microstructure,mechanical and electrochemical properties of Ti3AlC2 coatings prepared by filtered cathode vacuum arc technology

Ti₃AlC₂ MAX phases have attracted increasing attention due to their unique properties. However, high synthesis temperatures of Ti₃AlC₂ bulk materials limit their further development. In this work, Ti₃AlC₂ coatings were prepared by a two-step method with filtered cathode vacuum arc (FCVA) deposition at room temperature and annealing at 800 °C for 1 h. The structure and properties of coatings were investigated. The results showed that the formation of Ti₃AlC₂ phase in the annealed coating depended on the C₂H₂ flow rate during deposition. At low C₂H₂ flow rates (≤ 9 sccm), almost no Ti₃AlC₂ phase was formed. As the C₂H₂ flow rate increased, the annealed coatings mainly exhibited Ti₃AlC₂ phases, the texture of which transformed from (104) to (105) planes. Meantime, the hardness of Ti₃AlC₂ coatings continuously increased to a maximum of 20.7 GPa, and the corrosion resistance first increased and then decreased with the increase of C₂H₂ flow rate.     

A novel ZrB2-based composite manufactured with Ti3AlC2 additive

A novel ZrB₂–Ti₃AlC₂ composite was densified using spark plasma sintering at 1900 °C under pressure of 30 MPa for 7 min. The effect of Ti₃AlC₂ MAX phase on the densification behavior, microstructural evolutions, phase arrangement, and mechanical properties of the composite were investigated. The phase analysis and microstructural studies revealed the decomposition of the MAX phase at the initial steps of the SPS process. The structural characteristics and surface morphology of the in-situ synthesized reinforcements were verified using X-ray diffraction and scanning electron microscopy, respectively. The formation mechanism of each reinforcement phase was also investigated using thermodynamical assessments. The prepared ZrB₂–Ti₃AlC₂ composite not only possessed a near fully-dense characteristic having an excellent hardness of 31 GPa, but also unexpectedly presented high fracture toughness. The indentation fracture toughness of the composite was calculated as 7.8 MPa m^1/2, which is unprecedented compared with the same class of hard ZrB₂-based composites. Indeed, the superior mechanical properties of the composite achieved in this study was obtained by the homogenous distribution of Al-based reinforcements, formation of hard interfacial ZrC grains, and solid solutions provided by Ti-based phases. The correlations between the phase arrangement, microstructure, and the attained mechanical properties of the composite were comprehensively discussed.     

Oxidation behaviour of the MAX-phases Ti2AlC and (Ti,Nb)2AlC at elevated temperatures with and without fluorine treatment

MAX-phase materials have shown great potential for different technical applications due to their mechanical properties. If the main group element is aluminium their excellent oxidation resistance also makes them attractive for several high temperature applications. As an example the thermodynamically stable MAX-phase Ti₂AlC forms a thin, protective alumina layer in oxidising atmospheres at elevated temperatures. This alumina layer is formed due to the high Al activity within the material and prevents further attack by the environment. However, high temperature oxidation tests at 900 °C in air of “technical” Ti₂AlC which is not pure single-phase Ti₂AlC led to the formation of a non-continuous alumina scale which is intersected by a mixed TiO₂/Al₂O₃ scale. Furthermore, internal oxidation was observed. This “technical” material consists of two phases namely Ti₂AlC plus γ-TiAl due to the manufacturing route. Such γ-TiAl-grains are preferentially oxidised. This type of internal attack can be suppressed by a preceding fluorine treatment.