Structural investigation of substoichiometry and solid solution effects in Ti2Al(Cx,N1−x)y compounds

The milling, cold compaction and thermal annealing (4 h–1400 °C–Ar flow) of Ti, TiC, Al and AlN powders were used to produce Ti₂Al(C_xN_(1?x))_y compounds with x = 0, 0.25, 0.5, 0.75, 1 and 0.7 ≤ y ≤ 1. X-ray diffraction analysis, scanning electron microscopy observations combined with microanalysis confirmed the formation of the almost pure Ti₂AlC_xN_(1?x) carbonitrides for y = 1 whereas increasing amounts of titanium aluminides were formed when y decreases. Proportions of the different phases deduced from Rietveld refinements of the X-ray diffractograms indicate that no or very poor substoichiometry in carbon was possible in carbide whereas C and N deficiency can be achieved in nitrides and carbonitrides Ti₂AlC_xN_(1?x). Electron Energy Loss Spectroscopy investigations confirm that carbonitrides can have at least 20% of vacancies on the C or N site. The a lattice parameter varies linearly with x whereas it is not the case for the c lattice parameter, its values being lower for the carbonitrides. Furthermore, a strong broadening of the carbonitrides’ XRD peaks is observed, a phenomenon that can be mainly attributed to C and N concentration gradients inside the samples.     

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.     

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.     

High strengthening effects and excellent wear resistance of Ti3Al(Si)C2 solid solutions

The Ti₃Al_1.2−xSi_xC₂ (x = 0, 0.2, 0.4) powders were synthesized from Ti, Al, Si, and TiC powders, and nearly pure Ti₃Al_1.2−xSi_xC₂ bulks were fabricated by the means of two-time hot-pressing method. Significant strengthening effect in bulks was found after the addition of 0.2 Si and 0.4 Si to form Ti₃Al(Si)C₂ solid solutions. The flexural strengths of Ti₃AlSi_0.2C₂ and Ti₃Al_0.8Si_0.4C₂ were 485 and 554 MPa, 14% and 30% larger than the strength of Ti₃AlC₂, respectively. The Vickers hardness of these compounds were separately, 6.95 and 7.57 GPa, representing the enhancements of 37% and 49% over those of Ti₃AlC₂. The tribological behavior was studied by dry-sliding method with a S45C steel at the sliding speed of 30 m/s and the normal load of 20-80 N. The results showed that after incorporating different contents of Si, the friction coefficient was between 0.22 and 0.30, correspondingly lower wear rate was 3.19-2.61 × 10⁻⁶ mm³/Nm. These excellent tribological performances were attributed to the presence of continuous self-generated oxidized films during tribological examination. Finally, the phase compositions and microhardness of the oxidized films were analyzed and characterized.     

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.     

Determination of interdiffusion coefficients of Si and Al in Ti3SiC2–Ti3AlC2 diffusion couple at 1373-1673 K

In the diffusion couple of Ti₃SiC₂ and Ti₃AlC₂, only interdiffusion of Si and Al occurred during diffusion treatment process. Based on the concentration profiles of Si and Al measured by electron probe microanalysis (EPMA), the interdiffusion coefficients of Si and Al at 1373-1673 K in Ti₃SiC₂–Ti₃AlC₂ diffusion couple were determined by both the Boltzmann-Matano (B-M) method and the Saucer-Freise (S-F) method. At the position of Matano plane with the composition of Ti₃Al_0.5Si_0.5C₂, the interdiffusion coefficient could be expressed as D_int (m²/s) = 5.6 × 10⁻⁴⋅exp [−246 ± 14 (kJ/mol)/RT]. Based on the two methods, the calculated interdiffusion coefficients increased with increasing temperature, and the magnitudes of their absolute values were on the order of 10^–13-10^–11 m²/s at 1373-1673 K. At 1373-1573 K, the calculated interdiffusion coefficients decreased monotonously with the increase of Si concentration, that is, x_Si/(x_Al + x_Si). But at 1673 K, the variation trend of interdiffusion coefficients with x_Si/(x_Al + x_Si) was no longer monotonous, probably due to the presence of Ti₅Si₃ phase and voids on Ti₃AlC₂ side.     

Effects of Al substitution with Si and Sn on tribological performance of Ti3AlC2

Since their discovery, the MAX phases have elicited engineering interest as potential choices for wear resistant parts. One such compound is Ti₃AlC₂ with nano-layered structure, low density (4.25 g/cm³), good oxidation resistant and self-lubrication properties. The purpose of this investigation was to evaluate the dominant effect of the A-site solid solution elements addition on dry sliding characteristics of Ti₃AlC₂ against 0.45% C steel (S45C) disk, a material which is widely used in wear-critical applications such as impeller, gear and axles. Dry sliding tribological behaviors of hot-pressed Ti₃Al_0.94C₂, Ti₃Al_0.78Sn_0.22C₂ and Ti₃Al_0.67Si_0.28C₂ solid solutions were conducted using a block-on-disk type tester at surface sliding speed range from 10 to 30 m/s and in the normal load range from 20 to 80 N. The results show that friction coefficient of Ti₃Al_0.94C₂ is higher than that of Ti₃Al_0.78Sn_0.22C₂, but lower than that of Ti₃Al_0.67Si_0.28C₂. However, the change in wear rate as a function of normal load for different sliding speed shows almost reversed trend. Difference in the phase composition of friction films were found to be responsible for observed change in tribological behaved after partial substitution of Al in Ti₃AlC₂ with Sn and Si. This study shows that friction coefficient of Ti₃AlC₂ can be adjusted from 0.2 to 0.38 by partially substituting Al with Si and/or Sn.     

Synthesis,Microstructure, and Mechanical Properties of Ti3Sn(1−x)AlxC2 MAX Phase Solid Solutions

Ti₃Sn_(1−x)Al_xC₂ MAX phase solid solutions are successfully synthesized from different reactant mixtures. Rietveld refinement allows to carefully characterize their structures and the ocathedra and trigonal prims distortion parameters as a function of the Al content. Mechanical properties of solid solutions are studied from nanoindentation experiments and dynamic resonant method. It is shown that solid solution hardening is not operative in this system. Elastic modulus is found to increase from Ti₃SnC₂ to Ti₃AlC₂, and such a result is discussed in terms of Ti–A bond stiffness.     

Preparation of Ti3AlC2 bulk ceramic via aqueous gelcasting followed by Al-rich pressureless sintering

Ti₃AlC₂ bulk ceramics were prepared via aqueous gelcasting followed by C-rich and Al-rich pressureless sintering. The optimized pH value, zeta potential, dispersant content, and solid loading content were determined to be 10, 72.6 mV, 1.6 wt%, and 52 vol%, respectively. Impurities at ppm level containing in the flowing argon could cause severe decomposition of gelcasted bulk Ti₃AlC₂, forming whiskers of Al₂OC and Al₄O₄C and floccule of AlN. C-rich pressureless sintering resulted in the delamination of a duplex layer of Ti(CO) and Ti₃(AlO)C_x-Ti(O,C). The channels formed after debinding facilitated the outward diffusion of Al and the inward diffusion of O and C, and thereby promoting the decomposition of C-rich sintered Ti₃AlC₂. The combined effect of the unclosed channels and the porous reaction Ti₃(AlO)C_x-Ti(O,C) layer brought a catastrophic reduction in the density and mechanical properties of the C-rich sintered Ti₃AlC₂ ceramic. While the Al-rich pressureless sintering system isolated C, CO and N₂ and supplied a closed Al-rich atmosphere, thereby suppressing the decomposition reactions and promoting the sintering densification and ultimately leading to the superior in mechanical properties. The density, hardness, flexural strength and fracture toughness of the Al-rich sintered ceramic reaches 4.13 g/cm³, 4.36 GPa, 345 MPa, 4.79 MPa m^1/2, respectively.     

Fabrication and thermal stability of two-dimensional carbide Ti3C2 nanosheets

Two-dimensional Ti₃C₂ nanosheets were obtained by exfoliation of synthesized Ti₃AlC₂ powders in 40% HF solution at room temperature. The influence of etching time on the synthesis, structural evolution during heating, and thermal stability of as-prepared Ti₃C₂ nanosheets were studied. The graphene-like structure of as-prepared Ti₃C₂ nanosheets was confirmed by XRD and SEM. TG–DTA and elemental analysis suggested that the OH and F groups attached on the surface of Ti₃C₂ nanosheets could be eliminated by heat treatment. It is noteworthy that Ti₃C₂ nanosheets are thermal stable up to 1200 °C in Ar atmosphere. In the heating process, a small quantity of TiO₂ anatase emerges at 500 °C and then reacts with Ti₃C₂ to form a solid solution of TiC_xO_1−x (0<x<1) with the increase of temperature.     

Synthesis and characterization of quarternary Ti3Si(1−x)AlxC2 MAX phase materials

Ti₃Si_(1−x)Al_xC₂ (x=0–1) quarternary MAX phase materials were prepared by spark plasma sintering of TiC, Ti, Si and Al powder mixtures at 1200 °C. Effect of Al addition on lattice parameters, density and hardness were investigated. Impurities are limited to binary phases of TiC and Ti₅Si₃. No multinary compound other than Ti₃Si_(1−x)Al_xC₂ can be detected. TiC exists as impurity in all samples and trace amount of Ti₅Si₃ can be detected in Samples x=0.1–0.6. Oxidation of Al cannot be avoided although all sintering were performed under vacuum and trace amount of Al₂O₃ can be found in all samples with Al addition. Experimental results show that the lattice parameters a and c increase linearly with increasing Al content for x=0–1. The lattice variations are strongly anisotropic and follow Vegard׳s law. Both density and hardness decrease as Al content increases. The linear variation of lattice parameters, d spacings of crystalline faces and density against Al concentration suggest that continuous solid solutions of Ti₃Si_(1−x)Al_xC₂ (x=0–1) may have been formed between Ti₃SiC₂ and Ti₃AlC₂.     

Synthesis and characterization of a new (Ti1-ε,Cuε)3(Al,Cu)C2 MAX phase solid solution

A new (Ti_1-εCu_ε)₃(Al,Cu)C₂ MAX phase solid solution has been synthesized by sintering at 760 °C compacted Ti₃AlC₂-40 vol.% Cu composite particles produced by mechanical milling. Using XRD and TEM-EDXS, it has been demonstrated that Cu can enter the crystallographic structure of the Ti₃AlC₂ MAX phase, whereas a Cu(Al,Ti) solid solution is also formed during thermal treatment. TEM-EELS analyses have demonstrated that Cu is mainly located on the A site of the MAX phase. The composition of the MAX phase solid solution, determined after selective chemical etching of the Cu(Al,Ti) matrix, by analyzing the filtrate and the solid phase using ICP-OES end EDXS methods respectively, is (Ti_0.93–0.97Cu_0.07–0.03)₃(Al_0.49–0.52Cu_0.51–0.48)C_2.     

Effect of Al atomic layer on the wetting behavior,interface structure and electrical contact properties of silver reinforced by Ti3AlC2 ceramic

Ti₃AlC₂ ceramic exhibits potential in Ag-based composite electrical contact materials, but its interface characteristic with Ag matrix remains unexplored. In this work, sessile drop experiment is carried out to investigate the high-temperature wetting behavior of molten Ag with Ti₃AlC₂. Stable Ti₃AlC₂ is hardly wetted by molten Ag below 1000 °C(contact angle of 148.5°), but the wettability of Ag/Ti₃AlC₂ improves with the increasing temperature(final 14° at ～1130 °C). In contrast, the Ti₃C_2, a MXene with Al layer removed from its parent Ti₃AlC₂, exhibits inferior wettability with Ag(final 56.5° at ～1130 °C). Wetting mechanism of Ag/Ti₃AlC₂ is proposed on the basis of the interfacial structure and chemical composition. Increasing temperature accelerates dissociation of Ti₃AlC₂, and outward-escaping Al reacts with Ag to form interface layer with a composition of Ag_4.86Ti_8.66AlC_7.59, Ag also diffuses along Ti₃AlC₂ grain boundaries and forms gradient reactive products(Ag–Ti–Al–C), which promotes their wettability. Finally comprehensive properties of Ag/Ti₃AlC₂ and Ag/Ti₃C₂ are compared. Al–Ag interdiffusion slightly decreases the electrical conductivity of Ag/Ti₃AlC₂ bulk material, but strengthen the interface bonding of composite and promote the viscosity of the molten pool, leading to the superior mechanical and anti-arc erosion properties. Absence of Al–Ag interdiffusion does remarkably improve the electrical conductivity of Ag/Ti₃C₂ bulk materials, but lack of Al layer damages the mechanical core and wettability with Ag, resulting in a drastic decrease of anti-arco erosion property.     

Experimental synthesis and density functional theory investigation of radiation tolerance of Zr3(Al1‐xSix)C2 MAX phases

Synthesis, characterization and density functional theory calculations have been combined to examine the formation of the Zr₃(Al_1–xSi_x)C₂ quaternary MAX phases and the intrinsic defect processes in Zr₃AlC₂ and Zr₃SiC₂. The MAX phase family is extended by demonstrating that Zr₃(Al_1–xSi_x)C₂, and particularly compositions with x≈0.1, can be formed leading here to a yield of 59 wt%. It has been found that Zr₃AlC₂ ‐ and by extension Zr₃(Al_1–xSi_x)C₂ ‐ formation rates benefit from the presence of traces of Si in the reactant mix, presumably through the in situ formation of Zr_ySi_z phase(s) acting as a nucleation substrate for the MAX phase. To investigate the radiation tolerance of Zr₃(Al_1–xSi_x)C₂, we have also considered the intrinsic defect properties of the end‐members. A‐element Frenkel reaction for both Zr₃AlC₂ (1.71 eV) and Zr₃SiC₂ (1.41 eV) phases are the lowest energy defect reactions. For comparison we consider the defect processes in Ti₃AlC₂ and Ti₃SiC₂ phases. It is concluded that Zr₃AlC₂ and Ti₃AlC₂ MAX phases are more radiation tolerant than Zr₃SiC₂ and Ti₃SiC₂, respectively. Their applicability as cladding materials for nuclear fuel is discussed.     

Subsize Ti3C2Tx derived from molten-salt synthesized Ti3AlC2 for enhanced capacitive deionization

Ti₃C₂T_x is a promising intercalation-type electrode material for capacitive deionization (CDI). However, Ti₃C₂T_x, obtained from traditional synthesized Ti₃AlC₂, is with large particle size and undersized interlayer space, which can easily lead to the longer ion diffusion path, fewer adsorption sites, and higher ion diffusion barrier in CDI process. In this work, subsize and Na⁺ intercalated Ti₃C₂T_x (Na⁺-Ti₃C₂T_x-MS) was prepared by HF etching KCl-assisted molten-salt synthesized Ti₃AlC₂ and following NaOH treatment. The Na⁺-Ti₃C₂T_x-MS achieves a high electrosorption capacity of 14.8 mg/g and a high charge efficiency of 0.81 at the applied voltage of 1.2 V in 100 mg/L NaCl solution. Besides, the stable desalination performance of Na⁺-Ti₃C₂T_x-MS has been confirmed. The superior performance of Na⁺-Ti₃C₂T_x-MS can be attributed to the subsize particle and larger interlayer space. Both two factors can effectively increase ions adsorption sites, shorten diffusion path lengths, and reduce diffusion barriers in the CDI process.     

Intermediate phases in synthesis of Ti3SiC2 and Ti3Si(Al)C2 solid solutions from elemental powders

In this paper, we investigated the reaction path to synthesize Ti₃SiC₂ by the in situ hot pressing/solid–liquid reaction method. The effect of different content of Al addition on this process was also examined. Ti₃SiC₂ mainly formed from the reaction between Ti₅Si₃C_x, TiC_x, TiSi₂ and graphite at 1400–1500 °C. As an inescapable impurity in Ti₃SiC₂, TiC_x was removed by addition of small amounts of Al. This was owing to the fact that the addition of a minor quantity of Al increased the amount of “effective TiC_x” and relatively decreased that of “invalid TiC_x”. Further increasing Al content, however, resulted in the presence of TiC_x again in the final product. This was due to the fact when significant amounts of Al was added, the stoichiometric ratio of silicon and graphite has been deviated from that for Ti₃SiC₂. More Si and less graphite were needed to prepare a monolithic Ti₃Si(Al)C₂ solid solution with high Al content.     

Influence of Mo doping on the tribological behavior of Ti3AlC2 ceramic at different temperatures

(Ti_0·8Mo_0.2)₃AlC₂ solid solutions were successfully synthesized from Ti, Al, TiC, and Mo powders using the in situ hot-pressing sintering method. The tribological properties of (Ti_0·8Mo_0.2)₃AlC₂ and the reference Ti₃AlC₂ in the temperature range 25–800 °C were evaluated in ambient air with the counterpart of Al₂O₃ balls. The results show that (Ti_0·8Mo_0.2)₃AlC₂ has improved lubricating properties and wear resistance above 400 °C compared with Ti₃AlC₂. This can be contributed to the formation of tribo-oxidation films containing MoO₃ and MoO_3-x. Structural characterization of the tribo-oxidation films was conducted using SEM, EDS, Raman spectroscopy, and XPS to evaluate the effect of Mo doping on the wear mechanisms of Ti₃AlC₂ in detail.     

Investigation of Ti3AlC2 formation mechanism through diffusional reaction between carbon and Mo-modified Ti6Al4V

Carbon and Mo-coated Ti6Al4V alloy diffusion couple was used to investigate Mo-modified diffusion reaction between carbon and Ti6Al4V. Randomly dispersed carbide particles were observed in Ti6Al4V alloy after 900℃ exposure. Carbide particles were found evolving from defective TiC_x (x<1) to mixture of TiC_x and defective Ti₃AlC₂. Although Mo atoms were hardly detected in particles, their dilution effect along Ti alloy grain boundary (GB) is beneficial to carbon diffusion and carbide formation along GB. Based on high-resolution TEM (HRTEM) imaging, high density stacking faults (SFs) and nanotwins were observed in TiC_x particles which explain Raman activation of defects in TiC. Formation of SFs and nanotwins in TiC_x is attributed to carbon vacancies, which inversely promotes incoherent twin boundary (ITB) formation. Transformation from TiC_x to Ti₃AlC₂ is considered driven by Al indiffusion along ITBs. Intergrowth of TiC_x in defective Ti₃AlC₂ is the compromise to low Al concentration in Ti6Al4V.     

Multilayer Ti3C2Tx: From microwave absorption to electromagnetic interference shielding

Ti₃C₂T_x MXene has attracted extensive attention in the field of electromagnetic (EM) protection over recent years. Multilayer Ti₃C₂T_x (M-Ti₃C₂T_x), as an intermediate product of MXene ultra-thin structure, has potential advantages in the field of EM protection. Herein, the M-Ti₃C₂T_x was obtained by HCl/LiF etching Ti₃AlC₂. The microwave absorption (MA) and electromagnetic interference (EMI) shielding performance of Ti₃AlC₂ and M-Ti₃C₂T_x were compared. The mechanism research of MA and EMI shielding indicates that the construction of local conductive network plays a leading role in the EM wave attenuation. The sample with 30% M-Ti₃C₂T_x display RL_min of ?50.26 dB, and corresponding bandwidth of 4.64 GHz at the thickness of 1.7 mm. Especially, the metastructure based on the EM parameters of M-Ti₃C₂T_x/wax exhibits ultra-wide bandwidth (15.54 GHz). Our research will provide a basis for the design of MXene-based EM protection performance.     

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.