Probing the oxidation behavior of Ti2AlC MAX phase powders between 200 and 1000 °C

Oxidation of commercial Ti₂AlC MAX phase powders at 200–1000 °C has been investigated by XRD, XPS, SEM, STA and TGA coupled with FTIR. These powders are a mixture of Ti₂AlC, Ti₃AlC₂, TiC and Ti_1.2Al_0.8. Oxidation at 400 °C led to disappearance of carbide phases from Ti 2p, Al 2p and C 1s XPS spectra. At 600 °C, powders changed from dark grey to light grey with a significant volume increase due to crack formation. Powders were severely oxidized by detecting rutile with minor anatase TiO₂. At 800 °C, α-Al₂O₃ was detected while anatase transformed into rutile TiO₂. The cracks were healed and disappeared. At 1000 °C, the Ti₂AlC powders were fully oxidized into rutile TiO₂ and α-Al₂O₃ with a change of powder color from light grey to yellow. FTIR detected the release of C as CO₂ from 200 °C onwards but with additional CO above 800 °C.     

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.     

Improving the Cyclic-Oxidation Resistance of Ti3AlC2 at 550°C and 650°C by Preoxidation at 1100°C

Preoxidation of Ti₃AlC₂ at 1100°C for 2 h was conducted to improve its cyclic-oxidation resistance at the testing temperature of 550°C and 650°C in air. The cyclic oxidation of the preoxidized Ti₃AlC₂ was found to follow a parabolic rate law rather than the linear oxidation rate for that without preoxidation. Through the X-ray diffraction and SEM analysis, the remarkable improvement of the cyclic-oxidation resistance of preoxidation Ti₃AlC₂ is suggested due to the existence of protective α-Al₂O₃ layers formed during the preoxidation treatment, which inhibits the formation of amorphous Al₂O₃, which can result in larger thermal stress and stress-induced microcracks.     

Effects of Xe+ irradiation on Ti3SiC2 at RT and 500 °C

In this study, effects of a 400 keV Xe⁺ irradiation on Ti₃SiC₂ were systematically investigated by transmission electron microscopy (TEM). At RT, results show that the Xe⁺ irradiation induced the dissociation of Ti₃SiC₂ to polycrystalline TiC first, and then the polycrystalline to TiC nanograins with the increasing fluence. However, there is no significant microstructure change observed on the sample irradiated at 500 °C. It is demonstrated that Ti₃SiC₂ had not been completely amorpherized even up to 116.9 displacements per atom (dpa).     

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.     

Oxidation behavior of ZrB2-SiC-ZrC at 1700 °C

The oxidation behaviors of four compositions of ZrB₂-SiC-ZrC and one composition of ZrB₂-SiC were studied at 1700 °C in air and under low oxygen partial pressure. Volatility diagrams for ZrB₂-SiC-ZrC and ZrB₂-SiC were used to thermodynamically elucidate the oxidation mechanisms. SiO₂ and ZrO₂ layers formed on the surfaces of ZrB₂-SiC-ZrC and ZrB₂-SiC oxidized at 1700 °C. A SiC-depleted layer only formed on the surface of the ZrB₂-SiC oxidized under low oxygen partial pressure. The oxide layer thickened with increasing ZrC volume content during oxidation in air and under low oxygen partial pressure. The ZrB₂-SiC-ZrC oxide surface exploded in air when the ZrC volume content was more than 50%. Under low oxygen partial pressure, the oxide surfaces of all the ZrB₂-SiC-ZrC specimens bubbled.     

Phase evolution of a novel silicon-alumina-fused mullite-containing Ti2O3 refractory at 1300 °C in N2

A novel silicon-alumina-fused mullite-containing Ti₂O₃ composite refractory is prepared and sintered in the presence of solid carbon at 1300 °C in N₂. The sintered samples exhibit a functional gradient characteristic. The phase evolution can be described as follows: Passive and active oxidation of Si to form SiO₂ and SiO to reduce the partial pressure of oxygen. SiO(g) and Si react with N₂ to form Si₃N₄ respectively. As the temperature increases and the partial pressure of oxygen decreases, Ti₂O₃ reacts with CO and N₂ to form Ti(C,N)ss, which is accompanied by the release of O₂. Si₃N₄ fixes the O₂ and reacts to form Si₂N₂O, and Si₂N₂O reacts with Al₂O₃ to form O′-Sialon, thereby realizing the transformation from Si₃N₄ to Sialon. CO and residual carbon from the pyrolysis of phenolic resin react with SiO(s) and Si to form SiC. The dense layer formed by SiC and SiO₂ blocks the diffusion of external gas to the central parts of the samples, there is still free Si which can continue to react and transform into a non-oxide reinforcing phase. In this paper, the reaction models are presented.     

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.     

Effect of YSZ-incorporated glass-based composite coating on oxidation behavior of K438G superalloy at 1000 °C

A glass-based composite coating incorporating YSZ particles was prepared by sintering on K438G superalloy substrates. The YSZ additions increased the cyclic oxidation resistance at 1000 °C, while the formation of zircon resulting from interfacial reactions between YSZ and the glass matrix worked reversely. Besides, the YSZ inclusions changed the crystallization behavior of the glass matrix, and only anorthite precipitated during cyclic oxidation. Due to the synergy of sand-blasting and sealing effect of the glass-based coating, the oxidation behavior of K438G was changed and a layer of alumina instead of chromia formed at the substrate/coating interface. Furthermore, a gahnite layer formed at the alumina/gahnite interface because of interfacial reactions between alumina and the glass matrix, leading to the formation of a bi-layered thermally grown oxide. Thus, the alumina layer was protected from the attack of the active glass matrix. Accordingly, the coated K438G superalloy exhibited satisfactory oxidation resistance at 1000 °C.     

Oxyacetylene ablation of (Hf0.2Ti0.2Zr0.2Ta0.2Nb0.2)C at 1350 − 2050 °C

Ablation resistance of a multi-component carbide (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta)₂O₅, (Hf, Zr)TiO₄, and (Hf, Zr)O₂ were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O_x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics.     

Microstructure and mechanical properties of titanium aluminum carbides neutron irradiated at 400–700 °C

This work reports the first mechanical properties of Ti₃AlC₂-Ti₅Al₂C₃ materials neutron irradiated at ∼400, 630 and 700 °C at a fluence of 2 × 10²⁵ n m⁻² (E > 0.1 MeV) or a displacement dose of ∼2 dpa. After irradiation at ∼400 °C, anisotropic swelling and loss of 90% flexural strength was observed. After irradiation at ∼630–700 °C, properties were unchanged. Microcracking and kinking-delamination had occurred during irradiation at ∼630–700 °C. Further examination showed no cavities in Ti₃AlC₂ after irradiation at ∼630 °C, and MX and A lamellae were preserved. However, disturbance of (0004) reflections corresponding to M-A layers was observed, and the number density of line/planar defects was ∼10²³ m⁻³ of size 5–10 nm. HAADF identified these defects as antisite Ti_Al atoms. Ti₃AlC₂-Ti₅Al₂C₃ shows abrupt dynamic recovery of A-layers from ∼630 °C, but a higher temperature appears necessary for full recovery.     

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf_0.84Y_0.16O_1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m^-1 K^-1 at as-sprayed state to 1.6 W m^-1 K^-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.     

Hot corrosion behavior of CoWSi/WSi2 coating exposed to Na2SO4+NaCl salt at 900 °C

In this study, the high temperature hot corrosion behavior of a CoWSi/WSi₂ composite coating was investigated. Hot corrosion studies were performed on CoWSi/WSi₂ coated nickel specimens after exposure to a molten Na₂SO₄+NaCl salt environment at 900 °C under cyclic conditions. Thermogravimetric technique was used to establish the kinetics of corrosion. XRD and SEM/EDS techniques were used to analyze the corrosion products. The oxide scale formed on the coating surface was complex and the hot corrosion resistance of coating may be attributed to the formation of oxides and spinels of silicon, cobalt and tungsten. Also, NaCl accelerated the degeneration of the coating because of producing the volatile CoCl₂ and thereby oxygen and sulfur could easily penetrate into the coatings and caused the formation of internal oxide and sulfide.     

Microstructural evolution,swelling and hardening of CVD-SiC induced by He ions irradiation at 650 °C

In the present study, chemical vapor deposited (CVD) SiC samples were irradiated by 500 keV He ions to different doses at 650 °C. The microstructural evolution, surface swelling and nanohardness change caused by irradiation versus He ion dose were investigated using TEM, AFM and nanoindentation. Results showed that a high number density of He bubbles and dislocation loops were formed in the sample irradiated to the high dose, resulting in a swelling of 1.53%. Meanwhile, the hardness of the samples increased after He ions irradiation, which was attributed to the pinning effect of the irradiation induced defects and bubbles. The irradiation hardening degree increased and tended to be saturated with the increasing He ion dose.     

Hot corrosion behavior of YSZ/ZrW2O8 composites exposed to V2O5 at 700 °C and 850 °C in air

The hot corrosion behavior of YSZ/ZrW₂O₈ composites as a promising thermal barrier coating system exposed to V₂O₅ at 700 °C and 850 °C was investigated in order to better understand the influence of the incorporated ZrW₂O₈ with isotropic negative thermal expansion performance on the corrosion resistance. Results indicate that the ZrW₂O₈ incorporation could retard the degradation of YSZ from V₂O₅ attack and the corrosion process is significantly related to the inclusion content and the temperature. The corrosion resistance could be determined by the incorporation content, while the reaction products are only temperature dependent. At 700 °C, ZrV₂O₇, YVO₄ and m-ZrO₂ were the main corrosion products, while ZrW₂O₈ recrystallized under the acidic environment provided by V₂O₅. At 850 °C, ZrW₂O₈ decomposed and only WO₃, YVO₄ and m-ZrO₂ could be detected as final corrosion products. The corrosion mechanisms of YSZ/ZrW₂O₈ composites at 700 °C and 850 °C were discussed based on the phase diagrams and Lewis acid-base rule as well as the volume compensation of the positive and negative expansion ceramics.     

Influences of SiC infiltration and coating on compressive mechanical behaviours of 2D C/SiC composites up to 1600 °C at wide-ranging strain rates

The influences of the SiC infiltration and coating on the compressive mechanical behaviours of 2D C/SiC composites were determined up to 1600 °C at 0.001 and 1000/s strain rates in argon and air. In addition, the failure mechanisms responsible for the compressive mechanical behaviours were elucidated through in-situ observation and micro-analysis-based methods. The 2D C/SiC composite compressive strength was highly sensitive to temperature, loading rate, and oxidation, and was enhanced by the change in the thermal residual stress and decreased by oxidation. In argon, because of the extra infiltrated SiC matrix, SiC treated 2D C/SiC specimens exhibited higher compressive strengths and lower strain rate sensitivity factors than SiC untreated 2D C/SiC specimens. The SiC coating effectively improved the oxidation resistance of the 2D C/SiC composites in air, regardless of the temperature, strain rate, and oxidative damage-which depends on SiC coating, strain rate, and temperature.     

Isothermal phase diagram of CaO–SiO2–Nb2O5-5wt% Fe2O3–TiO2 system at 1200 °C

The lack of thermodynamic data such as the phase diagram of CaO–SiO₂–Nb₂O₅–Fe₂O₃–TiO₂ system has seriously hampered the comprehensive utilization of niobium, titanium and other resources in the Bayan Obo tailing. In this study, phase equilibria of the CaO–SiO₂–Nb₂O₅–Fe₂O₃–TiO₂ system at 1200 °C were investigated using high temperature equilibrium experiment for the first time, and the CaTiO₃–Ca₁₀Nb₂Si₆O₂₇–Ca₂SiO₄–Ca₂Nb₂O₇ solid phase coexistence region was determined. Afterwards, based on the high temperature equilibrium experiments, the liquidus surfaces of the liquid + CaTiO₃ and liquid + SiO₂ equilibrium coexistence regions in CaO–SiO₂–Nb₂O₅-5wt% Fe₂O₃–TiO₂ system at 1200 °C were calculated using mathematical methods of interpolation and fitting. Finally, the 1200 °C isothermal phase diagrams of CaO–SiO₂–Nb₂O₅-5wt%Fe₂O₃–TiO₂ system were plotted. The results of the study can provide theoretical guidance for the enrichment and extraction processes of niobium and titanium resources in the Bayan Obo tailing.     

Solubility limit of Si in YAG at 1700 °C in vacuum

Measurement of the solubility limit of Si in yttrium aluminum garnet (YAG-Y₃Al₅O₁₂) is crucial for understanding the mechanisms by which Si influences grain boundary mobility, and the mechanisms by which grain boundaries migrate. In the present work, the solubility limit of Si in YAG at 1700 °C in vacuum (5 × 10⁻⁶ Torr), which are the most common sintering temperature and environment for YAG, was measured for the first time. Measurements were conducted by wavelength dispersive spectroscopy (WDS), using polished YAG specimens with 3700 ppm Si (0.8 wt% SiO₂). Si content to ensure saturation with Si. The accuracy of the WDS result was confirmed by using a series of doped specimens and by comparing to inductively coupled plasma mass spectrometry ICP-MS results. The results indicate that the solubility limit of Si in YAG at 1700 °C (5 × 10⁻⁶ Torr), is 980 ± 60 ppm. The measured Si solubility was found to significantly depend on the cooling rate, where for furnace cooled specimens the measured Si solubility was 650 ± 60 ppm. A second phase in triple junction was repeatedly observed when higher content of Si was used, confirming this work results.     

Reaction of smectites with iron in aerobic conditions at 75 °C

The stability of six dioctahedral smectites with different crystal chemistry was studied in the presence of iron to simulate the possible reactions of clay minerals in contact with Fe canister in a nuclear waste repository. The batch experiments were performed at 75 °C for 35 days in air. The reaction products were examined by XRD, QXRD, FTIR, BWA analyses and UV–Vis spectroscopy. The reaction of bentonites with metal iron led to the consumption of iron and the formation of magnetite and 7-Å phyllosilicate. The original smectites were partially transformed from Al-rich to Fe-bearing. Appearance of the dioctahedral–trioctahedral domains in the octahedral sheets of smectites resulted in the partial destabilization and/or partial dissolution of the smectite structure indicated by the increase of the layer charge, splitting of the smectite particles and partial disruption of the smectite layers. Benefits of NIR spectroscopy were found in identification of FeMgOH and FeFeOH groups, which were not recognized in the MIR region of reacted iron–bentonite mixtures due to overlapping with other absorption bands. The comparison of the present study with the paper of Osacký et al. (2010) showed that smectites reacted differently with iron in air and nitrogen atmosphere. In the aerobic conditions more iron was consumed and less amount of magnetite was formed. The distinct experimental conditions (aerobic versus anaerobic) had the greatest effect on the layer charge and thickness of the smectite particles. Less pronounced changes of the layer charge and thickness of the smectite particles were observed for the iron–bentonite mixtures reacted in the aerobic conditions. It indicated that smectite destabilization was inhibited in the aerobic conditions.     

Ablation mechanisms of Ti3SiC2 ceramic at 1600 °C in nitrogen plasma flame

Present work showed that Ti₃SiC₂ ceramic has good ablation resistance at 1600 °C. The mass ablation rate and linear ablation rate after exposure to plasma flame for 120 s are ?0.23 mg/s and 5.58 μm/s, respectively. After ablation for 120 s, the micro-morphologies of cross section of the ablation fringe show that the ablation layer was divided into four-layers: an oxide outer layer, a porous layer, a decomposition layer, and the matrix layer. The formation of porous layer is mainly related to the Ti vacancies caused by the outward diffusion of Ti ions, and outward diffusion of gaseous SiO and CO. The dense and large grain-sized TiO₂ oxide layer is dismembered by the generated SiO₂ to form a fine-structured TiO₂ and SiO₂ mixture. The solid-liquid mixed oxide layer is quickly blown away under the shearing force of plasma flow, resulting in a significant increase of linear ablation rate.