An experimental approach to delineate the reaction mechanism of Ti3AlC2 MAX-Phase formation

A comprehensive reaction mechanism of Ti₃AlC₂ MAX-phase formation from its elemental powders while spark plasma sintering has been proposed. Microstructural evaluation revealed that Al-rich TiAl₃ intermetallic forms at around 660 °C once Al melts. Gradual transition from TiAl₃ to Ti-rich TiAl and Ti₃Al intermetallic phases occurs between 700 °C and 1200 °C through formation of layered structure due to diffusion of Al from periphery toward the centre of Ti particles. Formation of TiC and Ti₃AlC transient carbide phases were observed to occur through two different reactions beyond 1000 °C. Initially, TiC forms due to interaction of Ti and C, which further reacts with TiAl and Ti and gives rise to Ti₃AlC. Later, Ti₃AlC also forms due to diffusion of C into Ti₃Al above 1200 °C. Above 1300 °C, Ti₃AlC phase decomposes into Ti₂AlC MAX-phase and TiC in presence of unreacted C. Finally, Ti₂AlC and TiC reacts together to from Ti₃AlC₂ MAX-phase above 1350 °C and completes at 1500 °C.     

Microstructure and dielectric properties of perovskite-structured high-entropy ceramics of Pb(Zr0.25Ti0.25Sn0.25Hf0.25)O3

A dielectric high-entropy ceramic with a composition of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ was designed through B-site doping, and then prepared by solid phase reaction method combined with conventional sintering in air for 3 h at 1200 ^°C, 1250 ^°C and 1300 ^°C, respectively. All the high-entropy ceramics of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ possess a perovskite structure with uniform elemental distribution and their average grain size falls within the range of 3.19–5.5 μm. For the sample sintered at 1250 ^°C, the dielectric loss is less than 0.07 in the testing frequency of 1 kHz∼1 MHz in 30–350 ^°C, and the dielectric constant reaches a peak of 14356 at about 270 ^°C at 1 kHz. At room temperature, the remnant polarization P_r reaches 28.8 μC/cm². The results demonstrate that the high-entropy ceramic of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ has great potentials in the dielectric and ferroelectric field.     

Synthesis,sintering, and effect of surface roughness on oxidation of submicron Ti2AlC ceramics

Submicron Ti₂AlC MAX phase powder was synthesized by molten salt shielded synthesis (MS³) using a Ti:Al:C molar ratio of 2:1:0.9 at a process temperature of 1000°C for 5  hours. The synthesized powder presented a mean particle size of ~0.9 µm and a purity of 91 wt. % Ti₂AlC, containing 6 wt. % Ti₃AlC₂. The Ti₂AlC powder was sintered by pressureless sintering, achieving a maximal relative density of 90%, hence field-assisted sintering technology/spark plasma sintering was used to enhance densification. The fine-grained microstructure was preserved, and phase purity of Ti₂AlC was unaltered in the latter case, with a relative density of 98.5%. Oxidation was performed at 1200°C for 50 hours in static air of dense monolithic Ti₂AlC with different surface finish, (polished, ground and sandblasted) which resulted in the formation of an approx. 8 µm thin aluminum oxide (Al₂O₃) layer decorated with titanium dioxide (rutile, TiO₂) colonies. Surface quality had no influence on Al₂O₃ scale thickness, but the amount and size of TiO₂ crystals increased with surface roughness. A phenomenon of rumpling of the thermally grown oxide (TGO) was observed and a model to estimate the extent of deformation is proposed.     

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.     

(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics with low thermal conductivity

A novel high‐entropy carbide ceramic, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C, with a single‐phase rock salt structure, was synthesized by spark plasma sintering. X‐ray diffraction confirmed the formation of a single‐phase rock salt structure at 26‐1140°C in Argon atmosphere, in which the 5 metal elements may share a cation position while the C element occupies the anion position. (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C exhibits a much lower thermal diffusivity and conductivity than the binary carbides HfC, ZrC, TaC, and TiC, which may result from the significant phonon scattering at its distorted anion sublattice. (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C inherits the high elastic modulus and hardness of the binary carbide ceramics.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

The phase transformation and microstructure of TiAl/Ti2AlC composites caused by hot pressing

Two series of raw materials were adopted to form TiAl/Ti₂AlC composites: Ti/Al/TiC and Ti/Al/C. Differential thermal analysis (DTA) of starting powers and X-ray diffraction (XRD) of samples sintered at different temperatures from 600 °C to 1300 °C by hot pressing were utilized to analyze the phase transformation and the mechanism of synthesis. Scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) was utilized to investigate the morphology characteristics of the products. The experimental results showed that Ti reacted with Al to form TiAl intermetallics below 900 °C; and above 900 °C, TiAl reacted with TiC to produce dense TiAl/Ti₂AlC composites. The products sintered at 1200 °C had fine crystals and dense fibres, and the distribution of Ti₂AlC particles in TiAl matrix was homogeneous.     

Synthesis and sodium ionic conductivity for perovskite-structured Na0.25La0.25NbO3 ceramic

In this work, a perovskite-structured sodium ion conductor, Na_0.25La_0.25NbO₃ (NLNO) was developed from analogous Li_0.25La_0.25NbO₃ ceramic. NLNO ceramic was successfully synthesized by solid state reaction. The sodium ionic conduction in Na_0.25La_0.25NbO₃ ceramic was studied and the effect of sintering temperature on the microstructure, phase structure, density and sodium ionic conductivity for Na_0.25La_0.25NbO₃ was also discussed. Single phase of perovskite was successfully obtained from NLNO sintered at 1200 °C and 1250 °C, and the result shows high sintering temperature leads to a large grain size, large lattice parameters and high density. With an increase of sintering temperature from 1150 °C to 1250 °C, the conductivity of samples increases gradually. NLNO sintered at 1250 °C presents a high sodium ionic conductivity of 1.06 × 10^?5 S cm^?1 at 30 °C, which is much higher than that of electronic conductivity in NLNO sintered at 1250 °C.     

Effects of SiC nanowires and whiskers on high-entropy carbide-based composites sintered at low and high temperatures

This study aimed to investigate the toughening effects of SiC nanowires (SiC_nw) and SiC whiskers (SiC_w) on high-entropy carbide based composites prepared at different temperatures (1600°C and 2000°C). At low temperature (1600°C), SiC_nw and SiC_w maintain their original morphology and properties, and exhibit the good toughening effects. The SiC_nw with larger aspect ratio and more curly wires exhibit a much stronger toughening effect on the (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites reinforced with 15 vol.% SiC_nw, which shows the highest value of fracture toughness about 6.7 MPa∙m^1/2. However, at high sintering temperature (2000°C), SiC_nw and SiC_w are prone to thermal-induced damages, which significantly reduces their mechanical properties, and thus, toughening effects on (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites. The addition of SiC_w, which have better thermal stability at 2000°C, results in the (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8–15 vol.% SiC_w composite exhibiting relatively better fracture toughness, about 3.7 MPa∙m^1/2. Based on the results of the current study, the critical influence of SiC_nw and SiC_w on the toughening of (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites is highly dependent on their high-temperature thermal stability.     

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.     

A novel approach to the rapid synthesis of high-entropy carbide nanoparticles

A novel strategy for the rapid synthesis of high-entropy carbide particles is proposed that involves the transformation of multicomponent intermetallic intermediates to multicomponent carbides (high-entropy carbide precursors). (Ti_0.25V_0.25Nb_0.25Ta_0.25)C nanoparticles with a uniform solute distribution were successfully synthesized in an Al matrix by heating Al-Ti-V-Nb-Ta-C powder mixtures at 1500°C for 10 minutes. The multicomponent aluminide intermediates led to the rapid formation of multicomponent carbides during heating to 1100°C, which transformed into a high-entropy solid solution during heating to 1500°C. We developed a new rapid approach for the synthesis of high-entropy ceramic particles.     

ZrB2-based solid solution ceramics and their mechanical and thermal properties

Zirconium diboride ceramics as one of the main members of ultrahigh-temperature ceramics are capable of being used as structural components at ultrahigh temperatures. Entropy adjusting is a newly developed approach to improving the properties of ceramics. In this work, a series of ZrB₂-based solid solution ceramics with different mixing entropies, formulated (Zr_xTi_yNb_yTa_y)B₂ (x = .25, .85, .925, .9625, 1; x + 3y = 1), were prepared by adjusting the content of other diborides. Diboride solid solution powders were synthesized by boro/carbothermal reduction process and then densified by spark plasma sintering. The results show that the formation of a single-phase solid solution is independent of the mixing entropy in (Zr_xTi_yNb_yTa_y)B₂ system. The addition of other diborides into ZrB₂ is beneficial to reduce the particle size of the synthesized powder and promote the densification process. The dense sintered samples with higher mixing entropy have finer grain size, higher hardness, and modulus. The (Zr_0.25Ti_0.25Nb_0.25Ta_0.25)B₂ ceramic has the highest hardness of 31 GPa and a modulus of 682 GPa. Severe lattice distortion in samples with higher mixing entropy will result in increased phonon scattering and lower thermal conductivity.     

The effect of submicron grain size on thermal stability and mechanical properties of high-entropy carbide ceramics

(Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramics (HEC) with a submicron grain size of 400 to 600 nm were fabricated by spark plasma sintering using a two-step sintering process. Both X-ray and neutron diffractions confirmed the formation of single-phase with rock salt structure in the as-fabricated (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C samples. The effect of submicron grain size on the thermal stability and mechanical properties of HEC was investigated. The grain growth kinetics in the fine-grained HEC was small at 1300 and 1600°C, suggesting high thermal stability that was possibly related to the compositional complexity and sluggish diffusion in HEC. Compared to the coarse-grain HEC with a grain size of 16.5 µm, the bending strength and fracture toughness of fine-grained HEC were 25% and 20% higher respectively. The improvement of mechanical properties in fine-grained HEC may be attributed to micromechanistic mechanisms such as crack deflection.     

Synthesis of MAX phase-based ceramics from early transition metal hydride powders

MAX phase ceramics are typically prepared by the reactive sintering of elemental powders that are often coarse, expensive, and prone to oxidation. The temperature-driven dehydrogenation of metal hydride powders offers an alternative synthesis approach, as the hydrides decompose into phase-pure, dimensionally fine elemental powder particles. The increased reactivity of these in situ formed, fine powder particles drastically reduces the formation temperature of the antecedent intermetallic phases, without forming excess binary carbides or facilitating powder oxidation in the Ti-Al-C and Zr-Al-C systems. This work elucidates the effect of metal hydrides on the sequence of formation reactions in MAX phase ceramics. In the Zr-Al-C system, the use of coarse, oxidation-prone elemental Zr powders prevented MAX phase formation, whereas spark plasma sintering of ZrH₂ powders at 1500 °C produced ceramics containing 60 wt% Zr₃AlC₂. Similarly, in the Ti-Al-C system, spark plasma sintering of TiH₂ powders at 1200 °C produced phase-pure Ti₃AlC₂ ceramics.     

Sintering behavior of Cr2AlC MAX phase synthesized by Spark plasma sintering

In the current study, the sintering and mechanical properties of the Cr₂AlC MAX phase synthesized by Spark plasma sintering at 1000, 1100, and 1200°C were investigated. The X-ray diffraction (XRD) patterns showed that the synthesis of the Cr₂AlC MAX phase was associated with the presence of impurities such as Cr₇C₃ and Al₂O₃. On the basis of the FESEM images equipped with energy dispersive spectrometer, the MAX phases had been formed successfully and the length of these layers increased by enhancing the sintering temperature. The results of the density showed that by increasing the temperature, the density increases from 5.10 to 5.33 g/cm³ and finally decreases to 5.25 g/cm³. Vickers hardness method applied to determine the hardness of the samples showed that the hardness decreases from 8.52 to 8.07 GPa for the prepared samples.     

Low temperature fabrication of Cf/BNi/(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C-SiCm high entropy ceramic matrix composite by slurry coating and laminating combined with precursor infiltration and pyrolysis

In this study, the low temperature fabrication of a C_f/BN_i/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C-SiC_m high entropy ceramic (HEC) ceramic matrix composite (CMC) was achieved through slurry coating and laminating (SCL) combined with precursor infiltration and pyrolysis (PIP). Firstly, the (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C HEC powder was synthesized by pressureless sintering and ball milling. Then, a C_f/BN_i/HEC_m CMC preform was obtained by the SCL process. At last, the composite was densified by PIP of SiC at 1200 °C and a C_f/BN_i/HEC-SiC_m CMC was the final result. The density and open porosity of the HEC-CMC were 2.7 g/cm³ and 10%, respectively. The composite had a relatively high flexural strength (269 ± 25 MPa) and flexural modulus (53.3 ± 7.9 GPa). Fiber degradation was scarcely detected and fiber pullout was clearly observed. Most importantly, the fabrication method is simple and the fabrication temperature is rather low. This study opens a new insight for high entropy ceramic matrix composites fabrication.     

Synthesis and enhanced mechanical properties of compositionally complex MAX phases

A novel strategy of fabricating compositionally complex MAX phases was successfully developed. Multicomponent 413 MAX phase solid solutions (Ti_0.36Nb_0.27Ta_0.37)₄AlC_2.8 and (Ti_0.28Nb_0.26Ta_0.28V_0.18)₄AlC_2.9 simultaneously containing 3 and 4 transition-metal elements at M site were experimentally synthesized via hot pressing equimolar mixture of 211 type MAX phase powders. By elemental analysis and structural characterization, it can be verified that those uniform compositionally complex solid solutions can be obtained only in the presence of Cr₂AlC in raw powders. Those compositionally complex MAX phase solid solutions exhibit typical layered structures with distinct elongated grains. This discovery further enriches the MAX phases family and provide a new avenue for tailoring the properties of these materials. MAX phase composites containing around 87.5 vol.% (Ti_0.36Nb_0.27Ta_0.37)₄AlC_2.8 exhibit a high flexural strength of 720 MPa and a high fracture toughness of 9.5 MPa m^0.5.     

The synthesis and mechanical properties of high pure Ti2Al(Sn)C solid solution

High pure Ti₂Al_(1?x)Sn_xC (x = 0‐1) powders were synthesized using Ti, Al, Sn, and TiC powders as raw materials by pressureless sintering method. The influence of sintering temperature and raw material ratio on the purity of Ti₂AlC and Ti₂Al_0.8Sn_0.2C powders were investigated. The results show that pure Ti₂AlC and Ti₂Al_0.8Sn_0.2C powders were obtained from the mixed raw materials ratio of Ti:1.1Al:0.9TiC and Ti:0.9Al:0.2Sn:0.9TiC at 1450°C, respectively. Subsequently, fully dense Ti₂AlC and Ti₂Al_0.8Sn_0.2C bulks were prepared using mechanically alloying and hot pressed sintering method. The Vickers hardness of Ti₂AlC and Ti₂Al_0.8Sn_0.2C approaches approximately about 6 GPa and 4 GPa, the flexural strength was measured to be 650 ± 36 MPa and 521 ± 33 MPa, respectively. Microstructural analysis reveals that grain delamination, kink bands, and crack deflection occurred around the indentation area and at the fracture surface.     

Oxidation Behavior of MAX Phase Ti2Al(1−x)SnxC Solid Solution

MAX phase Ti₂Al_(1?x)Sn_xC solid solution with x = 0, 0.32, 0.57, 0.82, and 1 was synthesized by pressureless sintering of uniaxially pressed Ti, Al, Sn, and TiC powder mixtures. Annealing in air atmosphere at 200°C–1000°C triggered a sequence of oxidation reactions which reveal a distinct influence of solid solution composition on the oxidation process. With decreasing Al/Sn ratio, the characteristic temperature of accelerated oxidation reaction of A‐element was reduced from 900°C (x = 0) to 460°C (x = 1). SnO₂ was formed at temperatures significantly lower than TiO₂ (rutile) and Al₂O₃. Substitution of A‐element in MAX phase solid solution by low‐melting elements such as Sn may offer potential for reducing oxidation‐induced crack healing temperatures.     

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.