Helium effects and bubbles formation in irradiated Ti3SiC2

Ti₃SiC₂ is a potential structural material for nuclear reactor applications. However, He irradiation effects in this material are not well understood, especially at high temperatures. Here, we compare the effects of He irradiation in Ti₃SiC₂ at room temperature (RT) and at 750 °C. Irradiation at 750 °C was found to lead to extremely elongated He bubbles that are concentrated in the nano-laminate layers of Ti₃SiC₂, whereas the overall crystal structure of the material remained intact. In contrast, at RT, the layered structure was significantly damaged and highly disordered after irradiation. Our study reveals that at elevated temperatures, the unique structure of Ti₃SiC₂ can accommodate large amounts of He atoms in the nano-laminate layer, without compromising the structural stability of the material. The structure and the mechanical tests results show that the irradiation induced swelling and hardening at 750 °C are much smaller than those at RT. These results indicate that Ti₃SiC₂ has an excellent resistance to accumulation of radiation-induced He impurities and that it has a considerable tolerance to irradiation-induced degradation of mechanical properties at high temperatures.     

The damage evolution of He irradiation on Ti3SiC2 as a function of annealing temperature

The radiation damage response of Ti₃SiC₂ irradiated by 110 keV helium ions at room temperature (RT), the subsequent evolution of damage including helium bubble growth as a function of annealing temperatures are investigated using grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy and transmission electronic microscopy (TEM). In addition to collision cascade effects leading to TiC nanocrystal formation near the surface of Ti₃SiC₂, He ion irradiation produces damage due to the growth of He bubbles, which cause a structural transformation into a large grain TiC crystalline phase at high temperatures. The displacement of matrix Si atoms adjacent to the He bubbles along the Si layer in Ti₃SiC₂ either via bubble growth or the production of inter-bubble fracture is the reason for the structural transformation. Depending on the He damage level, a significant recovery of the He irradiation damage can occur at moderate temperatures. This property may play a positive role in the damage resistance of Ti₃SiC₂, making it a potential candidate for future nuclear reactor applications.     

Effect of controllable decomposition of MAX phase (Ti3SiC2) on mechanical properties of rapidly sintered polycrystalline diamond by HPHT

Sintered polycrystalline diamond (PCD) was prepared via high temperature and high pressure (HTHP) process using Ti₃SiC₂ and Si as the binder. The effect of decomposition of Ti₃SiC₂ on comprehensive properties of PCD was investigated at different temperatures. Results show that Si formed liquid phase infiltrated diamond surface at high temperatures to inhibit diamond graphitization and to reduce defects significantly. In addition, strong covalent bond of Si-diamond was generated, which increased the strength of PCD. At suitable temperature, Ti₃SiC₂ would partially decompose to TiC and SiC with high activity. Strong covalent bond of TiC and SiC also increased relative density and hardness of PCD, and residual Ti₃SiC₂ enhanced the toughness of PCD. At 1500 °C, the hardness and toughness of PCD reached 54.35 GPa and 8.6 MPa m^1/2, respectively, which are 19% and 82%, respectively, higher than PCD sintered at 1350 °C.     

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).     

Fast seamless joining of SiCw/Ti3SiC2 composite using electric field-assisted sintering technique

A pair of Ti₃SiC₂ reinforced with SiC whiskers (SiC_w/Ti₃SiC₂) composites was successfully joined without any joining materials using electric field-assisted sintering technology at a temperature as low as 1090°C (T_i) and a short time of 30 s. The microstructure and mechanical properties of the obtained SiC_w/Ti₃SiC₂ joints were investigated. The solid-state diffusion was the main joining mechanism, which was facilitated by a relatively high current density (~586 A/cm²) at the joining interface. The shear strength of the sample joined at 1090°C was 51.8 ± 2.9 MPa. The sample joined at 1090°C failed in the matrix rather than at the interface, which confirmed that a sound inter-diffusion bonding was obtained. A rapid and high efficient self-joining process may find application in the case of SiC_w/Ti₃SiC₂ sealing cladding tube and end cap.     

Helium-driven element depletion and phase transformation in irradiated Ti3SiC2 at high temperature

Future nuclear reactors and advanced power generators require materials with good stability and damage tolerance under harsh conditions, including high temperatures and high-dose radiation. Ti₃SiC₂ MAX phase has good physical properties and mechanical strength. It can remain crystalline under serious microstructure damage due to the nanolaminate structure. In this study, the effects of helium in irradiated Ti₃SiC₂ at up to 1100 °C were investigated by microstructural and chemical composition analysis. The concentrated helium can grow into large bubbles without significant confinement or capture by the nano-laminated layers. A new hexagonal to fcc phase transformation mechanism, driven mainly by the evolution of the helium bubbles accompanied by Si diffusion and depletion, is found and investigated. Si interstitials are forced to move out from the peak helium region by the helium evolution and segregate at the outermost surface, forming a thin Si-O layer, at 1100 °C. The formation of the fcc phase is the result of chemical compositional changes and local compressive stress contributed by He bubbles.     

Phase Evolution of Ti3SiC2 Annealing in Vacuum at Elevated Temperatures

The thermal decomposition of Ti₃SiC₂ in vacuum furnace up to 1500°C has been investigated. The results show that the mild decomposition of Ti₃SiC₂ commences at 1300°C and the higher the holding temperature, the larger the volatilization of Si atoms. The Ti₃SiC₂ decomposition occurs simultaneously on the surface and in the bulk. Four phases coexist at 1400°C and 1450°C and the Ti₅Si₃C_x phase appears in the bulk and/or surface. Diffusion distance, rate, and volatilization of Si contribute to the porous structure and the presence of Ti₅Si₃C_x. The evolution of furnace pressure reflects the decomposition kinetics of Ti₃SiC₂.     

Green synthesis,formation mechanism and oxidation of Ti3SiC2 powder from bamboo charcoal,Ti and Si

Using low-cost and highly reactive bamboo charcoal, Ti and Si elemental powders as starting materials, Ti₃SiC₂ powder was synthesized via a simple and cost-efficient pressureless sintering technique in argon atmosphere. The influences of synthesis temperature, holding time and Si content on the Ti₃SiC₂ content of the synthesized products were investigated, and the analysis indicated that the relative content of Ti₃SiC₂ reached 98.9 wt% with a molar ratio of 3Ti/1.2Si/2.2C at 1400 °C for 1.5 h. The Ti₃SiC₂ with good crystallinity and homogeneous nanolayered structure was synthesized at lower temperatures due to the high reactivity and high specific surface area of bamboo charcoal. The non-isothermal oxidation behavior showed that Ti₃SiC₂ powder was stable in air below 540 °C. With the temperature increasing up to 1300 °C, continuous and dense TiO₂ and SiO₂ oxidation layers were formed on the surface of Ti₃SiC₂ particles, which conferred good oxidation resistance to Ti₃SiC₂ powder.     

Machinability of Ti3SiC2 with layered structure synthesized by hot pressing mixture of TiCx and Si powder

Nanolaminate Ti₃SiC₂ was synthesized from a mixture of TiC_x (x = 0.67)/Si powder by hot pressing to increase machinability. Ti₃SiC₂ was synthesized at temperatures of 1360 °C and 1420 °C for 90 min under a pressure of 25 MPa. The X-ray diffraction results showed that while mainly Ti₃SiC₂ with some unreacted TiC_x were detected in the synthesized samples at 1360 °C, no phases except Ti₃SiC₂ phases remained in the synthesized samples at 1420 °C. The cutting resistance of Ti₃SiC₂ was measured in terms of the principle, feed, and thrust forces and was compared with that of middle-carbon steel, SM45C. The values of the principal force of the synthesized Ti₃SiC₂ were lower than those of SM45C. After machining, the roughness of the Ti₃SiC₂ was lower than those of SM45C; however, the damage to the tool bit used for the machining of SM45C was less than the damage to those used for the machining of the Ti₃SiC₂.     

Microstructure and mechanical strength of transient liquid phase bonded Ti3SiC2 joints using Al interlayer

Based on the structure characteristic of Ti₃SiC₂ and the easy formation of Ti₃Si_1−xAl_xC₂ solid solution, a transient liquid phase (TLP) bonding method was used for bonding layered ternary Ti₃SiC₂ ceramic via Al interlayer. Joining was performed at 1100–1500 °C for 120 min under a 5 MPa load in Ar atmosphere. SEM and XRD analyses revealed that Ti₃Si(Al)C₂ solid solution rather than intermetallic compounds formed at the interface. The mechanism of bonding is attributed to aluminum diffusing into the Ti₃SiC₂. The strength of joints was evaluated by three point bending test. The maximum flexural strength reaches a value of 263 ± 16 MPa, which is about 65% of that of Ti₃SiC₂; for the sample prepared under the joining condition of 1500 °C for 120 min under 5 MPa. This flexural strength of the joint is sustained up to 1000 °C.     

Ti3SiC2 interphase coating in SiCf/SiC composites: Effect of the coating fabrication atmosphere and temperature

The SiC fibers were coated with Ti₃SiC₂ interphase by dip-coating. The Ti₃SiC₂ coated fibers were heat-treated from 900 °C to 1100 °C in vacuum and argon atmospheres to comparatively analyze the effect of temperature and atmosphere on the microstructural evolution and mechanical strength of the fibers. The results show that the surface morphology of Ti₃SiC₂ coating is rough in vacuum and Ti₃SiC₂ is decomposed at 1100 °C. However, in argon atmosphere, the surface morphology is smooth and Ti₃SiC₂ is oxidized at 1000 °C and 1100 °C. At 1100 °C, Ti₃SiC₂ oxidized to form a thin layer of amorphous SiO₂ embedded with TiO₂ grains. Meanwhile, defects and pores appeared in the interphase scale. As a result, the fiber strength treated in the argon was lower than that treated in vacuum. The porous Ti₃SiC₂ interphase fabricated under vacuum was then employed to prepare the SiC_f/SiC mini composite by chemical vapor infiltration (CVI) combined with precursor infiltration pyrolysis (PIP), and can effectively improve the toughness of SiC_f/SiC mini composite. The propagating cracks can be deflected within the porous interphase layer, which promotes fiber pull-outs under the tensile strength.     

High-temperature hydrogenation behaviour of bulk titanium silicon carbide

Thermal stability of Ti₃SiC₂ was investigated at 1200–1400°C in hydrogen atmosphere for 3 hours. The hydrogenation mechanism was clarified by a combination of X-ray diffraction, scanning electron microscope, Raman spectroscopy and first principles calculation. At 1200°C, a dense and uniform TiSi₂ layer formed on the sample surface, which originated from both the preferable lose of silicon from the Ti₃SiC₂ substrate and the dissociation of Ti₃SiC₂. As temperature increased to 1300°C, TiSi₂ layer began to scale off and presented laminated Ti₃SiC₂ grains beneath this layer, which indicated preferential hydrogenation occurred along the basal planes. This phenomenon was ascribed to the fact that the introduction of H interstitial atom weakened the combination between titanium and silicon interface layer, which was confirmed by first principles calculations. In addition, the formation of TiSi₂ owing to the dissociation of Ti₃SiC₂ caused the volume expansion after hydrogenation, resulting in that majority of TiSi₂ layer spelled off at 1400°C.     

Tribological behavior of Ti3SiC2 and Ti3SiC2/Pb composites sliding against Ni-based alloys at elevated temperatures

The Ti₃SiC₂ and Ti₃SiC₂/Pb composites were tested under dry sliding conditions against Ni-based alloys (Inconel 718) at elevated temperatures up to 800 °C using a pin-on-disk tribometer. Detailed tribo-chemical changes of Pb on sliding surface were discussed. It was found that the tribological behavior were insensitive to the temperature from 25 °C (RT) to 600 °C (friction coefficient ≈0.61–0.72, wear rate ≈10⁻³ mm³ N m⁻¹). An amount of Pb in the composites played a key role in lubricating with the temperature below 800 °C. The friction coefficient (≈0.22) and wear rate (≈10⁻⁷ mm³ N m⁻¹) at elevated temperatures were both decreased by the added PbO. The wear mechanisms of Ti₃SiC₂/Pb-Inconel 718 tribo-pair at elevated temperatures were believed to be the combined effect of abrasive wear and tribo-oxidation wear. During the sliding, two oxidization reactions proceed, 2Pb+O₂=2PbO (below 600 °C) and 6PbO+O₂=2Pb₃O₄ (800 °C). The friction coefficient and wear rate of the composites were reduced due to the self-lubricating effect of the tribo-oxidation products.     

Experimental investigation and mathematical modelling of water vapour corrosion of Ti3SiC2 and Ti2AlC ceramics and their mechanical behaviour

Commercially available Ti₃SiC₂ and Ti₂AlC ceramics were used in this study to investigate their wet corrosion and mechanical behaviour as they were under investigation for years for their applications in the field of nuclear as cladding materials and aerospace. The test coupons of dimension 3 × 4 × 40 mm³ and 3 × 4 × 20 mm³ were machined out from commercially available samples for the 3-pt bend test and wet corrosion test, respectively. The water vapour corrosion studies of these samples were carried out at 800 ℃, 1000 ℃, 1200 ℃ for 10, 20 and 100 h in gas flow condition containing 50 % steam + 50 % air. Phase analysis of the as-received Ti₃SiC₂ and Ti₂AlC ceramics revealed the presence of other impurity phases such as TiC and TiSi₂. The XRD patterns of the oxidised samples show the formation of rutile as the major phase in both materials. The oxidation layer formed on Ti₃SiC₂ sample was measured to be 280 μm after exposing the sample in steam for 100 h at 1200 °C. The water vapour corrosion studies reveal that Ti₂AlC has high oxidation resistance compared with the Ti₃SiC₂ due to the formation of protective layers of TiO₂ and Al₂O₃ which resulted in reduced weight gain and oxidation layer thickness. Three-point bend tests were conducted at room temperature for the samples after the water vapour corrosion test at 1000 °C/100 h. The TAC samples showed no degradation in the bending strength (244 MPa) whereas the TSC samples showed reduced strength of 320 MPa. The tensile strength of the samples was measured at room temperature and hydrothermal condition (250 °C and 250 bars pressure) and it was observed that Ti₃SiC₂ had high tensile strength (190 MPa) in hydrothermal conditions. The tensile strength results were validated using Finite element analysis (FEA) using ANSYS and the FEA results showed a negligible variance of 7 % compared with experimental method. Mathematical modelling based on one dimensional solution of diffusion equation combined with Deal-Grove model was employed to study and compare the oxidation thickness for the linear and parabolic models for the ceramics. The model was effective in validating the oxidation thickness of Ti₃SiC₂ showing that the experimental thickness was closer to that of mathematical model.     

In Situ Neutron Powder Diffraction Study of Ti3SiC2 Synthesis

The synthesis of Ti₃SiC₂ by pressureless reactive sintering of Ti/SiC/C mixtures under an Ar atmosphere has been studied using in situ neutron diffraction. The intermediate phases TiC_x and Ti₅Si₃C_x (x≤ 1) form first at ∼800–1400°C. These phases are consumed in the formation of Ti₃SiC₂, at ∼1500°C. After sintering, Ti₅Si₃C_x disappears but an amount of TiC_x remains in the sample primarily as a surface layer. The studies appear to support a suggestion that the intermediate phases react to form Ti₃SiC₂ through a diffusion-controlled process. Prolonged stepwise heating under argon in some experiments resulted in decomposition of Ti₃SiC₂ above ∼1400°C and significant disproportionation of the sample.     

Improving thermal stability of SiC whiskers via nanoscale Ti3SiC2 coatings

Effects of SiC whiskers (SiC_w) on the mechanical properties of composites largely depend on their thermal stability at high temperature. In this study, pure SiC_w and Ti₃SiC₂ coated SiC_w were thermal treated at 1600–1800°C for 1 h. Their phase assemblage, morphology, and structural evolution were investigated. Oxygen partial pressures in the graphite furnace resulted in the breakdown of SiC_w into particles at 1600°C, and the degradation became more pronounced with temperature increasing. The thermal stability of SiC whiskers at 1600–1700°C was significantly improved by a thin Ti₃SiC₂ coating on them, as both thermodynamic calculations and experimental observations suggest Ti₃SiC₂ coating could be preferentially oxidized/decomposed, prior to the active oxidation of SiC. At 1800°C, the protective role of the coating on the whiskers became weakened. SiC was converted into gaseous SiO and CO, with the remaining of interconnected TiC micro-rods and amorphous carbon.     

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.     

Al alloy/Ti3SiC2 composites fabricated by pressureless infiltration with melt-spun Al alloy ribbons

Al alloy/Ti₃SiC₂ composites with compressive strengths ranging from 743 to 932 MPa have been successfully fabricated by a new two-step pressureless infiltration method. 6061 Al alloy ribbons prepared by melt spinning were employed as the Al alloy matrix for melt infiltration. Shifts in phase constitution and reaction mechanisms of Ti₃SiC₂ preforms in molten Al at 950 °C were investigated, and the compression performance of Al alloy/Ti₃SiC₂ composites was tested. The Vickers hardness of the composites was enhanced to a maximum of 751 HV by increasing the Al content.     

High temperature properties of the monolithic CVD β-SiC materials joined with a pre-sintered MAX phase Ti3SiC2 interlayer via solid-state diffusion bonding

Monolithic high purity CVD β-SiC materials were successfully joined with a pre-sintered Ti₃SiC₂ foil via solid-state diffusion bonding. The initial bending strength of the joints (∼ 220 MPa) did not deteriorate at 1000 °C in vacuum, and the joints retained ∼ 68 % of their initial strength at 1200 °C. Damage accumulation in the interlayer and some plastic deformation of the large Ti₃SiC₂ grains were found after testing. The activation energy of the creep deformation in the temperature range of 1000 – 1200 °C in vacuum was ∼ 521 kJmol⁻¹. During the creep, the linkage of a significant number of microcracks to form a major crack was observed in the interlayer. The Ti₃SiC₂ interlayer did not decompose up to 1300 °C in vacuum. A mild and well-localized decomposition of Ti₃SiC₂ to TiC_x was found on the top surface of the interlayer after the bending test at 1400 °C in vacuum, while the inner part remained intact.     

Interface structure and wetting behaviour of Cu/Ti3SiC2 system

Wetting behaviour of a Cu/Ti₃SiC₂ system was investigated by the sessile drop technique under a vacuum atmosphere. Contact angles between Cu and Ti₃SiC₂ changed from 95 to 15° as temperatures increased from 1089 to 1270°C. Two distinct reaction layers consisting of different contents of Cu, TiC_x, Ti₃SiC₂ and Cu_xSi_y intermetallics were formed at the interface of Cu and Ti₃SiC₂. The formation of the interface layers contributes to the improvement of the wettability of the system. The dissolution of Si from theTi₃SiC₂ into the molten Cu at high temperature plays a dominant role in the wetting behaviour of Cu/Ti₃SiC₂ systems.