Precipitation induced crack healing in a Ti2SnC ceramic in vacuum

Self-healing ceramics are able to heal cracks through an oxidative healing mechanism at high temperature in oxidizing environments with the recovery of original performance and functionality. However, the oxidation induced repair may be impossible when the ceramics are used in vacuum or inert atmospheres with low oxygen partial pressures. So far little work has been done on crack healing in vacuum or inert atmospheres. Here we report on the crack healing of a Ti₂SnC ceramic in vacuum by a precipitation induced repair mechanism. Cracks induced by thermal shock in Ti₂SnC are completely filled by only metallic Sn at temperatures above 800 °C for only 1 h in vacuum. The electrical conductivity of healed materials is fully recovered, and it even exceeds the initial conductivity. This work might bring a new wave of research on crack healing behavior of ceramics in low oxygen partial pressure environments.     

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.     

Substitution behavior of Ag atoms in the Ti2AlC ceramic

In this study, the substitution behavior of Ag atoms in the Ti₂AlC ceramic was investigated using high‐angle annular dark field scanning transmission electron microscopy (HAADF‐STEM). The TEM sample of the Ag/Ti₂AlC interaction area was prepared following the wetting experiment, which was carried out at 1030°C for 5 minute using the sessile drop method. The Ag “single atoms” or “nanoclusters” were found in the crystal lattice of Ti₂AlC substrate. The characterizations along two typical crystallographic orientations suggest that both the Ti and Al atoms could be substituted by Ag atoms during the wetting process. In addition, the substitution behavior would contribute to the lattice expansion, which was coordinated by elastic distortion in the Ag nanoclusters with small size. However, when the Ag nanoclusters exceeded the critical size, the intact Ti₂AlC structure could no more sustain and defects like stacking faults formed, based on which the Ag nanotwins or polycrystals nucleated.     

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.     

Healing performance of Ti2AlC ceramic studied with in situ microcantilever bending

In situ microcantilever bending tests were carried out to evaluate the healing efficiency of pre-notched Ti₂AlC ceramic after annealing at 1200 °C for 1.5 h. Microcantilevers of different orientations were fabricated with focused ion beam method at different locations, i.e. in a singular Ti₂AlC grain, at a grain boundary or at the Ti₂AlC–Al₂O₃ interface after healing. Ti₂AlC microcantilever shows an anisotropic bending strength (ranging between 9.6 GPa and 4.6 GPa depending on the precise crystallographic orientations) that is closely related to the different atomic bonds in the layered structure. After healing, the Ti₂AlC–Al₂O₃ microcantilevers exhibit almost the same strength of about 5.2 GPa, i.e. slightly higher than the cleavage strength (4.6 GPa) of the initial Ti₂AlC microcantilevers. It suggests that the orientation of the matrix grain has no significant influence on the strength of healed microcantilevers. Furthermore, it turns out that the strength of the microcantilever with a healed grain boundary is at least twice the strength of the initial Ti₂AlC cantilever with a grain boundary. It is concluded that the oxidation dominated self-healing mechanism of Ti₂AlC ceramics can result in a perfect recovery of mechanical performance. The paper shows that the in situ microcantilever bending test provides a quantitative method for the evaluation of the strength of self-healing ceramics.     

热压烧结工艺制备Ti2AlC/Ti3AlC2陶瓷材料

以Ti，Al，C为原料，采用热压工艺制备出相组成为Ti2AlC/Ti3AlC2块体材料，合成材料的X—射线衍射和扫描电镜(SEM)分析的结果表明：当烧结温度为1400℃时，材料中的主晶相为Ti2AlC,大小为10μm的板状多晶体；而在1500℃的温度下烧结所得材料的主晶相为Ti3AlC2，其板状多晶体的晶粒尺寸平均约为20μm。     

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.     

Processing and thermal properties of SrTiO3 /Ti3AlC2 ceramic nanocomposites

Modulating the thermal conductivity has been a pragmatic approach for the development of high-performance thermoelectric material and thereby a step forward towards commercialization. Despite some efforts, the reduction in thermal conductivity of SrTiO₃ ceramic has not been fully realized. In this work, Ti₃AlC₂ in 3, and 7 vol% were uniformly incorporated in SrTiO₃ through nanostructured powder processing. The pristine SrTiO₃ and composites powders were consolidated by the spark plasma sintering at 1200 °C under uniaxial pressure of 50 MPa. Thermal properties of the bulk samples were evaluated from room temperature to 750 K through laser flash analysis. The thermal conductivity of SrTiO₃ based composites decreases substantially with the addition of nanostructured Ti₃AlC₂ from the pristine SrTiO₃ bulk sample. The reduction in thermal conductivity of 7 vol% composites is more than 30% at room temperature and even higher at elevated temperatures from the SrTiO₃. The interface thermal resistance was estimated which indicates a dominant role in diminishing the thermal conductivities of the composites. The results suggest that the addition of Ti₃AlC₂ as a second phase and nanostructuring through ball milling has significantly altered the phonon scattering mechanisms through multiple factors and thereby contributed to reducing effective thermal conductivities of the composites. This, work provide a scalable and economical route for the development of high-performance thermoelectric material.     

Reactive synthesis of porous Mo2Ti2AlC3 ceramic and its basic application properties

Porous Mo₂Ti₂AlC₃ was synthesized by reactive synthesis of the mixed powder of molybdenum, aluminum, titanium hydride and graphite at 1500 °C. The effects of sintering temperature on the phase transition and pore structure parameters of porous Mo₂Ti₂AlC₃ were deeply discussed, and the pore forming mechanism in the sintering process was further deduced. The results showed that the pore formation of porous Mo₂Ti₂AlC₃ consists of the following aspects: (i) At 500 °C, stearic acid was completely pyrolyzed; (ii) At 700 °C, titanium hydride was completely decomposed into titanium and hydrogen via endothermic reaction; (iii) The partial diffusion effect of aluminum in the formation of TiAl and Mo₃Al intermetallic compounds; (iv) The solid-solid reaction of titanium and molybdenum with graphite generated TiC and Mo₂C; (v) TiC, Mo₂C, Ti₂AlC, Mo₃Al and graphite further reacted to form Mo₂Ti₂AlC₃,where pore formation was mainly controlled by (iii) and (v). In addition, the corrosion resistance and mechanical properties of porous Mo₂Ti₂AlC₃ were also explored.     

Mechanical and oxidation behavior of textured Ti2AlC and Ti3AlC2 MAX phase materials

Highly textured Ti₂AlC and Ti₃AlC₂ ceramics were successfully fabricated by a two-step fabrication process, and the Lotgering orientation factors for {00l} planes of textured Ti₂AlC and Ti₃AlC₂ were calculated as 0.82 and 0.71, respectively. The effect of texturing was evaluated in terms of elastic modulus and hardness by macro- and micro-indentation. Moreover, the oxidation behavior of the MAX phases was investigated at 1300 °C in air, revealing that the oxidation was markedly anisotropic, where the textured side surface exhibited much better oxidation resistance, resulting from the rapid diffusion of Al element within its basal planes to form a protective Al₂O₃ scale on it. Furthermore, Ti₂AlC had larger difference regarding oxidation behavior between the top and side surface than Ti₃AlC₂, correlated to its higher Al ratio, leading to higher texturing degree and more diffusion pathways to the outer surface to produce an Al₂O₃ layer already at the initial oxidation stage.     

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.     

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.     

Comparative investigation of ultrafast thermal shock of Ti3AlC2 ceramic in water and air

In this work, ultrafast thermal shock of Ti₃AlC₂ ceramic was evaluated in water and air by utilizing the induction heating method. First, the annealed samples were heated to the set temperature in tens of seconds and dropped into the cooling water within 0.1 s which is rather short not to degrade the sample temperature. Compared to the traditional thermal shock method when quenching in water, the abnormal thermal shock phenomenon did not occur, which is owing to that no dense oxide layers were formed on the samples’ surface to act as the thermal barrier. The continuous decrease in residual flexural strength when quenched in water is associated with water infiltration, chemical reaction, and large surface tensile stress. The residual strength has 27.25 MPa upon 1250°C. Second, at the same testing temperature, the residual flexural strength when quenched in air maintains a high value of 388 MPa up to 1400°C. Dense oxide scales existed on the quenched surface of Ti₃AlC₂ samples. The results exhibit that Ti₃AlC₂ ceramic possesses excellent thermal shock resistance in water and air, suitable to be applied in extreme environments.     

Ti-Al-C三元系统中Ti3AlC2,Ti2AlC和TiC物相含量的测定

研究了Ti-Al-C三元体系中X射线衍射定量相分析方法,提出了Ti3AlC2,Ti2AlC和TiC这3种物相的特征衍射峰Ti3AlC2的(002)衍射峰,2θ=9.5°;Ti2AlC的(002)衍射峰,2θ=13.0°;TiC的(111)衍射峰,2θ=35.9°.采用混样无标样法推导了一套Ti3AlC2,Ti2AlC和TiC的定量计算公式,根据其特征衍射峰的强度就可计算这3个物相的相对含量.因为该公式推导过程本身就验证了其正确性(即自验证性),所以这个公式可以广泛用于Ti-Al-C三元体系中Ti3AlC2,Ti2AlC和TiC物相的定量测定.因此,这种方法非常简便、易行.     

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.     

Investigations on Radiation Tolerance of Mn+1AXn Phases: Study of Ti3SiC2, Ti3AlC2, Cr2AlC,Cr2GeC,Ti2AlC,and Ti2AlN

Nanolaminated M_n+1AX_n phases as candidate materials for next generation nuclear reactor applications show great potential in tolerating radiation damage. However, different M_n+1AX_n materials behave very differently when exposed to energetic neutron and ion irradiations. Based on first‐principle calculations, the radiation tolerance of two M₃AX₂ and four M₂AX phases were studied in this work, covering all the M_n+1AX_n phases previously investigated with experiments. We have calculated the formation energies of Frenkel pairs and antisite pairs in these materials. The improved radiation tolerance from Ti₃AlC₂ to Ti₂AlC observed by experiments can be understood in terms of different Al/TiC layer ratio as the A atomic plane in the nanolaminated crystal M_n+1AX_n accommodates radiation‐induced point defects. The formation of M_A–A_M antisite pair in M_n+1AX_n materials would provide an alternative way to accommodate the defects resulted from radiation damage cascades, whereas this ideal substitution channel does not exist for Cr₂GeC due to its pronouncedly higher M_A–A_M antisite pair formation energy. To further elucidate their radiation damage tolerance mechanism, we have made a detailed analysis on their interatomic M–X, M–A, and X–A bonding characters. Criteria based on the bonding analysis are proposed to assess the radiation tolerance of the six M_n+1AX_n materials, which can be further applied to explore other M_n+1AX_n phases with respect to their performances under radiation environment.     

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.     

三元层状Ti2AlC陶瓷材料的合成制备研究

以Ti，Al，C微粉为原料，分别采用放电等离子(SPS)烧结工艺和热压工艺(HP)烧结制备Ti2AlC层状陶瓷材料。X射线衍射和扫描电镜(SEM)分析的结果表明：采用放电等离子(SPS)烧结工艺时，能够在1100℃的温度下制备高纯、致密Ti2AlC材料。采用热压工艺时，则很难合成高纯的Ti2AlC陶瓷材料。     

Autonomous high‐temperature healing of surface cracks in Al2O3 containing Ti2AlC particles

In this work, the oxidation‐induced crack healing of Al₂O₃ containing 20 vol.% of Ti₂AlC MAX phase inclusions as healing particles was studied. The oxidation kinetics of the Ti₂AlC particles having an average diameter of about 10 μm was studied via thermogravimetry and/or differential thermal analysis. Surface cracks of about 80 μm long and 0.5 μm wide were introduced into the composite by Vickers indentation. After annealing in air at high temperatures, the cracks were filled with stable oxides of Ti and Al as a result of the decomposition of the Ti₂AlC particles. Crack healing was studied at 800, 900, and 1000°C for 0.25, 1, 4, and 16 hours, and the strength recovery was measured by 4‐point bending. Upon indentation, the bending strength of the samples dropped by about 50% from 402 ± 35 to 229 ± 14 MPa. This bending strength increased to about 90% of the undamaged material after annealing at 1000°C for just 15 minutes, while full strength was recovered after annealing for 1 hour. As the healing temperature was reduced to 900 and 800°C, the time required for full‐strength recovery increased to 4 and 16 hours, respectively. The initial bending strength and the fracture toughness of the composite material were found to be about 19% lower and 20% higher than monolithic alumina, respectively, making this material an attractive substitute for monolithic alumina used in high‐temperature applications.     

Ti3AlC2的制备与微观结构

热压烧结不同原料制备Ti3AlC2,结果表明热压烧结Ti,Al,C原始粉很难得到高纯致密的Ti3AlC2.当以TiC代替C和部分Ti,并掺加摩尔比为0.1～0.2的Si时,可以得到致密的单相材料.掺加的Si均匀分布在基体中,形成固溶体Ti2.76Al078Si0.22C2.烧结试样的晶体为层片状结构,1 300℃和1 400℃烧结试样的晶粒尺寸分别为6～8 μm和15～20 μm.材料的vicker硬度为3.3～5.0 GPa,弹性模量为289GPa,抗压强度为785 MPa,抗弯强度为375 MPa,断裂韧性为7.0 MPa·m1/2.