Insights into High‐Temperature Uniaxial Compression Deformation Behavior of Ti3AlC2

We report on strain‐rate‐dependent compression deformation behavior of Ti₃AlC₂ at 1000°C–1200°C. At 1000°C and high strain rate (10^?2 or 10^?3 s^?1), Ti₃AlC₂ deforms in a nonplastic manner. Upon increasing temperature and reducing strain rate, Ti₃AlC₂ exhibits a limited plasticity. For instance, the true plastic strain at 1200°C and 10^?4 s^?1 is only 3%, beyond which strain softening following a short hardening regime occurs. The softening results from the formation of localized microvoids and microcracks. Decreasing the strain rate further to 10^?5 s^?1 at 1200°C, strain hardening instead of softening is identified. Under such conditions, the plastic strain remarkably increases, reaching a value as high as 27%. Postdeformation microstructural analyses of the dislocation configurations explicitly evidence the dislocation reactions, formation of hexagonal dislocation networks and dislocation entanglements. These account for the strain hardening. The extraordinary plasticity at 1200°C and 10^?5 s^?1 benefits from the initiation of nonbasal slip systems. Finally, a complete high‐temperature deformation scenario for nanolaminated Ti₃AlC₂ is elaborated.     

Ti3AlC2材料的制备及其高温抗氧化性能研究

以TiC粉、Ti粉和Al为原料,按TiC:Ti:Al=2:1:1(原子比)混合,采用真空热压烧结法制备Ti3AlC2材料.XRD和SEM分析表明,合成产物中几乎全为Ti3AlC2相,TiC含量极少,层片状的Ti3AlC2发育良好,晶粒细小,分布均匀.该材料在900 ℃宅气中断续氧化30 h后的氧化动力学遵循抛物线规律,氧化层主要成分为TiO2和Al2O3,与基体粘结紧密,起到良好的保护作用.     

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.     

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.     

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。     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

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.     

Oxidation and Crack Healing Behavior of a Fine‐Grained Cr2AlC Ceramic

This work reports the oxidation and crack healing behavior of a fine‐grained (~2 μm) Cr₂AlC MAX phase ceramic. The oxidation behavior was investigated in the temperature range 900°C–1200°C for times up to 100 h. The material showed a good oxidation resistance, owing to the formation of a dense and thin α‐Al₂O₃ layer. The microstructure, composition and thickness of the oxide scale were characterized. Its oxidative crack healing behavior as a function of temperature, healing time, and initial crack size was studied systematically. The material showed excellent healing behavior. The main crack healing mechanism is the filling of the crack by oxides well adhering to the crack faces. The crack geometry before and after healing was characterized by X‐ray tomography. Three‐point bend tests showed the dependence of strength recovery at 1100°C as a function of initial crack length and healing time.     

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.     

Improving the High‐Temperature Oxidation Resistance of Nb4AlC3 by Silicon Pack Cementation

Niobium aluminum carbide (Nb₄AlC₃), as a member of the MAX phases, can retain its stiffness and strength up to over 1400°C. However, its applications are limited due to its poor oxidation resistance at high temperatures. In this work, silicon pack cementation has been applied to improve the oxidation resistance of Nb₄AlC₃. After Si pack cementation at 1200°C for 6 h, a dense and uniform silicide coating which was mainly composed of NbSi₂ and SiC and well bonded to the matrix was successfully formed on the surface of Nb₄AlC₃. The Si pack cemented Nb₄AlC₃ shows excellent oxidation resistance up to 1200°C due to the formation of protective Al₂O₃ layer. The oxidation kinetics of the cemented Nb₄AlC₃ obey parabolic law all the way to up to 1200°C, and the parabolic rate constants of cemented Nb₄AlC₃ are in the same order of magnitude as those of Ti₃AlC₂ in the temperature range 1000°C–1200°C. However, the oxidation of the cemented Nb₄AlC₃ was accelerated after oxidation at 1300°C for about 15 h due to the formation of NbAlO₄.     

Ti3SiC2/SiC复相陶瓷的抗氧化性研究

利用热等静压原位合成技术制备了Ti3SiC2/SiC复相陶瓷,对其高温氧化行为进行了研究.结果表明,Ti3SiC2/SiC复相陶瓷在空气中静态氧化时的氧化增重符合抛物线规律,有比纯Ti3SiC2更好的抗氧化性能,并且在1400℃的长时抗氧化性能优于1200℃.     

Influence of Ti3AlC2 content and load on the tribological behaviors of Ti3AlC2p/Al composites

In this study, Ti₃AlC₂ particles doped aluminum matrix composites were prepared by ultrasonic agitation casting method. Microstructure, mechanical properties, and tribological properties of pure aluminum and Ti₃AlC_2p/Al composites were characterized. Influence of different loads (10, 20, 30, and 40 N) and Ti₃AlC₂ contents (1.0, 2.0, 3.0, and 4.0 wt%) on the tribological behaviors of the composites were studied. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy dispersion spectroscopy (EDS), and 3D laser confocal were used to assist the analysis. The results indicated that fine and uniformly microstructure and the optimum comprehensive mechanical properties were exhibited on 2.0 wt%-Ti₃AlC_2p/Al composites. The abrasive grooves were widened and deepened with an increase in the load. The abrasion performance of composites improved distinctly with the addition of the Ti₃AlC₂ particles, which changed the wear mechanism from adhesive wear to abrasive wear. The 30 N load and the composites of 2.0 wt% Ti₃AlC₂ revealed the optimum tribological properties. The improvement of the tribological behavior of composites was attributed to the refinement of microstructure, the improvement mechanical properties and the three dimensional layered Ti₃AlC₂ phases with self-lubricating properties.     

Microstructure and mechanical properties of Zn based composites reinforced by Ti3AlC2

30?vol.-% Ti₃AlC₂ particle reinforced Zn based (Zn–27?wt-% Al, denoted as ZA27) composites have been prepared via three different mechanically activated sintering technologies: pressureless sintering, hot pressing and combination of pressureless sintering and hot pressing (two-step sintering) technology. The relationship between sintering conditions, microstructure and mechanical properties of the 30?vol.-% Ti₃AlC₂/ZA27 composites has been investigated. The Ti₃AlC₂/ZA27 composite with enhanced tensile strength of 335?MPa and bending strength of 570?MPa was achieved by the two-step technology. The improved mechanical properties are attributed to the good bonding between reinforcing particles and matrix as well as the fine grained microstructure. Raising the sintering temperature is more efficient to improve the mechanical properties of the composites than applying pressure.     

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.     

(Cr2/3Ti1/3)3AlC2 and (Cr5/8Ti3/8)4AlC3: New MAX‐phase Compounds in Ti–Cr–Al–C System

Two new MAX compounds, (Cr_2/3Ti_1/3)₃AlC₂ and (Cr_5/8Ti_3/8)₄AlC₃, were successfully synthesized by hot‐pressing elemental powders at 1500°C for 1 h under 30 MPa in a flowing argon atmosphere. Their crystal structures were indentified and characterized by X‐ray diffraction and transmission electron microscopy analysis. (Cr_2/3Ti_1/3)₃AlC₂ and (Cr_5/8Ti_3/8)₄AlC₃ have the same crystal structures with the well‐characterized Ti₃AlC₂ and Ti₄AlN₃, respectively.     

新型复合材料Ti3 SiC2/TiC的研究进展

Ti3SiC2/TiC具有陶瓷和金属的共同特性,是一种新型复合材料,它具有室温下优良的导热率、导电率、高弹性模量和抗弯曲能力等。本文重点介绍了Ti3SiC2/TiC复合材料的性能、制备工艺及后续研究方向。     

Oxidation resistance of Ti3AlC2 and Ti3Al0.8Sn0.2C2 MAX phases: A comparison

Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂ MAX phase powders are densified using Spark Plasma Sintering (SPS) technique to obtain dense bulk materials. Oxidation tests are then performed over the temperature range 800°C-1000°C under synthetic air on the two different materials in order to compare their oxidation resistance. It is demonstrated that, in the case of the Ti₃Al_0.8Sn_0.2C₂ solid solution, the oxide layers consist in TiO₂, Al₂O₃, and SnO₂. The presence of Sn atoms in the A planes of the solid solution leads to an easy diffusion of Sn out of the MAX phase which promote the formation of the nonprotective and fast growing SnO₂ oxide. Moreover, the small Al/Ti atom's ratio promotes the growth of a nonprotective rutile-TiO₂ scale as well. In the case of the Ti₃AlC₂ MAX phase, the oxide layer consists in a protective alumina scale; a few TiO₂ grains being observed on the top of the Al₂O₃ layer. The parabolic oxidation rate constants are about 3 orders of magnitude smaller for Ti₃AlC₂ compared to Ti₃Al_0.8Sn_0.2C₂.     

High‐temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity

Cr₂AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90‐180 μm, 180‐250 μm, and 250‐400 μm. Sacrificial template technique was used as the processing method, utilizing NH₄HCO₃ as a temporary pore former. Cr₂AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in‐situ formation of an outer thin continuous protective layer of α‐Al₂O₃. The in‐situ α‐Al₂O₃ protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter‐ to transgranular fracture mode. According to the excellent high‐temperature response, heat exchangers and catalyst supports are the potential application of these foams.     

Microstructure and mechanical strength of Ti3AlC2 joints bonded using oxides as interlayer

Ti₃AlC₂ ceramics were successfully joined with TiO₂ and Nb₂O₅ respectively as interlayers via solid-state diffusion bonding at 1300 °C. The joints bonded with TiO₂ and Nb₂O₅ exhibit distinct microstructures. Two continuous thin Al₂O₃ oxidation layers with a thickness around 1 μm were formed in the joints bonded with TiO₂. Between the oxidation layers there exists a dense oxycarbide (TiC_1-xO_x) layer. For the joint bonded with Nb₂O₅, a dense bonding layer with Nb₂AlC and Nb₄AlC₃ grains surrounded by thin Al₂O₃ oxidation layers at the grain boundaries were obtained. The shear strength of the final joints shows clear dependences on both the thickness and microstructure of the joints. Smaller joint thickness and the microstructure with complex phases favour for higher shear strength. Those result implies that bonding with oxides is a practical and efficient method for joining Ti₃AlC₂.     

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.