High Temperature Structural Materials Research Articles

Plastic evolution and phase transition mechanisms in γ-TiAl nano polycrystal by a molecular dynamics simulation

γ-TiAl based alloys are widely regarded as one important high-temperature structural material, however the deformation behaviors are still not clearly clarified. The microscopic plastic deformation and phase transition of γ-TiAl nano polycrystals under compression are investigated by molecular dynamics simulations. The results show that the initial plasticity is dominated by slip of partial dislocations whose continuous formation and secondary slip on the same slip plane result in the generation and disappearance of intrinsic stacking faults (ISFs). At the strain of about 0.2, a continuous structure transformation of “ISF → ESF → TB” is observed, during which the interaction between different ISFs causes the atomic misalignment corresponding to the change in atomic stacking order near ISF. As the compressive strain exceeds 0.2, the FCC-to-BCC phase transition occurs within polycrystals due to the high Mises stress, the high stress level within grains facilitates the expansion of BCC phase from initial FCC phase, and the newly formed BCC phase maintains the atomic-arrangement orderliness of original phase. The interface between FCC and BCC phases obeys the low-energy Kurdjumov-Sachs orientation relationship. This work is helpful for better understanding the atomic-scale deformation behaviors and provides a theoretical guidance for designing high-performance alloy materials.

Unveiling exotic multi-scale microstructure transformation and crack formation mechanisms in eutectic ceramic composite by laser powder bed fusion

Laser powder bed fusion (LPBF) represents an advanced and versatile technology. Exploiting its distinctive advantages for direct and rapid fabrication of complex-structured melt-grown oxide eutectic ceramic composite represents a pioneering yet challenging endeavor. In this work, LPBF is creatively employed to fabricate turbine blade-shaped, in-situ ternary eutectic ceramic composite. Through the integration of experimental procedures, Finite element method (FEM) simulations, and numerical analyses, an innovative design and manufacturing of oxide eutectic ceramic composites have been successfully established. Comprehensive FEM simulations, with detailed interface characteristic analysis, have revealed macro-scale cracks induced by intense maximum principal stress, and micro-cracks stemming from significant interfacial energy disparities among the three constituent phases. The applications of rapid solidification and nucleation theories have facilitated profound insights into the formation mechanisms of multi-scale exotic microstructures, including top-layer coarse dendrites, rosette-like spherical internally grown eutectic colony within layers, and columnar eutectic colonies with ultra-fine lamellar eutectic structures. Micro-mechanical property testing reveals enhanced performance in the inter-layer ultra-fine lamellar eutectic structure, which is attributed to a refined eutectic spacing of approximately 61 nm, coupled with distinct and robust bonding interfaces. These groundbreaking achievements, focusing on the processing-microstructure-property relationship in the fabrication of gas turbine blade-shaped solidified eutectic ceramic composite using LPBF, provide invaluable theoretical insights and data. This knowledge is crucial for the LPBF production of high-temperature structural materials, highlighting its significant potential applications in fields of aerospace and mechanical engineering.

Enhancing strength and γ’-coarsening resistance through micro-alloying additions to an oxidation resistant Al-Co-Cr-Fe-Ni-Ti compositionally complex alloy

The effect of alloying additions of Mo and Ta (0.4at.% each) on the microstructure, mechanical properties, oxidation resistance and coarsening resistance of the L12-strengthened (γ’) Al-rich Ni51Co18Fe5Cr10Al12Ti4 alloy was determined. The micro-alloyed composition exhibited improved yield strength (from room temperature to 1000 °C) and coarsening resistance (enhanced by 33% at 800 °C) compared to the base Ni51Co18Fe5Cr10Al12Ti4 alloy. The present Al-rich alloy compositions (both the micro-alloyed and reference compositions) exhibited superior oxidation resistance (800-1000 °C) compared to a Ti-rich Ni51Co18Fe5Cr10Al8Ti8 alloy (with similar precipitate volume fraction and (Al+Ti)-content) and commercial Inconel 718. The present work highlights the promise of Al-rich γ’-strengthened Al-Co-Cr-Fe-Ni-Ti alloys as candidates for high-performance, high-temperature structural materials, characterised by an attractive blend of strength, resistance to coarsening, and resilience against oxidation.

Internal damage evolution of C/SiC composites in air at 1650 °C studied by in-situ synchrotron X-ray imaging

Carbon fibre reinforced silicon carbide (C/SiC) ceramic matrix composites have attracted considerable attention due to their exceptional properties and extensive potential applications as high-temperature structural materials. However, due to their complex structure and manufacturing defects, C/SiC composites exhibit intricate mechanical behaviour under thermal-mechanical-oxidative coupling environments. To date, systematic studies on the internal damage evolution and failure mechanisms of C/SiC composites under high-temperature oxidative environments are lacking. In this study, a combination of synchrotron X-ray micro-computed tomography (SR-μCT) and in-situ experiments under thermal-mechanical-oxidative coupling environments at room temperature and 1650 °C in air was used to characterize the internal microstructures and damage evolution processes of C/SiC composites at different loading levels. Additionally, the 3D strain fields during in-situ loading were quantitatively analysed using the Digital Volume Correlation (DVC) method. The findings underscore the substantial impact of oxidative damage on the mechanical response of C/SiC composites, particularly concerning tensile properties and fracture modes. At room temperature, severe delamination, fibre bundle pull-out and interfacial debonding occurred internally. Whereas, under high-temperature atmospheric conditions, severe fibre oxidation reactions occurred at the specimen edges, resulting in rapid porosity escalation. Crack initiation from surface defects followed by rapid inward propagation is observed. Moreover, while the strain distribution remains relatively uniform until fracture, a pronounced concentration of strain is evident near the fracture zones at room temperature, with an even greater concentration observed at 1650 °C. Notably, the region of concentrated strain within the 3D deformation field corresponds closely to the final fracture location, as revealed by quantitative DVC analysis.

Self-generated B-MAX phase composites: The effect of sintering temperature on surface morphology and phase composition

The use of metal-based MAX phase composites is widespread in the production of high-temperature structural materials, anticorrosive materials and electrode materials. Herein, novel B-Ti3AlC2 and B-Ti4AlN3 composites were prepared using the hot-pressing method at varying sintering temperatures ranging from 950 °C to 1325 °C. This study also explores the effect of sintering temperature on the surface morphology and phase composition of both B-MAX phase composites. The morphological and crystal phase composition of MAX phase composites were investigated through SEM coupled with EDS, FE-SEM, AFM, and XRD test analysis. The results revealed that higher sintering temperatures increase the clear formation of MAX phases and composites as compared to lower temperatures. The findings also expose that the in-situ formation of B-MAX composites significantly changes the structure, contributing to transforming the overall performance and making it suitable for high-stress applications like electronics, aerospace, and cutting tools.

Full-scale insight into high-entropy ceramics from basic concepts, synthesis technologies, structural characteristics, and properties to application prospects

High-entropy ceramics (HECs) are an emerging material system that has gained significant attention and become a focal point of research due to their unique structure and outstanding performance. This paper provides a comprehensive examination of the basic concepts, synthesis methods, structural characteristics, unique properties, and application prospects of HECs. It begins with an overview of the basic concepts and historical context, followed by a detailed comparison of synthesis techniques, including both traditional and innovative approaches. The paper then analyzes the structural characteristics and phase compositions of HECs, particularly focusing on oxide, carbide, boride, and other non-oxide ceramics. In addition, it delves into the mechanical, thermal, electrical, and magnetic properties of HECs. The article also reviews the applications of HECs in high-temperature structural materials, functional materials, high-performance coatings, and biomedical implants. Finally, it discusses the future challenges and development pathways for HECs. By highlighting new applications and transformative possibilities, this study not only sheds light on cutting-edge research but also emphasizes the significant impact of HECs on sustainable material development. The integration of machine learning and artificial intelligence can further unlock the unique structural capabilities of HECs, offering substantial potential for advancements in emerging fields like new energy and biomedicine.

Development of Ti3SiC2 MAX phase tubular structures for solar receiver applications

Solar receiver tubes are key components of concentrating solar-thermal power (CSP) systems that harvest solar energy. For better efficiency, the Gen3 CSP receivers, which collect heat into a heat transfer fluid, require a temperature exceeding 700 °C during operation and need to perform under extreme conditions of high temperature and high thermal stress. Operators are seeking CSP designs using new high-temperature structural materials with high thermal conductivity and high creep resistance to achieve a design life of 30 years and thus help recover the plant capital cost sooner. MAX phase materials, which consist of an early transition metal element, an A-group element, and carbon or nitrogen, are expected to exhibit high creep resistance as well as high fracture toughness. In this paper, we describe fabricating both (1) dense Ti3SiC2 MAX phase disks and (2) short-length tubes using field-assisted sintering technology (FAST). First, the disk samples that we fabricated are fully dense and contain ≈90 % Ti3SiC2 MAX phase materials and ≈10 % TiC phase materials. We determined a flexure strength of 519 ± 32 MPa by conducting a four-point bending test at room temperature with rectangular bar samples of ≈100 % density. The thermal conductivity of the Ti3SiC2 MAX phase samples, measured by light-flashing analysis, decreases linearly from a value of 41 W·m−1·K−1 at room temperature to a value of 36 W·m−1·K−1 at 650 °C. A solar reflectance measurement of the Ti3SiC2 MAX phase revealed that, temperature increases from 400 to 1400 °C, thermal emittance increases from 0.39 to 0.49, while selectivity decreases from 1.8 to 1.4, respectively. Whereas the surface oxidized MAX phase samples after 100 h exposure to air at 1000 °C exhibit that of SiC.Next, we discuss fabrication of the crack-free Ti3SiC2 MAX phase tubular structures accomplished by using FAST processing in graphite bedding. A Ti3SiC2 MAX phase content of > 95 % with traceable ≈3% remaining TiC phase and ≈15 % porosity were demonstrated after high-temperature annealing. An average fracture strength of ≈250 MPa was determined with Ti3SiC2 MAX phase tubes of ≈85 % density by diametral compression testing at room temperature. Our work demonstrated that using FAST processing to produce Ti3SiC2 MAX phase tubular structures for CSP receiver applications is a viable approach.

A single-phase Nb25Ti35V5Zr35 refractory high-entropy alloy with excellent strength-ductility synergy

Refractory high entropy alloys (RHEAs) are promising candidates for high-temperature structural materials due to their excellent high-temperature performance. However, the room-temperature brittleness of most alloys results in poor plastic formability, thereby limiting their engineering applications. In this study, Nb25Ti35V5Zr35 alloy with cold-rollability was developed, and its phase stability, tensile mechanical properties, elastic properties, strengthening mechanism, and deformation mechanism were investigated by combining experiments, CALPHAD, DFT and molecular statics (MS) methods. The results demonstrate that Nb25Ti35V5Zr35 alloy possesses good phase stability, with both the as-cast and annealed states maintaining a single-phase BCC structure without undergoing phase transformation. The tensile yield strength of the alloy reaches its peak at 1152 MPa in the cold-rolled condition, while its annealed state exhibits an optimal balance of strength and plasticity, with a yield strength of 836 MPa and an elongation of 22.45 %, respectively. Solid solution strengthening is the primary source of its high strength, with V and Zr playing key roles in this strengthening mechanism. Dislocation slip plays a dominant role in plastic deformation. Additionally, compared to traditional BCC alloys, the significance of edge dislocations in the deformation process is notably enhanced, with the {112} plane serving as the preferred slip plane for dislocations. DFT results indicate that the alloy exhibits significant anisotropy in both Young's modulus and shear modulus. This work contributes to understanding the material properties of the Nb25Ti35V5Zr35 alloy and provides a reference for the design of new ductile RHEAs.

Carbon whiskers reinforced porous glassy carbon composites by synchronous growth

A crystalline and non-crystalline composite of pure carbon was proposed, which is a carbon whiskers reinforced porous glassy carbon composite, where crystalline carbon whiskers support the microscopic pores of non-crystalline glassy carbon matrix. Two allotropes of carbon were produced simultaneously and compounded together in a single high temperature treatment of thermosetting phenolic resin, with aluminum oxide (Al2O3) as a catalyst. A synchronous growth theory was proposed to explain the growth mechanism. In the pyrolysis process of phenolic resin to generate porous glassy carbon, hydrocarbon gas was produced and trapped in the generated pores. By the same time, the hydrocarbon gas in pores was decomposed and grew carbon whiskers by catalysis of Al2O3. With linear thermal expansion coefficient from 2.0∙10−6 to 3.8∙10−6 K−1, thermal conductivity from 1.763 to 2.518 W∙m−1 K−1, specific heat capacity from 0.396 to 0.752 J∙g−1 K−1, thermal diffusivity from 6.115 to 4.598 mm2 s−1, between room temperature (RT) and 500 °C, the Vickers hardness is at 65 MPa, and the fracture toughness is at 0.329 Mpa∙m1/2 in RT. The carbon whiskers reinforced porous glassy carbon composite keeps a balance between thermophysical and mechanical properties, showing its application value in thermal insulation materials and high temperature structure materials.

Anomalous increase of fracture toughness of TiAl-based alloys at high temperature

The anomalous increase of mechanical property of TiAl-based alloys at high temperature caters to the property requirements as the high-temperature structural materials, which is thus important and worth studying. However, people currently are not clear whether the fracture toughness of TiAl-based alloys has an anomalous increase at high temperature, and if so, how much it increases, and why it increases. In order to answer these questions, the forged Ti-44Al-8Nb model alloy with fine and uniform microstructure was used. The fracture toughness of the alloy at different temperatures was tested. The results indicated that the alloy did show an anomalous increase of fracture toughness at 700 °C–800 °C. The main crack deflected a large angle at 700 °C, and terminated at the upper part at 750 °C and 800 °C. The fracture morphology was fine, and the cleavage plane became smaller and less at high temperature. These characteristics corresponded to the anomalous increase of fracture toughness at high temperature. Through reverse thinking, we found that the high dislocation density was the root causing the anomalous increase of fracture toughness at high temperature. The forging-induced texture, such as B2 〈100〉//RD and γ-TiAl 〈011]//RD could also promote the dislocations slip on the main slip surfaces during the high temperature fracture toughness test, contributing to the final high dislocation density. The critical crack size at 700 °C–800 °C was about 4 times that of room temperature, corresponding to the higher fracture toughness of the alloy at high temperature.

The effect of TiSi2 healing improver in self-healing ability of ceramics

A method to enhance the temperature range in which continuous self-healing fiber-reinforced ceramics (shFRCs) can self-heal is proposed to obtain a new high-temperature structural material. The effect of TiSi2 oxidation on self-healing was investigated using SiC, a typical self-healing agent, and TiSi2, which oxidizes at a lower temperature than SiC. Mixtures of SiC and TiSi2 powders were prepared by wet-mixing, and changes in their high-temperature oxidation behavior were investigated using thermogravimetry/differential thermal analysis. The oxidation of TiSi2 at 1000 °C enhanced the oxidation rate of SiC by 2–3 times. A shFRC consisting of an Al2O3 matrix, an interface layer of SiC and TiSi2, and Al2O3 fiber bundles was fabricated by slurrying and filament-winding. The strength recovery of the shFRC following three-point bending was investigated, and results indicated that the prepared material recovered 50 times faster than conventional shFRCs at 1000 °C. The self-healing improver described in this study can promote the oxidation of self-healing agents via its reaction heat. Thus, this improver may be applied as a practical component of self-healing materials.

Multiscale oxide dispersion strengthened refractory high entropy alloys with superior mechanical properties

Refractory high-entropy alloy (RHEA) is one of the most promising materials for high-temperature applications. However, their application is inhibited by insufficient strength at elevated temperatures and brittleness at ambient temperatures. Herein, we proposed a multiscale oxide dispersion strengthened (ODS) NbTaTiV RHEA with an excellent combination of strength and plasticity at both room and elevated temperatures. The multilevel microstructures are composed of a fine-grained BCC phase matrix (∼1.2 μm), submicron Ti-(O, N) particles (∼330 nm), and nanoscale Y2Ti2O7 particles (∼18 nm). Owing to the strengthening of combinatorial multilevel microstructures, the yield strength of the RHEA was found as high as 1626 MPa, which exhibits a significant improvement (∼46%) in comparison with the NbTaTiV RHEA, without significant loss of ductility. The ODS NbTaTiV RHEA also shows superior thermal-softening resistance, with yield strengths of 945 MPa at 700 °C and 482 MPa at 1000 °C. The enhanced strength is mainly attributed to the multilevel microstructures that mediate the collaborative strengthening mechanism among dispersed strengthening, precipitation strengthening, and grain boundary strengthening. This work presents a new insight to develop multiscale microstructures in RHEAs for high-temperature structural materials.

Experimental determination and thermodynamic optimization of Nb-Al-Y ternary system

Niobium-aluminide-based alloys show excellent application potential as high-temperature structural materials. In the present work, the isothermal sections of the Nb-Al-Y system at 800 and 1050°C are constructed based on X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and transmission electron microscopy (TEM). Two new ternary compounds, designated as τ-phase (Nb8Al80Y12) and ρ-phase (Nb26Al58Y16), are identified and their homogeneity ranges and crystal structures have been described. The ρ-phase can exist at temperatures within the isothermal sections of 800 and 1050°C, while the τ-phase exists at 800°C and disappears as the temperature rises to 1050°C. In addition, the primary solidification phases were also identified, and five different primary solidification phases were found. Based on the experimental data, the thermodynamic optimization of the Nb-Al-Y ternary system is carried out using the thermo-calc method. The calculated results are in good agreement with the experimental results, and a set of reasonable and self-consistent thermodynamic parameters is obtained. The results provide a basis for the design of Nb-Al-Y-based alloys and enrich the database of Nb-based alloys.

AlxHfTaTi (0≤x≤0.5) refractory medium entropy alloys with excellent room-temperature tensile properties

Refractory high entropy alloys are strong candidates for high-temperature structural materials, because of their high strength, excellent thermal stability and exceptional softening resistance under high-temperature. However, majority of the existing refractory high entropy alloys are seriously limited in widespread application, due to their severe brittleness at room temperature. In this study, we proposed a novel AlxHfTaTi body-centered cubic (BCC) refractory medium entropy alloy with remarkable strength-ductility combination at room temperature. Adjusting the atomic ratio of Al (from 0 to 0.5 at.%), we investigated the effect of Al on the structure-property characteristics of the alloy under as-cast as well as cold-rolled and annealed conditions. Compared with the as-cast samples, the rolled and annealed samples exhibit more excellent tensile properties due to the remarkable reduction of grain size resulted from recrystallization. The Al0.3HfTaTi sample annealed for 5 min exhibits outstanding comprehensive properties at room temperature, with a yield strength higher than 1000 MPa and a fracture elongation of up to 24 ± 1 %, respectively. Quasi in-situ EBSD analysis results reveal that, during tensile deformation, the grain size has a significant effect on the grain rotation and dislocation gliding. Small grains are more likely to initiate multiple slip bands, promoting the synergistic effect of grain rotation and dislocation gliding, thereby improving the ductility of the alloy. Our findings can provide new ideas for the development of refractory high entropy alloys with outstanding room-temperature tensile properties.

Molecular dynamics simulation of the effect of tool parameters on nano-cutting of polycrystalline γ-TiAl alloys

As one of the most promising lightweight high-temperature structural materials in the future, the surface quality of γ-TiAl alloys has a great influence on the performance of the workpieces, and the tool parameters are an important factor affecting the machining results. In this study, molecular dynamics simulations of the nano-cutting process of polycrystalline γ-TiAl alloys with different tool parameters were carried out. The results show that increasing the tool rake angle and decreasing the tool edge radius within a certain range helps to reduce the average cutting force, cutting force fluctuation, cutting temperature, and stabilize the cutting process, while the change of the tool clearance angle has less influence on the cutting process. In contrast, negative rake angle cutting is more likely to produce grain rotation and grain boundary steps in the processed substrate and increase the processed surface roughness than positive rake angle cutting; increasing the tool rake angle within a certain range will weaken the elastic recovery effect of the substrate. During cutting at a positive rake angle, whether a portion of the substrate is prone to slip toward the surface of the substrate, thereby reducing the surface quality, depends on the relative state of grain orientation and force applied in the substrate.

Structure and mechanical properties of a refractory Al7.5(NbTiZr)92.5 medium-entropy alloy subjected to long-term annealing and oxidation

Recently, Al-containing refractory high/medium-entropy alloys (RH/MEAs) have attracted special attention from materials scientists worldwide. Some of these alloys demonstrated an excellent combination of high-temperature strength and room-temperature tensile ductility provided by the formation of a structure consisting of body-centred cubic (bcc) matrix and B2 nanodomains/particles. However, oxidation resistance and phase stability, and, more importantly, the resulting mechanical properties of the Al-containing RH/MEAs, which are critical characteristics for high-temperature structural materials, remain unexplored. Herein, these aspects were extensively investigated for the Al7.5(NbTiZr)92.5 (in at%) alloy having an initial bcc + B2 structure. The alloy was subjected to long-term (500 h) annealing or oxidation tests at 600, 700, and 800 °C followed by the evaluation of mechanical properties during uniaxial tension at 22 and 600 °C. Microstructural studies of the specimens exposed to long-term annealing showed the formation of Zr5Al3 particles that precipitated along the grain boundaries and within grain interiors. The volume fraction of these particles was the highest (∼16 %) after annealing at 600 °C and the lowest (∼0.5 %) after annealing at 800 °C. Oxidation tests revealed that the alloy was prone to pesting phenomena, which intensified with the temperature. The alloy could sustain pesting only up to 10 h at 600 °C, while it experienced a complete disintegration into powder after 5 h at 800 °C. The poor oxidation resistance originated from the formation of volumetric, non-protective (Al,Nb,Ti,Zr)O4 oxide. While insignificantly affecting the strength, long-term annealing or oxidation deteriorated the ductility of the Al7.5(NbTiZr)92.5 alloy both at 22 and 600 °C. The Zr5Al3 phase embrittled the alloy after long-term annealing at 600 or 700 °C, but marginally reduced the ductility in the 800°C-annealed specimens due to the low volume fraction and relatively homogeneous distribution of this phase. The oxidation for 1 h decreased the room-temperature ductility owing to the premature macroscopic localisation of plastic deformation induced by synergistic effects from the volumetric (Al,Nb,Ti,Zr)O4 oxide. This study highlights the necessity for the examination of a set of properties of promising RH/MEAs to critically assess perspectives of these alloys to find applications as new high-temperature materials in aircraft and aerospace sectors.

Understanding the hot deformation behavior of Mo[sbnd]42Re alloy: Insights from strain rate sensitivity, microstructure, and misorientation

Mo42Re alloy, as a high-temperature structural and functional material, has broad application prospects in the nuclear industry. In this study, the hot deformation behavior of Mo42Re alloy was systematically investigated using hot compression tests and electron backscatter diffraction (EBSD) techniques in the temperature range of 1200–1500 °C and strain rate range of 0.001–1 s−1. The results showed that the flow stress of Mo42Re alloy exhibited a strong dependence on temperature and strain rate. The strain rate sensitivity index (m-value) was high under the conditions of 1350–1450 °C and 0.001 s−1. With increasing deformation temperature and decreasing strain rate, dynamic recrystallization (DRX) was significantly promoted, and the recrystallized volume fraction increased from 2.79% at 1200 °C/1 s−1 to 20.3% at 1500 °C/0.001 s−1. Meanwhile, the occurrence of and DRX effectively reduced the grain reference orientation deviation (GROD) gradients and kernel average misorientation (KAM) levels within the grains. This study revealed the mechanisms of microstructural evolution during hot deformation of Mo42Re alloy, providing important guidance for optimizing its thermomechanical processing parameters.

Long-term oxidation of Ti3Al0.8Sn0.2C2 in air at 1200 °C

The addition of Sn into the raw materials is currently one of the most widely used methods to improve the purity and mechanical properties of Ti3AlC2. It follows in this work to investigate the oxidation behavior of the resulting Ti3Al0.8Sn0.2C2 at 1200 °C with the oxidation time up to 500 h. The oxidation kinetics shows that its oxidation resistance is much worse than that of common MAX phases including Ti3AlC2, Ti2AlC, and Cr2AlC. The oxide layer mainly consists of TiO2 and Al2O3, with only a small amount of SnO2 at the beginning of the oxidation stage. Cracks induced by the internal thermal stress penetrate through the entire oxide layer after only 1 h, which is an important reason for the rapid oxidation of the internal substrate layer. This work is expected to provide some inspiration and reference for Ti3Al0.8Sn0.2C2 used as the potential high-temperature structural material.

Al2O3 fiber-reinforced MAX phase ceramic matrix composite

In the aerospace industry, lightweight can reduce the weight of the engine and improve the thrust-weight ratio of the engine. Ceramic matrix composites have attracted widespread attention in the industry due to their much lower density compared to high-temperature alloys and excellent high-temperature performance. SiC fiber-reinforced SiC ceramic matrix composites are the most outstanding representatives, but their long manufacturing cycle and high manufacturing costs limit their widespread use. Ti2AlC is a new type of ceramic material with the characteristics of low density, high melting point, high-temperature resistance, oxidation resistance, and corrosion resistance of ceramic materials, as well as the conductivity and machinability of metals. However, its mechanical properties are slightly inadequate. The use of chopped fibers for toughening can compensate for these deficiencies to a certain extent and improve mechanical properties. This article first explored the influence of various process parameters on the mechanical properties of pure-phase Ti2AlC ceramic materials in SPS preparation technology and then prepared Ti2AlC composites reinforced with 10 vol%, 20 vol%, and 30 vol% Al2O3 chopped fibers. The results showed that the samples prepared at 1300 °C, 40 MPa, and 10 min had a flexural strength of 697 MPa and a fracture toughness of 9.83 MPa m1/2 when the volume ratio of the reinforcement phase was only 10 vol%, which was the same as that of the continuously fiber-reinforced SiCf/SiC composites. This study showed that the MAX phase matrix composites reinforced with chopped fibers have the potential to become high-temperature structural materials by adjusting the composite composition and optimizing the preparation process.

TiAl Alloy Fabricated Using Election Beam Selective Melting: Process, Microstructure, and Tensile Performance

TiAl alloy is one of the most attractive candidates for a new generation of high-temperature structural materials and has broad application prospects in the aerospace field. As a typical intermetallic material, TiAl is inevitably difficult to process using conventional methods. Election beam selective melting (EBSM) is an effective method of addictive manufacturing to prepare TiAl alloy with a complex structure. However, the microstructure of TiAl alloy formed using EBSM often contains defects such as pores, which seriously reduces the mechanical properties of the material. In this work, the effects of EBSM and post-processing procedures on the microstructure and mechanical properties of Ti-48Al-2Cr-2Nb alloy were studied. The results show that the microstructure of Ti-48Al-2Cr-2Nb alloy formed using the EBSM process was dense and composed of equiaxed γ-phase and double-phase regions. A large number of dislocations that formed due to thermal stress were clearly observed inside the Ti-48Al-2Cr-2Nb alloy. When the EBSM process parameters were 13.5 mA, 4.0 m/s, and 40.50 J/mm3, as the current intensity increased, the Al content decreased, the content of α2 phase increased, and the microstructure of the material was coarse. The results of the tensile test fracture morphology indicate that the Ti-48Al-2Cr-2Nb alloy exhibited brittle fracture during tensile deformation, lacking the typical yield deformation of metal materials. As the energy density of the EBSM process increased, the mechanical properties of the Ti-48Al-2Cr-2Nb alloy first increased and then decreased. The samples prepared with an energy density of 34.50~40.50 J/mm3 had excellent mechanical properties, of which the maximum tensile strength and maximum elongation reached 643 MPa and 2.09%, respectively. The phase composition of the Ti-48Al-2Cr-2Nb alloy after hot isostatic pressing (HIP) treatment remained unchanged from the EBSM samples, but there was a slight difference in content. There was an increase in the amount of γ phase and a decrease in B2 phase, accompanied by the generation of a massive γ phase after HIP treatment. Moreover, the number of dislocations inside the material increased. The Ti-48Al-2Cr-2Nb alloy after HIP treatment exhibited obvious plastic deformation characteristics, with a tensile strength of 679 MPa and elongation of 2.5%. A heat treatment of 900 °C/5 h was performed on the Ti-48Al-2Cr-2Nb alloy after HIP. The dislocation density of the Ti-48Al-2Cr-2Nb alloy decreased, and the B2 phase transformed from massive to lamellar.