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Porous intermetallic membrane with extensive interconnected pores are potential candidates as functional materials for high-temperature particulate matter (PM) capturing. However, fabrication of intermetallic membrane with a combined performance of high filtration efficiency and high-temperature oxidation resistance remains a challenge. To tackle this issue, a hierarchical micro-/nano-dual-scale sized pores was constructed on the inner cell walls of a porous support through mutual diffusion and chemical reaction. Benefited from its hierarchical micro/nano-dual-scaled pore structural features, the high Nb containing TiAl-based porous composite microfiltration membrane demonstrates ultrahigh PM>2.5 removal efficiency (99.58%) and favorable oxidation/sulfidation performance at high temperature. These features, combined with our experimental design strategy, provide insight into designing high-temperature PM filtration membrane materials with enhanced performance and durability.

Intermetallic Ti-45Al-8Nb-(W, B, Y) (at.%) and Ti-46Al-5Nb alloys are directionally solidified at a constant growth rate of 30 µm·s−1 using a Bridgman type apparatus. The quenched microstructures and lengths of different phase regions were observed and measured after various growing times of 0–30 min. Results show that the phase transformations in different phase regions are mainly depending on the high temperature microstructure and the supercooling degree during quenching process. After isothermal holding, the primary phase grows into the liquid phase, the dendrites change from equiaxed to columnar grains, and the length of the L+β phase region, L+β+α phase region and mushy zone varies, indicating that the entire directional solidification process can be described by a static equilibrium — nonequilibrium — dynamic equilibrium evolution process. In addition, the gap between the original growth interface and front interface shows that the actual crystal growth rate is not equal to the drawing velocity during directional solidification.

In order to improve corrosion resistance of the 316L stainless steel in liquid zinc, four different kinds of TiAl–Nb coatings were deposited by high velocity oxygen fuel spraying. The results indicated that the TiAl–Nb coatings have dense and uniform microstructure and good mechanical properties. The TiAl–Nb coatings exhibit excellent corrosion resistance. Four different kinds of TiAl–Nb coatings have a incubation time, which do react with liquid zinc. The Ti28.15Al63.4Nb8.25Y0.2 coating possesses the longest lifetime, which can reach up to 1080 h. The TiAl–Nb coatings can provide a long-term corrosion protection for 316L stainless steel. The Al content has an important effect on the corrosion resistance of the coating, and the addition of the Y element may benefit the corrosion resistance to some extent. The corrosion process of the coating consists of the following steps: physical barrier of the TiAl–Nb coatings (incubation period), the corrosion of bond coat and the failure of the coatings.

Low cycle fatigue of lamellar TiAl with 8.5 at.-%Nb was studied with a total strain amplitude of 0.28% at three temperatures: room temperature, 750°C and 900°C. At room temperature, the material exhibited cyclic hardening and the fracture mode was mainly interlamellar. At 750°C and 900°C, the material showed cyclic softening and the fracture mode was translamellar. The lattice strain in γ phase was almost tensile and larger tensile lattice strain in γ phase seems detrimental. Besides, the opposite direction of {201}γ and {100}α2 lead to crack propagation along α2/γ interfaces. B2/βo phase always suffered compressive lattice strain in the tests. The destruction of lamellar microstructure was the reason for colony refinement at 750°C and 900°C. Accepted version

Tensile behavior of an equiaxed-grained Fe-6.5 wt.%Si alloy, which was deformed into Φ6 mm bar by hot rotary swaging, was investigated at various temperatures (300–400 °C) and stretching rates (0.42–1 mm/min). The results revealed an enhancement in the intermediate-temperature tensile ductility after heat treatments. Deformation twinning was found in the equiaxed-grained Fe-6.5 wt.%Si bars during the tensile test, and heat treatments can enhance the deformation twinning. More twins can be observed in the necking areas than other regions. The high Schmid factor values above 0.4 after heat treatments demonstrated that deformation twinning can easily occur in the equiaxed-grained Fe-6.5 wt.%Si alloy. Higher deformation temperatures, higher strain rates, and larger degree of order suppressed the formation of deformation twinning, while the grain sizes had little effect on the deformation twinning. The twinning stress of the Fe-6.5 wt.%Si alloy increased with the increasing grain size, which did not agree with the Hall–Petch type relationship. The deformation twinning resulted in the improved ductility of the Fe-6.5 wt.%Si alloy.

The paper describes the microstructure evolution by high-temperature compression of a high Nb-containing TiAl alloy. The paper extends a previous publication [Song, et al. Intermetallics 109 (2019) 91–96], in which a unique twin-like morphology in the α2 (Ti3Al) phase was reported. However, the origin of these structures could not be clarified without doubt. The present study is focused on phase transformations that in this multiphase alloy can be associated with deformation. Particular attention is paid to local transformations of the α2 phase into O phase or ω-related phases, which, because of structural and chemical similarity of these phases with α2, can easily occur and could mistakenly be considered as a twin structure. The details of the atomic processes involved are elucidated by electron microscopy. Given the large shufflings and the atomic site interchanges required for the operation of this twinning system, it is concluded that twinning of the α2 phase is a diffusive-displacive process. Within the α2 phase, ωo is heterogeneously nucleated. The nucleation sites are defect-rich areas, which are subjected to high local stresses. The study strongly emphasizes the close relationship between high-temperature deformation and phase transformations in multiphase titanium aluminide alloys.

TiAl porous alloys exhibit potential for application in high-temperature flue gas filtration, because of their satisfactory mechanical properties and excellent high-temperature creep resistance. However, poor oxidation resistance and corrosion resistance at temperatures exceeding 800 °C considerably limit the practical applications of TiAl porous alloys. The disadvantages of TiAl alloys can be mitigated by adding a specific amount of alloying elements to delay oxygen diffusion. In this study, two kinds of porous alloys, Ti-48Al and Ti-48Al-2Nb-2Cr, are used to evaluate the high-temperature oxidation resistance and sulfidation resistance of TiAl-based porous alloys. With the addition of alloying elements Nb and Cr, the high-temperature oxidation resistance and sulfidation resistance of the TiAl alloys have greatly improved. The mechanisms underlying phase transformation are also explained using the first-principles theory. The excellent antioxidation and anti-sulfidation properties of the Ti-48Al-2Nb-2Cr porous alloy are mainly attributed to the markedly reduced free energy of Al2O3 formation and suppress the presence of TiO2 at elevated temperatures, as explained by first-principles calculations. The Ti-48Al-2Nb-2Cr porous alloy has advanced the application of multiple alloying in high-temperature flue gas filtration.

A new hatted 1T/2H-phase MoS2 on Ni3 S2 nanorods, as a bifunctional electrocatalyst for overall water splitting in alkaline media, is prepared through a simple one-pot hydrothermal synthesis. The hat-rod structure is composed mainly of Ni3 S2 , with 1T/2H-MoS2 adhered to the top of the growth. Aqueous ammonia plays an important role in forming the 1T-phase MoS2 by twisting the 2H-phase transition and expanding the interlayer spacing through the intercalation of NH3 /NH4+ . Owing to the special "hat-like" structure, the electrons conduct easily from Ni foam along Ni3 S2 to MoS2 , and the catalyst particles maintain sufficient contact with the electrolyte, with gaseous molecules produced by water splitting easily removed from the surface of the catalyst. Thus, the electrocatalytic performance is enhanced, with an overpotential of 73 mV, a Tafel slope of 79 mV dec-1 , and excellent stability, and the OER demonstrates an overpotential of 190 mV and Tafel slope of 166 mV dec-1 .

The dieless roll forming is a special technology for the manufacture of discal parts. This paper will describe metallurgical evaluations of the rolling thermal deformation. The deformation behavior and microstructure of 6061 aluminum alloy was reseached by the numerical simulation and the EBSD (electron-backscattered diffraction). The results show that the grain sizes increase with the distance increase from surface, and the surface of discal part shows the small grain. The number of small grain decreases with the increased distance from surface, and the discal part surface has large number of deform grains. The metal flow and effective strain during forming were also studied by numerical simulation.

Journal of alloys and compounds 918, 165626 (2022). doi:10.1016/j.jallcom.2022.165626, In-situ synchrotron-based high energy X-ray diffraction (HEXRD) and ex-situ Gleeble tests were conducted to investigate the deformation behavior of the $β_{o}(ω_{o})$ phase in a Ti$_4$Al$_3$Nb alloy. In samples compressed at 600 and 800 °C, the brittle $ω_o$ phase deforms mostly elastically, resulting in the failure of the Ti$_4$Al$_3$Nb alloy by premature fracture. In a sample deformed at 900 °C, the $ω_o$ phase mostly transforms into the $β_o$ phase under uniaxial loading. Moreover, the ductility of a Ti$_4$Al$_3$Nb alloy is largely enhanced at this temperature. In a sample deformed at 1000 °C, dynamic recrystallization (DRX) of the $β_o$ phase extensively takes place. Direction 1 (D1) and Direction 2 (D2) deviating from the loading direction with an angle of 65° ± 5° and 15° ± 5° are selected to analyze the lattice strain evolution of (110)$_{β_{o}}$ lattice planes. At the late stage of macro strain hardening, deformed < 110 > $_{β_{o}}$//D1 oriented grains bear a higher load due to the occurrence of DRX in < 110 > βo//D2 oriented grains. Subsequently, DRX continues in < 110 > $_{β_{o}}$//D1 oriented $β_o$ grains. The coordinated deformation of $β_o$ grains ensures the good deformability of a Ti$_4$Al$_3$Nb alloy at 1000 °C., Published by Elsevier, Lausanne

The element distribution and the microstructures of directionally solidified ingots of Ti-45Al-8Nb and Ti-46Al-8Nb alloys were studied by scanning electron microscope (SEM) and electron probe microanalyzer (EPMA) equipped with wavelength-dispersive X-ray spectroscope (WDS). At high solidification rates, e.g., more than 50 μm/s, the ingot solidified in columnar β dendrites, while at low solidification rates, e.g., less than 30 μm/s, the solidification path changed from initial β solidification to L + β→α peritectic solidification, forming cellular dendrites with the β phase matrix surrounded by the α phase. The difference of Ti content in dendritic arms and interdendritic regions was not pronounced. The composition segregation was mainly caused by the mutual conversion of Al and Nb contents. Therefore, it was difficult to distinguish the variation of Ti in microstructure by EPMA-WDS map and line profiles. The composition of the peritectic α phase was different from that of the α phase transformed directly from the β phase. The Al content of the former was about 1 at% higher than that of the latter, while the Nb content was about 1 at% lower. The change of solidification path in the final solidified part resulted from the more severe segregation caused by slow solidification.

The 8Nb isopleth section of a Ti-Al-Nb system is experimentally determined based on thermal analysis and thermodynamic calculation methods to obtain the phase transformation and equilibrium relations required for material design and fabrication. The phase transus and relations for the 8Nb-TiAl system show some deviations from the calculated thermodynamic results. The ordered βo phase transforms from the disordered β/α phases at 1200–1400 °C over a large Al concentration range, and this transformation is considered to be an intermediate type between the first- and second-order phase transitions. Moreover, the βo phases are retained at the ambient temperature in the 8Nb-TiAl microstructures. The ωo phase transforms from the highly ordered βo phase, rather than from α2 or βo with low degree of atom ordering B2 (LOB2) structure, with Al concentration of 32–43 at.% at approximately 850 °C. From the experimental detection, the transition of the ωo phase from the βo phase is considered to be a further ordering process.

Hot isostatic pressing (HIP) and subsequent heat-treatments (HT) are necessary for titanium aluminide (TiAl) casting components. But there are few studies carefully comparing the microstructure changes from the initial as-cast condition to the final heat-treated condition. In this study, the microstructures of Ti-47Al-2Cr-2Nb (at%) alloy in the as-cast, as-HIPed and as-heat-treated conditions were characterized by optical microscopy and scanning electron microscopy. The mechanical properties after HTs were determined by the tensile tests at 700 °C. The results show that after HIP and HTs, all the microstructures exhibit a nearly lamellar (NL) structure and can be divided into an edge region and a central area. The microstructure after HIP in the edge region is normal, while distorted lamellae and many fine recrystallized grains exist in the central area. The yield strengths after three HTs are nearly the same, but the elongation after the HT at 1310 °C is much more than that after HTs at 1185 °C and 1280 °C. A refinement of colony size induced by distorted lamellae in as-HIPed condition is considered responsible for the great improvement in elongation.

Nano-Y2O3 reinforced Ti-47.7Al-7.1Nb-(V, Cr) alloy was fabricated by a powder metallurgy route using spark plasma sintering (SPS), and the influence of nano-Y2O3 contents on the microstructure and mechanical properties were investigated systematically. The results revealed that the ultimate tensile strength and elongation of the alloy were 570 ± 28 MPa and 1.7 ± 0.6% at 800 °C, 460 ± 23 MPa and 6.1 ± 0.4% at 900 °C with no nano-Y2O3, 662 ± 24 MPa and 5.5 ± 0.5% at 800 °C, and 466 ± 25 MPa and 16.5 ± 0.8% at 900 °C with 0.05 at% nano-Y2O3 addition, respectively. Due to the fine-grain strengthening and the second-phase strengthening, both tensile strength and elongation of the high-Nb TiAl alloy were enhanced with the addition of nano-Y2O3.

Conventional circulating activated sludge techniques (CASS) are time consuming (72 h) and energy intensive, all of which greatly limits their use. Although advanced oxidation techniques (e.g., photocatalysis, photoelectrocatalysis UV/•OH, and Electro-Fenton) can reduce the treatment time by several hours, the slow generation and fast coupling of electron and hole make the low degradation efficiency. In this work, an intact route using a two-step strategy is developed to eliminate organic dyes from wastewater in only a few minutes. The electron and hole exhibit “fast generation and slow coupling” by using the new technique via electrolytic discharge plasma (EDP) combined with a core-shell structure Au@SiO2 nanocatalyst for [Rhodamine (RhB)/Eosin yellowish (EY)] dyes degradation in wastewater. Results demonstrate that the synergy of EDP and Au@SiO2 nanocatalyst enhances degradation kinetics, and it is effective in degrading different concentrations of RhB and EY dyes in the range of 50–1.5 mg/L. Then, the Au@SiO2 nanocatalyst (over 99%) and carbon impurities are filtered by a porous nanocomposite ultrafiltration membrane. Favorable contributions of the two-step strategy are further ascertained based on chemical oxygen demand (COD) and relative removal efficiency. This two-step strategy provides an unprecedented rapid approach for industrial wastewater treatment.

Ti–48Al–2Cr–2Nb and TiC/Ti matrix composites were successfully joined using Ti–28Ni eutectic filler metal at different brazing temperatures. Microstructure and mechanical properties of the joints were systematically studied. The results showed that the joints all showed integral interfaces and the microstructures of the joints for 980 °C/15 min were detected as Ti-48Al-2Cr-2Nb/α2-Ti3Al + τ3-Al3NiTi2/α2-Ti3Al/Ti(s,s) + δ-Ti2Ni + TiC/Ti matrix composites. As brazing temperature increased, continuous Ti2Ni layer diminish and Ti2Ni phase became more discrete. However, α2-Ti3Al rich layer has thickened which deteriorates the properties of the joints. The highest shear strength achieved 469.5 MPa as the joint brazed at 1010 °C for 15 min. The evolution of microstructures brazed with different temperatures was studied and its relationship with the shear strength was also revealed in details.

Melt-spinning was successfully used to fabricate continuous Fe–6.5wt.%Si ribbons of 35 μm in thickness and 50–60 mm in width. Equiaxed grains measuring 2.4 ± 1.0 μm and columnar grains measuring 1.69 ± 0.72 μm were observed on the wheel and free surfaces, respectively. The as-spun ribbons showed strong 〈100〉 fiber texture, excellent room-temperature ductility, and very low core losses at high frequencies above 10 kHz. Compared with other fabricating techniques, the present work verified the feasibility for melt-spinning technique to fabricate ribbons of Fe–6.5 wt%Si with large-scale and thus cost-effective.

Powder metallurgy is an important method for preparing TiAl alloys due to its ability to minimize grain size and eliminate compositional segregation. However, the main obstacle to industrial applications is the large number of Kirkendall voids are inevitably introduced in the sintering process because of the diffusion rate discrepancy between Ti and Al atoms. The growth of these cavities in the matrix weakens the oxidation and corrosion resistance under service conditions. Here, a composite with an improved high-temperature antioxidant property and corrosion resistance was fabricated by manipulating chemical reaction, which benefit for higher-temperature applications, such as in aeronautics and aerospace.

Friction and wear properties of a high Nb-containing TiAl alloy against WC-8Co, Si3N4, and GCr15 in an unlubricated contact were investigated under various friction and wear conditions. The counterface material plays a key role in the friction and wear properties of the high Nb-containing TiAl alloy. The Si3N4/TiAlNb pair had the highest average friction coefficient caused by the abrasive wear and adhesive wear. The applied temperature had a strong effect on the friction coefficient of the Si3N4/TiAlNb and GCr15/TiAlNb pair. The WC-8Co/TiAlNb pair has the stable average friction coefficient under various friction and wear conditions due to the abrasive wear.

The solidification pathway and peritectic transition characteristic related to the solute concentration in TiAl-Nb system are calculated using the Thermo-Calc software. The results show that there are four typical solidification modes in the TiAl-Nb system, which are single β, hypo-peritectic, hyper-peritectic and single α solidification. The mole fraction of peritectic growth αp can be used to describe the undergoing extent of the peritectic tranformation (EPT) directly for the alloys with the peritectic transition, and βp/βmax can also be used to describe the EPT for the hypo-peritectic and Lp/(1 − βmax) for the hyper-peritectic. The mole fraction of αp shows a parabolic shape variation on a plot of solute Al at constant Nb content. The composition of the complete peritectic transformation (PT) is around 46.5 at.% Al for the TiAl binary alloy, and it gradually shifts to the direction of high Nb content with increase of solute Al. A temperature gap (△Tp) corresponding to the peritectic transition exists in Nb containing TiAl alloys and the △Tp shows a parabolic shape on a plot of solute Al at constant Nb content. The experimental results are generally in accordance with the theoretical rules.

Novel micro-/nano-dual-scale porous composites have been fabricated for nanopollutant filtration by powder metallurgy via four-step sintering of Ti, Al, and Nb powders incorporated with SiO2/ZrO2/Y2O3/MgO/ZnO nanoparticles, respectively. Afterward, phase formation, surface morphology, and pore features of the porous composites have been characterized with the changes of heat-treatment temperatures and additive nanoparticles. The separation efficiency of as-prepared Ti-48Al-6Nb porous alloy and porous composite membranes for 50 mg/L TiO2 nanopollutants from pure water can be calculated through determining the change of TiO2 concentrations after filtration. Compared with the Ti-48Al-6Nb porous alloy membrane with a separation efficiency of 89.91%, all porous composite membranes can enhance the separation efficiency up to 99.80%. In particular, favorable contributions of the porous composite membrane with nano-SiO2 are further ascertained by maintaining a high separation efficiency of above 99.95% after repe...

Seldom could metals and alloys maintain excellent properties in cryogenic condition, such as the ductility, owing to the restrained dislocation motion. However, a face-centered-cubic (FCC) CoCrFeNi high-entropy alloy (HEA) with great ductility is investigated under the cryogenic environment. The tensile strength of this alloy can reach a maximum at 1,251±10 MPa, and the strain to failure can stay at as large as 62% at the liquid helium temperature. We ascribe the high strength and ductility to the low stacking fault energy at extremely low temperatures, which facilitates the activation of deformation twinning. Moreover, the FCC→HCP (hexagonal close-packed) transition and serration lead to the sudden decline of ductility below 77 K. The dynamical modeling and analysis of serrations at 4.2 and 20 K verify the unstable state due to the FCC→HCP transition. The deformation twinning together with phase transformation at liquid helium temperature produces an adequate strain-hardening rate that sustains the stable plastic flow at high stresses, resulting in the serration feature.

A pre-deformation process was employed for a TiAl alloy via high-temperature torsion, in which the stability of the constituent phases was tailored, resulting in enhanced hardening capability and ductility via a gradient microstructure. A sample with a pre-torsion of 360° exhibited a yield strength of 475 MPa and an ultimate tensile strength of 592 MPa, with a tensile ductility of 47% at 850 °C. The tensile properties were significantly enhanced compared with the as-forged sample, which exhibited values of 395 MPa, 494 MPa, and 4.6%, respectively. The physical mechanisms for the significant enhancement of the mechanical property of the TiAl alloys were studied in-depth via of transmission electron microscopy, electron-backscattered diffraction, and high-energy X-ray diffraction techniques. The high strength is mainly attributed to the twin structure formed during torsion, while high fracture elongation correlates to the recrystallization of the γ phase at twin-twin sections and the load partitioning regulated by a hierarchical microstructure. When the tensile micro-strains along the loading direction and transverse direction in the γ phase of the pre-deformed TiAl alloy, a higher mechanical performance was obtained. Moreover, the fracture mode of the pre-torsional tensile sample is a combination of pores and cleavage facets which resulted from the microstructure after torsion. The gradient twin structure approach in this study provides a strategy for developing TiAl alloys with exceptionally high-temperature tensile property, and the results of the micromechanical behavior-microstructure-property relationship may improve the understanding of the plastic deformation of TiAl alloys.

Electron beam melting (EBM) has been applied to fabricate a high Nb-TiAl alloy with a fully dense microstructure and good tensile properties at both room and high temperatures. The effects of preheating and melting parameters on melting, solidification, phase transformation and resulting microstructure formation in as-EBM high Nb-TiAl alloy were investigated by performing a design-of-experiments. Results show that the limited EBM processing window can be broadened to produce different characteristic microstructures ranging from nearly fully lamellar γ/α2 to equiaxed γ grains. Such a broadened processing window has been achieved by using stronger preheating beam current. A numerical simulation was performed to understand temperature evolution at a fixed point of interest where electron beam passed several times with a certain line offset within one build layer. Both the preheating and melting stages were considered in the model. Modelling results show that a higher preheating beam current resulted in a longer hold time within the temperature range between 1300 and 1380 °C (i.e. single α-phase region). This helped to produce fine lamellar microstructure in the high Nb-TiAl alloy. Fundamental principles are thus proposed in terms of controlling microstructure formation and fabricating fully dense high Nb-TiAl alloy in as-EBM condition. Keywords: Electron beam melting, Additive manufacturing, Titanium aluminide, Microstructure, Mechanical property

Si-carbon composites have been considered as next generation lithium-ion battery anodes, with a view to sufficiently exerting the respective superiorities of high specific capacity of Si as well as excellent mechanical flexibility and electrical conductivity of carbon. However, direct blending of carbon with Si cannot obtain a synergy composite, resulting in inferior cycle properties during charge–discharge due to huge volume changes and deficient electron-conducting channels from the unavoidably aggregated Si. Herein, the composition of carbon fibers (CNFs) with Si nanoparticles (SiNPs) has been performed through UV-ozone surface modification followed by electrostatic self-assembly. It is found that solvent-free UV-ozone exposure of CNFs for 20 min successfully introduces carboxylic groups, as conventional acid treatment for 12 h. Besides UV-ozone surface modification provides an efficient and scalable route, the distribution and functionalization of CNFs can be also modified to effectively combine with amino-functionalized SiNPs. As a result, such Si-CNF composites containing 70.0 wt% SiNPs are able to exhibit excellent cycle performance with high coulombic efficiency of 74.8% at the 1st cycle and high specific discharge capacity of 1063 mAh g−1 at the 400th cycle.

For the self-supporting catalysts of Hydrogen evolution reaction (HER), the rational design and fabrication of substrate with porous structure and sufficient mechanical strength are critical to promoting the industrial HER application. In this work, a novel porous sintered Ni metal as the matrix for supporting catalysts was successfully prepared by powder metallurgy, and a self-supporting electrode for HER was fabricated via a simple sulfurization process on this sintered Ni metal matrix. For instance, MoS2/Ni3S2 nanorods (NRs) were vertically grown on the as-sintered porous Ni matrix, and the morphology of the sintered Ni matrix significantly affected the growth of MoS2/Ni3S2 NRs. The NRs@Sintered Ni electrode shows a low overpotential of η10 = 56 mV and Tafel slope of 82 mV dec−1. Compared with NRs@Ni foam, the NRs@Sintered Ni exhibits superior stability, where no exfoliation and cracks are observed on the surface of the electrode after 5000 CV cycles. Ultra-high stability and long-term durability are obtained over 100 h even at high potentials of 200 and 300 mV. Moreover, the tensile strength of the as-obtained electrode based on the sintered Ni manifests nearly 260 times higher than that of NRs@Ni foam, evidencing an excellent mechanical property. This work provides an idea for the preparation of self-supporting metal-based catalytic electrodes in large-scale hydrogen production.

Due to the lack of independent slip systems and the difficulty in mechanical twinning, D019 ordered α2-Ti3Al phase is known to be of substantial mechanical anisotropy, which partly accounts for the low ductility of two-phase (α2+γ) TiAl alloys. However, recent studies have shown that in Al-rich α2 phase compression deformation twins can be activated, which could be beneficial for the ductility of the alloys. In the present study, for the first time, we have revealed a novel internal twinning mechanism of α2 phase in a TiAl alloy deformed at high temperature. Three twin generations of a common type {20 2 ¯ 1} 1 ¯ 014> were revealed: Secondary twins formed within the primary twin, and tertiary twins formed in the secondary twin. It is important to note that this internal twin structure can only be observed in the 1 ¯ 1 ¯ 6> direction but not the commonly used 2 ¯ 0> direction. The shear plane P of all twin variants is of type {1 2 ¯ 10}. Due to the variation of the twinning elements between the twin variants, the twinning shear is blunted. The interfaces between the twin variants are characterized by high-resolution electron microscopy. Special attention was paid to the structure of the coherent and incoherent twin boundaries. The effect of chemical ordering is discussed. Our results also shed light on the complex twinning mechanisms in hexagonal structures.

High Nb-containing TiAl alloys have good oxidation resistance and mechanical properties, but the microstructure and the properties are substantially affected by the segregation. To quantitatively investigate the segregation behavior of Al during solidification, microstructures of directionally solidified (DS) Ti-45Al-8Nb (in atomic percent) alloy prepared at withdrawing rates of 30 μm/s and 200 μm/s and a temperature gradient of 4200 K/m were observed by optical microscope and electronic probe microanalyzer. The microsegregations were characterized by wave dispersive spectroscopy. The results show that the DS ingots include the no melting zone, directionally solidified zone with columnar grains, mushy zone, and quenched liquid zone. The primary dendritic arm spacings are 353 μm and 144 μm, respectively, for the two ingots. But the solidified microstructures of the ingots are large lamellar colonies, which contain a few B2 patches and γ bands induced by microsegregation. From dendritic zone to columnar zone, the volume fractions of B2 patches and γ bands decrease. The segregation extents of Al and Nb decrease with the increase of solidification rate. There exists an obvious back diffusion process of Al during solidification and cooling after solidification. According to evolution of Al concentration profiles from mushy zone to columnar grain zone, interdiffusion coefficient for Al in β-Ti at near solidus temperature is semiquantitatively calculated, and the value is (6 – 11) × 10−11 m2/s.

In this paper, the microstructure characteristic of a β solidifying TiAl alloy Ti-44Al-4Nb-4V-0.3Mo-Y (at. %) was investigated systematically. Cylindrical blanks were isothermal forged at 1250 °C within (α + β) two-phase-field and 1200 °C within (β + γ) two-phase-field under the strain rate of 0.02 s−1 and 0.03 s−1, with a total reduction of 75%. The microstructure of the alloy forged in α + β two-phase-field consists of lamellar colonies (only represents α2/γ lamellar colonies), mixture structure of β/B2 phase and γ phase as well as β/γ lamellae resulted from cellular reaction. However, a large amount of equiaxed γ grains appear with no β/γ lamellae available under forging in β + γ two-phase-field, and the average size of γ grain is about 10 μm. By further annealed at 900 °C for about 0.5 h, the structure of hot forging at 1250 °C is composed of β/B2 phase and equiaxed γ grains with the mean size about 5 μm. After heat treated at 1200 °C for 2 h, the microstructure of this annealed samples is characterized by fine grained fully lamellar (FFL) structure with size of 20 μm. A specially designed hot forging process is proposed, which prevents the existence of inhomogeneous lamellar structure.

A Ni-doped Al2O3 coating was prepared on Ti-45Al-8.5 Nb alloys by the cathodic plasma electrolysis (CPE) process in a 1.2 mol/L Al(NO3)3 ethanol solution containing different concentrations of Ni(NO3)2. The coatings deposited in these solutions mainly consisted of γ-Al2O3, NiAl2O4 and some metallic Ni. The Ni-doped Al2O3 coating exhibited increased oxidation resistance and adhesion compared with that of a pure Al2O3 coating. The mass gain of the Ti-45Al-8.5 Nb alloys after oxidation was significantly reduced if Ni was added to the coating. A large number of craters on the coating surface were filled with NiAl2O4 and metallic Ni that inhibited the oxide from contacting the substrate through the craters and pores. The metallic Ni and Al2O3 were also transformed to NiAl2O4 during oxidation at 900 °C. The structure of the coating was also stabilized by the formation of NiAl2O4. Therefore, the addition of Ni can be used as a sealing technology for ceramic coatings prepared by the CPE process.

LiBr has been incorporated into Li6.25La3Zr2Al0.25O12 garnet matrix to develop a new garnet-based composite electrolyte through high-energy ball milling followed by solid-state reaction. Of importance to note in the composite materials is joint occurrence of anion doping at Li6.25La3Zr2Al0.25O12 bulks and lithiated interface at Li6.25La3Zr2Al0.25O12 grain boundaries. Lattice distortion based on Br− substitution at partial O2− sites and high densification resulted from mergence effect of LiBr have been detected in the composite electrolytes. Li-ion conductivity and transport kinetics are thereby enhanced after LiBr incorporation. Also, LiBr addition into Li6.25La3Zr2Al0.25O12 causes the appearance of finely microporous feature for the composite materials without compromising pellet densification, which can favor interfacial contact and Li-ion motion between the composite electrolytes and electrode. A full cell with a Li metal anode, the newly developed solid electrolyte, tiny amount of liquid electrolyte, and a LiCoO2 cathode has been assembled in sequence. High cycle stability of the full cell can be derived from capacity retention of 91.8% and high discharge capacity of 111.6 mAh g−1 after 70 cycles at room temperature. Consequently, favorable contributions of LiBr addition towards solid-state cell system can be further ascertained.

Microarc oxidation (MAO) electrolysis plasma deposition was used to prepare Al2O3 coatings on Ti-45Al-8.5 Nb alloys to improve high temperature oxidation resistance. The surface and cross-section morphologies before and after high-temperature oxidation, the chemical composition, and the phase identification of the coatings were investigated by scanning electron microscopy (SEM), electron probe microanalyses (EPMA), and X-ray diffraction (XRD). The results show that Al2O3 coatings with a thickness of approximately 8 µm can be obtained on the Ti-45Al-8.5 Nb alloys by MAO for 600 s. The samples with the Al2O3 coatings exhibited better high-temperature oxidation resistance. A minimal weight gain of only 0.396 g/m2 after 100 h oxidation at 900 °C was observed for the coatings formed with a deposition voltage of 400 V and using a duty cycle of 3%. The deposition mechanism of the Al2O3 coatings and the effect of the MAO parameters are also described.

Porous TiAl alloys are anticipated to become next-generation high temperature structural filtration materials due to their promising applications in the field of high temperature gas/solid and liquid/solid two-phase separation. However, poor high temperature oxidation resistance at elevated temperatures greatly limits the use of the alloys for practical applications. Here, a novel TiAl/Ti3Al/Nb2Al/Ti5Si3/Al2O3 porous composite with hierarchical micro-pore structure has been developed by manipulating chemical reactions to improve the anti-oxidation properties of high Nb-TiAl-based alloys at elevated temperatures. Several characterization methods were used to investigate the phase transformation mechanism of the porous composites, including scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). The results show that the porous composites possess slow oxidation kinetics and the weight gain during oxidation declines as the Nano-SiO2 content increases after the 900 °C/100 h isothermal treatment. A minimum weight gain of 3.68 g/m2 is achieved for the composite with 6 wt% Nano-SiO2 particles after the isothermal treatment. The porous composite's excellent anti-oxidation properties are mainly attributed to the synergistic effect of Al2O3 and Ti5Si3 generated by the Nano-SiO2 reaction of TiAl and Ti3Al, which suppresses the O inward diffusion and Ti outward diffusion. This study provides additional insights into high temperature applications of TiAl-based porous alloys.

Martensitic transformations are utilized to refine the microstructure of high Nb containing TiAl alloys by rapid quenching from β phase region. Additions of β stabilizing elements, such as Cr and V, are verified to promote the β→α martensitic transformation. The morphologic and crystallographic characteristics of martensites in Ti-42.5Al-6Nb-x(Cr, V) alloys indicate that the martensitic α has a perfect Burgers orientation relationship with the parent β phase, and the lateral size of the martensite decreases with increment of β stabilizing elements. With subsequent heat treatment, a series of refined fully lamellar (FL) and nearly lamellar (NL) microstructures can be obtained, where the FL colony size can reach 30 μm with 2 at.% Cr and 50 μm with 3 at.% V respectively.

A constitutive equation based on the hyperbolic sinusoidal Arrhenius-type model has been developed to describe the hot deformation behavior of a β-γ Ti-Al alloy containing 8 at.% of Nb. Experimental true stress-true strain data were acquired from isothermal hot compression tests conducted across a wide range of temperatures (1273 K~1473 K) and strain rates (0.001 s−1~1 s−1), and the changes in the experimental conditions were reflected in the values of the Zener-Hollomon parameter. The impact of true strain was expressed through material constants (A, α, n and Q), and it was found that a 7th order polynomial is appropriate to express the relations between the true strain and these material constants. The average absolute relative error (AARE) and correlation coefficient (R) were used to evaluate the accuracy of the constitutive equation, and the values obtained were 6.009% and 0.9961, respectively. These results indicate that the type of constitutive equation developed here can predict the flow stress for this alloy with good accuracy over a wide range of experimental conditions. Thus, equations of this form could be applied more widely to analyses of hot deformation mechanism and microstructure evolution.

This study systematically investigated the low cycle fatigue deformation of a high Nb-containing TiAl alloy with a nominal chemical composition of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y at 850 °C by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and synchrotron-based high-energy X-ray diffraction (HE-XRD) techniques. Cyclic stress-strain (CSS) behavior, lattice strain, and peak broadening of {100}α2, {201}γ, and {202}ωo planes, phase transformations, and crack propagation behavior were obtained for samples with three total strain amplitudes: Δet/2 = ±0.25%, Δet/2 = ±0.28%, and Δet/2 = ±0.30%. At early deformation stages, α2lamellae transformed into ωo phase with a distinct orientation relationship, and a certain orientation relationship (OR) between them was observed after the following cyclic deformation. Furthermore, γ particles precipitated within the single ωo area. In addition, according to the peak intensity and peak broadening results, the ωo → B2 phase transformation occurred, leading to the appearance of single B2-phase areas. The lattice strains in the ωo phase were always in tension during the cyclic deformation and large differences of the lattice strains were found in the γ phase and α2 phase, not only the values but also the directions, which resulted in crack nucleated at and propagated along the α2/γ lamellar interface. This study provides a better understanding of the low cycle fatigue deformation of TiAl alloys.

Creep rupture tests of the as-forged Ti-45Al-8.5Nb-(W,B,Y) alloy with a duplex microstructure were carried out at 750–850 °C and 130–300 MPa. An investigation was performed to study the effects of temperature and stress on the microstructure and texture evolution, as well as on the creep properties of the forged high Nb-containing TiAl alloy with a duplex microstructure. The results show that the stress exponent n = 2.8–3.3, and the creep activation energy values are in the range of 300–400 kJ/mol. After creep, a regular arrangement of ωo precipitations was observed in the βo region. The un-transformed βo phase, which remained in the ωo interval, was examined using high resolution transmission electron microscope (HRTEM). It is shown that the growth of ωo precipitations and the morphology of residual βo phases are related to the creep temperature and stress. Certain βo stabilizer elements, such as Nb, segregate into α2 plates and cause ωo precipitation during dissolution of α2 plates at high temperature. The interfacial structure between the ωo and α2 phases was studied in detail using HRTEM. Meanwhile, texture evolution and dynamic recrystallization of the γ phase were studied using electron backscatter diffraction (EBSD). Observations on the deformation structure also demonstrate that ωo precipitation acts as an obstacle inhibiting crack propagation and growth of the γ phase. The corresponding formation mechanism and possible effects on creep are discussed.