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Journal of materials science & technology 102, 89 - 96 (2022). doi:10.1016/j.jmst.2021.06.041, In modern ��-solidified TiAl alloys, the decomposition of ��$_2$ phase is frequently observed during heat treatment or high-temperature deformation of the alloys. In this study, high-temperature deformation and decomposition mechanisms of ��$_2$ phase in a Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y alloy are investigated. In a sample deformed at 800 ��C, the precipitation of ��$_o$(��$_o$) phase is observed within the equiaxed ��$_2$ phase. The nucleation of ��$_o$ particles within the ��$_o$ matrix indicates the ��$_2$ ��� ��$_o$ ��� ��$_o$ transformation. In addition, numerous �� phase precipitates form within the ��$_o$(��) areas. The ��$_2$ lamellae decompose into ultrafine (��$_2$+��) lamellae and coarsened �� lamellae via ��$_2$ ��� ��$_2$ + �� and ��$_2$ ��� �� transformation, respectively. Moreover, the ��$_o$ phase nucleates within the ultrafine lamellae via ��$_2$ ��� ��$_o$ transformation. However, in a sample deformed at 1000 ��C, the nucleation of ��$_o$ particles is sluggish, which is caused by the efficient release of the internal stress via dynamic recrystallization (DRX). These results indicate that complex phase transformations can be introduced by the decomposition of ��$_2$ phase in TiAl alloys with a high amount of ��-stabilizing elements., Published by Ed. Board, Journal of Materials Science & Technology, Shenyang

The transformations between the phases α2 (Ti3Al) and ωo were investigated in a lamellar multiphase titanium aluminide alloy based on γ (TiAl). The paper complements an earlier investigation performed on the same material in which the importance of deformation-induced twin structures for the α2→ωo transformation was demonstrated. The present study shows that the reverse transformation ωo→α2 can also occur during high-temperature deformation. The transformation is probably triggered by constraint stresses, which exist between the different constituents due to the crystalline mismatch. The combined operation of mechanical twinning of the α2 phase and the reversible transformation fully converts the α2 lamellae into a mixture of α2 and ωo. This conversion greatly reduces the mechanical anisotropy existing in former α2 lamellae. Regarding the technical use of the alloy, the stability of the converted structure with respect to further annealing was also examined. The reported processes occur at the nano-meter and sub nano-meter scale, thus, advanced characterization techniques were applied, such as high-resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT).

This research provides a new insight into designing next-generation filtration materials for Nano-pollutants removal from wastewater, and potentially for gas/solid or liquid/solid separation in corrosive environment. Considering the high corrosion and oxidation resistance, Ti–40Al–10Nb–10Cr porous alloy was prepared by powder metallurgy and used as a mechanical support with a pore size of around 10 ​μm. Subsequently, metallic Zn nanoparticles was coated on the Ti–40Al–10Nb–10Cr porous framework by means of chemical vapor deposition, obtaining a novel microfiltration membrane with three-dimensional reticular architecture. Furthermore, the influence of Zn coating on surface morphology, pore feature, and filtration capability of Ti–40Al–10Nb–10Cr porous alloy was investigated. It is of note that the as-prepared microfiltration membrane exhibits an excellent Nano-pollutants filtration efficiency of greater than 99.989% from wastewater. Most importantly, such a three-dimensional reticular architecture provides excellent structural stability, allowing the reutilization of the microfiltration membrane for at least 200 filtration cycles of wastewater treatment.

High niobium content TiAl alloys and Al2O3 ceramics were successfully joined using novel Nb particle-reinforced Ag–Cu composite filler. The changes in microstructure brazing at different temperatures were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The corresponding mechanical properties were studied by shear tests. The results showed that the brazed joints prepared at 900 °C can be divided into three zones. The typical microstructure of the brazed joints is high niobium content TiAl/AlCuTi + AlCu2Ti/Ag(s,s)+Cu(s,s)+Nb/Ti3(Cu,Al)3O/Al2O3. The brazing temperature was shown to have a significant influence on the microstructure and reaction layer thickness. After brazing at 810 °C, macrodefects could be observed in the brazing seam. At higher brazing temperatures, brazed joints without any defects were obtained. The addition of Nb particles and the suitable thickness of the Ti3(Cu,Al)3O reaction layer are the main reasons for the high shear strength. When brazing at temperatures higher than 900 °C, the volume fraction of the brittle AlCu(Ti, Nb) phase was increased and a thick Ti3(Cu,Al)3O reaction layer was formed, which deteriorated the shear strength of the joint. The maximal shear strength joint reached 228 MPa at 870 °C for 10 min, which is a substantial increase compared with conventional brazing processes.

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.

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.

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.

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.

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.

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.

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

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.

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.

A CuTiZrNi amorphous filler metal was fabricated and utilized for brazing high Nb–TiAl alloys and GH4169 superalloys. The microstructure and mechanical properties of the high Nb–TiAl/GH4169 joints brazed from 900 °C to 1020 °C temperature range were studied. Results showed that the high Nb–TiAl/GH4169 joints consisted of three zones. The interfacial microstructure of the joints was high Nb–TiAl alloy/Ti3Al + Al3(Ni, Cu)Ti2 + Al(Ni, Cu)2Ti/AlCu2(Ti, Zr) + (Ti, Zr)(Ni, Cu) + Ti2(Ni, Cu)/Cr-rich (Cr, Ni, Fe)ss + Ni-rich (Ni, Cr, Fe)ss/GH4169 alloy. The element diffusion between the substrates and the filler alloy was intensified as the brazing temperature increased, and the interfacial reaction layer thickened. At 960 °C, the joint exhibited a maximum shear strength which was approximately 241.9 MPa. At higher brazing temperatures, the excessive AlCu2(Ti, Zr) phase and coarse Ti2(Ni, Cu) deteriorated the joint properties due to their inherent brittleness.

The high temperature torsion deformation of a duplex microstructure high Nb-TiAl alloy was investigated at 850 °C. The experimental results showed that the hardness and the density of twins increased with radial distance from the center to rim of 180° torsion sample. Besides, the “recrystallized”, “substructured” and “deformed” microstructures were also presented gradient and forming sub-grains was the transition stage between recrystallization and deformation. Moreover, γ grain recrystallization at twin-twin junctions occurred and the observation of dislocation-twin interactions revealed the formation of secondary twins by dissociating of the dislocation at twin-twin boundary. The dissociation of such dislocations interacting with twin boundaries and γ grain recrystallizations at twin-twin boundaries avoided excessive stress concentration, which allowed the alloy to continue plastic deformation.

We demonstrate excellent superelasticity and giant elastocaloric effect in a directionally solidified Co50.7V33.3Ga16 alloy. Owing to the formation of [001]A preferentially oriented columnar grains, the as-grown alloy displays nearly perfect superelastic behaviors with fully reversible compressive strain of up to 5.4%, narrow stress hysteresis of about 40 MPa and a wide operating temperature window from 258 to 373 K. Simultaneously, a giant adiabatic temperature change ΔTad of -18.7 K is obtained on removing a uniaxial stress of 400 MPa at room temperature, which is in good agreement with the theoretical values based on Maxwell relation (-19.0 K) and calorimetric measurements (-19.9 K). A large |ΔTad| of greater than 12.5 K is obtained over a wide temperature span of at least 115 K. Moreover, both superelasticity and elastocaloric effect remain good functional stability without any evident degradation for 100 cycles.

Electrical steel used in high speed motors and motors of electric vehicles and hybrid electric vehicles requires both good magnetic properties and high strength. Existing High-strength electrical steel (HSES) usually sacrifices magnetic properties to increase its strength. In this work, a new state-of-the-art HSES with excellent comprehensive performance is presented in which Cu-rich nanoprecipitates are used to strengthen the alloy. The precipitation behavior of Cu-rich precipitates was investigated by solution treatment and aging processes, and the results showed that the Cu-rich nanoprecipitates dramatically improved the yield strength without deteriorating the magnetic properties. A yield strength of 728 MPa and an iron loss P1.0T/400Hz of 15.05 W/kg were obtained, and the ratio of the yield strength to iron loss in this work is over twice that of commercial HSES materials.

Here we present a novel concept that uses a low volume fraction of nanoparticles to nano-treat Fe-Ti-B melts for a new high performance high modulus steel (HMS). Microstructures of the nano-treated Fe-Ti-B HMS were investigated. The nano-treated HMS consists of fine TiB2 particles and a strengthening phase of unexpected ultrafine Fe2B lamellas. While Fe2B phase is traditionally considered to be detrimental in steel making due to its inherent embrittlement effect, this study shows a successful conversion of the detrimental Fe2B phase into an advantageous one. The nano-treated HMS has significantly higher tensile strength compared to its conventionally made counterpart, 950 MPa vs. 510 MPa, while retaining the high Young's modulus, low density, and ductility. This new concept effectively demonstrates the successful structural refinement in Fe-Ti-B system using nanoparticles, without changing the solidification rate, which eliminates the dilemma between achieving enhanced mechanical properties versus manufacturing for mainstream applications and enables the use of stiff HMS for lightweighting of practical applications. A new mechanism of nanoparticle-induced solute enrichment at nanoscale in alloy melts was also proposed and has implications for new manufacturing routes of metals.

In this work, the hot deformation behavior of high Nb-containing TiAl alloy with β + γ phases was investigated. Elongation, twisting and bending of the microstructure are the primary deformation phenomena at low temperatures and high strain rates, whereas high temperatures and low strain rates are beneficial for dynamic recrystallization, and the grains are equiaxed. Based on the experimental flow stress data, the Arrhenius constitutive modeling was established. In addition, an artificial neural network was innovatively utilized to propose a high temperature constitutive modeling. The results denote that the accuracy of artificial neural network is higher than the Arrhenius equation. The proposed artificial neural network modeling provides high accuracy, and the average absolute relative error and correlation coefficient are 1.28 % and 0.99988, respectively. Thus, this is a new approach to establish a constitutive modeling for TiAl alloys.

The ωo phase transformation within βo areas in high Nb-containing TiAl alloys has been a popular research topic in recent years. However, a single ωo phase alloy is difficult to prepare because of the difficulty in reaching the equilibrium state at lower temperatures and the ambiguous position of the ωo phase in the phase diagram. In this study, a Ti–34Al–13Nb alloy based on the composition of the βo areas in high Nb-containing TiAl alloys is prepared. The phase transformation behaviors in the Ti–34Al–13Nb alloy during annealing are examined by electron microscopy. The results show that the α2 phase in different variants precipitates during annealing at 900 °C and follows the orientation relationship [11 2 ¯ 0]α2//[0001]ωo; (0001)α2//(11 2 ¯ 0)ωo with the ωo phase. The interfaces between different α2 variants are examined by high-resolution transmission electron microscopy (HRTEM). A unique twin-like interface between α2 grains is observed. The formation mechanism of this interface is considered to be related to the growth of two [11 2 ¯ 0]/60° α2 variants after self-accommodation. A stereographic projection analysis is also applied to explain the corresponding mechanisms.

The microstructural degradation of the high Nb-TiAl alloy during the fabrication process by electron beam melting (EBM) is reported. The lamellar structure of as-EBM samples in the bottom part of the build shows significant microstructure degradation, resulting in deterioration of tensile properties at both ambient and high temperatures. Microstructural analysis has been conducted by electron backscattered diffraction and transmission Kikuchi diffraction microscopy. The results show that the degradation of the lamellar structure is not only caused by the coarsening under the high-frequency thermal cycling during the fabrication of following layers but also attributed by the discontinuous dynamic recrystallization of the unstable initial lamellar structure resulting from the rapid solidification.

Fe-6.5wt.%Si alloy is an excellent soft magnetic material. Due to the appearance of ordering structures, the alloy shows poor ambient temperature ductility, and can only be cold rolled by specific rolling process. The rolling process with heat treatment is complex and time consuming. Meanwhile, high-energy electropulsing shows electroplastic effect. Heat treatment effect of the high-density electropulsing on microstructure and mechanical properties were investigated. The hot rolled sheet with 1 mm in thickness could be uniformly recrystallized in 33 s at 690 oC by appropriate high density pulses and the ductility was improved in comparison to the conventional heat treatment in a furnace. After the electropulsing treatment, the hot rolled sheet could be warm rolled by 50% reduction of the thickness after one pass without edge crack.

Ordered omega (ω o ) phases are considered to be equilibrium phases in high Nb-TiAl alloys at intermediate temperatures. However, most studies have focused on the evolution behaviors of these phases during annealing, while the precipitation behavior has not been systematically studied. In the present work, the precipitation behavior of the ω o phase (B8 2 structure) in the Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y (at.%) alloy during annealing at 850 °C was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After annealing at 1150 °C for 2 h followed by water quenching, the ω o phase in the as-cast alloy transformed to the β o phase. At the initial stage of annealing processing at 850 °C, the ω o phase re-precipitated at the γ/β o boundaries and the size of the ω o phase increased with the annealing time. Moreover, the γ plates precipitated within the β o phase at this temperature became coarser and tended to globalize during annealing. In addition, the α 2 and D8 8 -ω phases were also observed at the γ/β o boundaries. Energy dispersive X-ray spectroscopy (EDS) and Scanning transmission electron microscopy (STEM) analysis showed that the D8 8 -ω phase is rich in W and Nb elements. The corresponding mechanisms of the transformation mentioned above were discussed.

During the processing of high Nb-containing TiAl (Nb-TiAl) alloys, the cooling rate at certain parts of the components can be very high, especially at thin parts. Moreover, annealing treatment must be applied to TiAl alloys to improve their mechanical properties. In this study, the phase transformation mechanisms in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy during subsequent annealing were characterized using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results show that βo, α2, and massively transformed γ phases co-exist in the as-quenched microstructure. Some fine γ laths nucleated in the primary α2 phase in the quenched samples. After annealing at 800 °C for 1 h, numerous extremely fine γ laths precipitated in the bulk α2 phase and could only be recognized using TEM imaging. The ωo particles at sizes of 0.5–1 μm precipitated in the retained βo phase and nearly consumed all of the βo areas. More interestingly, some coarsened γ grains in true-twin relationship were observed at the boundaries of the lamellar colony and βo(ω) regions. The orientation relationship between βo(ω) and coarsened γ was confirmed to be the following: [110]β//[11 2 ¯ 0]ω//[111]γ, (11 1 ¯ )β//(0001)ω//(1 1 ¯ 0)γ. After annealing at 800 °C for 100 h, the βo phase region transformed into small ωo particles and equiaxed γ grains and still followed the above-mentioned orientation relationship. The α2 phase only existed as thin laths in the lamellar structures in a small volume fraction. These results indicate that the ωo phase is stable at 800 °C. Possible mechanisms of these phase transformations are discussed.

Ordered ω phases are equilibrium phases at intermediate temperatures in TiAl alloys. However, a systematical research on the effects of alloying elements on the ordered ω phase transformations is still lacking. The effects of alloying elements on the ordered ω phase formation should be considered in future design of TiAl alloys. In this study, the effects of Cr, Mn and Ni on the ordered ω phase formation are investigated. The results show that the precipitation of ordered ω phase is promoted by addition of Ni, i.e., the sizes of ordered ω phase were in micron-level. However, the addition of Mn greatly suppresses the growth of ordered ω phases. During thermal exposure at 850 °C, a Ti-Al-Ni ternary phase formed within the original βo phase areas in the Ni-containing alloy. Meanwhile, no ordered ω phase existed in an annealed Mn-containing alloy. In both as-cast and annealed conditions, the effects of Cr seemed to be similar to those reported in Mo and W-containing alloys. Differential scanning calorimetry (DSC) experiments suggested that the precipitation temperatures of ordered ω phase in these three alloys were affected by minor alloying elements, which fitted well with the observed phenomena in the electron microscopy study. Keywords: TiAl, Phase transformation, Microstructure, Electron microscopy

High Nb-containing TiAl (Nb–TiAl) alloys possess mechanical properties at elevated temperatures superior to conventional TiAl alloys. However, the strengthening mechanisms induced by Nb addition have been discussed controversial for a long time. In the present study, the dislocation structures in a polycrystalline high Nb–TiAl alloy after tensile tests at 700 and 900 °C were investigated by transmission electron microscope (TEM) observation. The results show that abundant double cross slip of ordinary dislocations is activated in the samples deformed at 700 °C. The dislocations are pinned at the jogs and numerous dipoles are observed. Debris can be commonly observed in the vicinity of screw dislocations. Trace analysis shows that the cross-slip plane is (1 1 0)γ at 700 °C but (1 1 1)γ octahedral plane at 900 °C. Three-dimensional (3D) dislocation structures, caused by cross-slip and annihilation of ordinary dislocations, were observed along the screw orientation. The dipoles and debris produced by...

The friction weldability of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy has been investigated by optimizing process parameters and analyzing the microstructures and tensile properties of the joints. The as-cast alloy with a nearly lamellar (NL) microstructure and the as-forged alloys with a duplex (DP) microstructure have been successfully welded. All the joints have a severe deformation zone (SDZ) and a transition zone (TZ) between the parent metal (PM) and SDZ. SDZ, showing a biconcave lens geometry, has a maximum thickness of hundreds of micrometers at the periphery. TZ is hundreds of micrometers thick. All SDZs have a fine-grained DP microstructure with a grain size of a few micrometers. For the joint of the as-cast alloy, the TZ consists of deformed lamellar colonies as the major constituent and partially recrystallized grains as the minor constituent. For the joint of the as-forged alloy, the TZ is similar to both the PM and SDZ, showing a DP microstructure. The grain size, volume fraction of &gamma, grains, and the remnant lamellar colonies all increase with the distance from the SDZ. All joints presented perfect metallurgical bonding. The strengths of the joints are higher than those of the corresponding PMs. This indicates that the studied alloy has good friction weldability.

The properties of Ti–Al–Nb alloys strongly depend on alloy compositions and microstructures. Proper design of these important aspects requires detailed knowledge of the solidification sequences and liquidus projection. However, there are still some conflicts between previous experimental and calculated results about the liquidus projections in the composition range of Ti-(40–60)Al-(5–30)Nb and this has hindered Ti–Al–Nb alloy development. In this present work, we explicitly investigated the as-cast microstructures and phase constitution of 50 alloys with the composition range of Ti-(40–60)Al-(5–30)Nb and clarified the solidification sequences. We re-constructed the partial liquidus projection and found that the primary β phase region extend to just below 55 at.% Al content and the primary α phase region extend to just below 24 at.% Nb content. Moreover, it was observed that the invariant reaction between Liquid, β, α and γ phases is a peritectic reaction (Liquid+β+γ→α) and all the Liquid+β+γ, Liquid+β+α and Liquid+α+γ three-phase regions are of peritectic type. The present work provides essential information necessary for optimizing the Ti–Al–Nb thermodynamic database to accelerate alloy development.

Plate-like η-Nb(Ti,Al)3 precipitates with hierarchical microstructure comprised of alternating arrangement of γ-(Ti,Nb)Al and η plates in the Ti-55Al-22Nb alloy have been investigated using transmission electron microscopy. In addition, the orientation relationship (OR) between η precipitates and γ matrix is determined to be tetragonal-tetragonal. The habit plane of η precipitates is along {022) crystal planes of γ matrix. And the interface structure between them can be described using the classical ‘terrace-disconnection’ theory. Furthermore, the phenomenon of phase separation responsible for the formation of this hierarchical microstructure can be well explained by the chemical free energy, that is, Al-depletion and Ti-enrichment in η precipitate drive the splitting of η precipitate. To the authors' knowledge, this is the first work which demonstrates such a hierarchical microstructure in η precipitate in high Nb-containing TiAl alloy.

Selective electron beam melting (SEBM) is considered to be a promising method to fabricate TiAl alloy components with complex geometries. In this work, the effect of annealing on the microstructure and anisotropy of tensile properties of SEBM-produced Ti–47Al–2Cr–2Nb alloys was systematically investigated. The results indicated that the SEBM-produced TiAl alloy samples exhibited equiaxed and elongated grains in the transverse and longitudinal directions, respectively, and presented anisotropy of the tensile properties. For the sample produced at an energy density of 24.07 J/mm3, the microstructure in the vertical cross section presented an elongated lamellar structure consisting of (α2/γ) lamellar colonies, after annealing at 1310 °C for 3 h; on the other hand, for the same sample, elongated grains were almost invisible after annealing at 1370 °C for 2 h. Further, for the sample produced at an energy density of 35 J/mm3, the elongated grains remained after annealing at 1370 °C for 2 h and were destroyed by cyclic heat treatment. The texture intensity and anisotropy of the tensile properties of the two samples fabricated with different processing parameters were weakened by annealing at 1370 °C for a duration of 2 h and cyclic heat treatment between the α+β+γ and α phase regions, respectively. Both the tensile strength and elongation increased as a result of heat treatment. Moreover, the phase transformation during annealing was analyzed.

The large scale ingot of Ti-45Al-8.5 Nb-(W, B, Y) alloy, which was manufactured by plasma arc cold hearth melting process, was successfully forged by a two-step canned forging process with a total deformation of 85%. The pancake is crack-free with smooth surface and presents recrystallized duplex microstructure. Additionally, the fraction of β/B2 phase in the as-forged microstructure has dropped. The high angle grain boundaries are dominant among recrystallized grains. After the heat treatments at different temperatures, the fine duplex, nearly lamellar and fully lamellar microstructures can be obtained respectively. Compared with the as-cast condition at room temperature, the ultimate tensile strength increases from 633 MPa to 897 MPa and the elongation improves from 0.23% to 2.2%. The strengths of as-forged TiAl alloy are relatively high at high temperatures. Moreover, superplastic behavior appears above 900 °C.

Microstructural instability induced by shot peening was investigated in a Ti-45Al-8.5Nb-(W, B, Y) alloy following high temperature exposure. After shot peening and thermal exposure at 1000 °C for 300 h, fine grains are formed in the outermost (FG layer). Underneath, coarse grains (CG layer) are formed. The FG layer is composed of completely recrystallized γ grains. The CG layer is composed of incompletely recrystallized γ grains, where the critical strain for recrystallization is not reached. During long term thermal exposure, α 2 lamellae can undergo dissolution, precipitation and growth. After thermal exposure for 300 h, large α 2 grains precipitated at the γ/γ interface or inside the γ grains in both FG and CG layers. The precipitated α 2 particles almost have the same orientation with primary α 2 lamellae, indicating that nearly no recrystallization phenomena occur for α 2 phase. So the γ lamella is easier to recrystallize than that of α 2 lamella at the same temperature and residual strain.

The isothermal sections of the phase diagram and the liquid projection of the Ti-Al-Nb ternary system as well as the phase transformations of a Ti-46.5Al-16.5Nb alloy were calculated using Pandat software. The solidification sequence and subsequent phase transformations of the alloy were verified by microstructural observations and thermo-analysis (TA). The tie-triangle of the three-phase region of α + β + γ at 1300 °C and the tie line of the α and β phases at 1400 °C were identified using electron probe microanalysis (EPMA). Neither the phase equilibria of the α, β and γ phases at 1300 °C nor the tie line of the α and β phases at 1400 °C are in agreement with the calculated phase diagrams reported in the literature. The solidification sequence and subsequent solid-state phase transformations of this alloy were determined as follows: liquid → liquid + β → β → α + β → α + β + γ → β + γ (The β phase and the B2 phase were not differentiated in the present paper.) The phase transformation temperatures were also measured by differential scanning calorimetry (DSC).

Phase transformation behavior and kinetics of Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at. %) alloy during continuous cooling under the cooling rates of 3 °C/min, 5 °C/min, 10 °C/min, 20 °C/min, 40 °C/min and water quenching (WQ) were investigated in this study. The phase composition and microstructure of the alloy were studied using X-ray diffraction (XRD) and optical microscopy (OM), respectively. The expansion and the volume fraction evolution of phases during continuous cooling were investigated by dilatometry (DIL), and the microhardness of the alloy suffered from cooling was also tested through micro-sclerometer. The results indicate that the phase transformation from parent α phase to γ and α2 take place during continuous cooling, the α→γ phase transformation occurs in the temperature range of 1150 °C–1310 °C while the α→α2 phase transformation takes place between 1100 °C and 1220 °C. The transformation temperature decreases with the cooling rate increasing, and the higher the cooling rate is, the higher the phase transformation rate is. The relative fraction of γ phase and α2 phase much depend on the cooling rate, and the lamellar colony sizes and interlamellar spacing of crystal structure decrease with the cooling rate increasing. The microhardness of high Nb–TiAl alloy increases with the cooling rate increasing due to the variations of phase composition and crystal structure.

Porous materials have always attracted extensive attention owing to their low density, tunable porosity and high surface area. Generally, porosity is introduced in amorphous materials through dealloying or electrochemical dealloying processes. In this work, an iron-based surface porous network was successfully fabricated utilizing selective electrochemical dissolution of Fe-Si alloy ribbons based on the cellular structure prepared by melt-spinning technique. After 30 s, the surface of the ribbon gradually becomes flat and grains can be observed in the first stage of electrochemistry; after an extra 10 s, the pores spread throughout the surface of the ribbon in the second stage. The average size of pores is about 310 nm and the average size of the ligament is 150 nm. The associated dissolution mechanism has been proposed based on the inhomogeneous composition of the center and edge of the cell. The entire process of electrochemical dissolution has been divided into two stages and the entire duration of synthesis does not exceed one minute. This method is extremely feasible and provides a promising strategy for preparing surface porous materials for selective electrochemical dissolution of cellular structure.

A series of β-γ Ti-44Al-8Nb-0.2B-0.2Y alloys have been modified with 0.5 to 2.0 at.% of the β-stabilizing elements Mn, Cr, Mo and V. Additions of Cr and Mo alone result in a decrease in the flow stress, whereas the opposite effect was observed for additions of V. For alloys with Mn additions, a minimum value of the flow stress was achieved in the alloy with 1.5% Mn. For alloys with combined additions, optimum hot deformation behavior was obtained for the alloy with 1.5% Mn and 1.0% Cr.

Microstructure and microsegregation of directionally solidified Ti–45Al–8Nb alloy were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalyzer (EPMA). For the alloy solidified at the solidification rates (v) ranging from 10 to 400 μm·s−1, the microstructure of the mushy zone exhibits a cellular-dendritic structure at lower growth rate (v = 10–50 μm·s−1) and a typical dendritic morphology at higher growth rate (v = 100–400 μm·s−1). The relationship between primary dendrite arm spacing (λ 1) and v is λ 1 = 1.08 × 103 v −0.35. Al and Nb elements segregate at interdendritic zone and in the dendritic core, respectively. In solid of mushy zone, a relatively flat concentration profile is observed for the typical dendrite structure, and Nb enriches in B2 phase induced by β → α + β transformation. The content of B2 phase is hardly affected by v. The extent of microsegregation in steady-state zone decreases at a lower growth rate because holding the samples at higher temperature after solidification for a long time can homogenize the solid effectively.

Intermetallic Ti–xAl–8Nb (x = 41, 43, 45, 47, 49; at%) alloys were solidified unidirectionally upwards with a constant temperature gradient of G = 3.8 K·mm−1 at wide range of growth rates of v = 10–400 μm·s−1 using a Bridgman directional solidification (DS) furnace. Microstructural parameters including the primary dendrite arm spacing (λ 1), secondary dendrite arm spacing (λ 2), dendrite tip radius (R) and mushy zone depth (d) were measured statistically. The values of λ 1, λ 2, R and d decrease as the growth rate increases for a given composition (x). The values of λ 1, λ 2, R and v increase with the increase in x value, while the value of d firstly increases and then decreases with the increase in x value for a given v. The relationships between λ 1, λ 2 and R were analyzed by the linear regression. The average growth rate exponent of λ 1 is 0.29, which is in accordance with the previous experimental observations, and that of λ 2 is close to the previous experimental results, while those of R and d are lower than the results in other alloy systems. In addition, theoretical models for λ 1, λ 2 and R were compared with the experimental observations, and a comparison of the present experimental results with the theoretical models and previous experimental results was also made.