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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.

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.

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.

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.

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.

Cathodic plasma electrolysis (CPE) has been used as an environmentally-friendly process to deposit metal coating on metal substrate. In this work, a coating of zinc was deposited via CPE on a low carbon steel wire using direct current (DC) and square wave pulsed current (SWPC) power. The experimental results showed that application of continuous DC power did not produce zinc deposition, but a dense coating of zinc was easily applied to the substrate with the application of SWPC power at 120 V, 4000 Hz and an 80% duty cycle. The resulting coating was porous with some imperfections on the surface, but the morphology and quality of the coating could be controlled by adjusting the parameters of the SWPC power. Increasing both the duty cycle and frequency of the power resulted in a lower deposition rate, but the coating was denser. The steel wire surface was cleaned using plasma generated in the CPE process and the Zn coating was found to accumulate during the rise in the power pulse. A linear regression model was established to describe the relationship between coating thickness and power parameters.

γ-TiAl alloys have attractive properties such as low density, high creep resistance, high modulus, good oxidation and burn resistance. The main concern for the fabrication and use of TiAl alloys is the low deformability at room and high temperatures. Alloying and processing have been developed during the last three decades to improve the high-temperature deformability. Fabrication methods have been developed, including powder metallurgy and ingot metallurgy, to provide sheet materials for aerospace applications. This paper reviews the current status of TiAl sheet materials in terms of fabrication methods, microstructural control and mechanical properties. Future challenges and opportunities are also discussed.

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.

The cyclic stress-strain (CSS) behavior of high Nb containing TiAl alloy with a duplex microstructure was investigated at 850 °C. Transmission electron microscope (TEM) observations and electron backscattered diffraction (EBSD) techniques were carried out to obtain insight into the microstructure evolution governing this behavior. At low strain amplitude, the material exhibits a rapid saturation of stress amplitude. At intermediate and high strain amplitude, the CSS behavior is characterized by generally cyclic softening. The changes of microstructure are strain-induced phase transformations and dynamic recrystallization, which lead to a degradation of lamellar microstructure. Twin boundaries can promote discontinuous dynamic recrystallization of γ phase, for processing relatively high energy. α2 + γ → B2, α2 → B2, and α2lamellae → γ phase transformations are detrimental to fatigue life of the material, because fracture propagation are usually along B2 phase boundaries and γ grain boundaries, thus, the fracture mode is a combination of ductile fracture and intergranular cleavage fracture.

This work is devoted to coating perovskite-type La 2/3−x Li 3x TiO 3 (LLTO) grains with a Li 2 O-B 2 O 3 (LIB) glass, considering good wetting properties, low melting point, and no grain boundary of LIB, to obtain novel LLTO/LIB composite electrolytes used for all-solid-state batteries. Perspectives with regard to the effect of LIB glass on mitigating the LLTO-related issues, such as low grain boundary conductivity, have been illustrated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and other techniques. From the characterization observations, the LLTO/LIB composites with the LIB phase filling at grain boundaries of LLTO show great densification, local lattice distortion and Li-ion redistribution. Obvious improvement on conduction properties both along LLTO grains and across LLTO electrolyte/LiCoO 2 (LCO) electrode interface occurs by using appropriate amount of LIB materials. Specifically, LLTO/1.5 wt% LIB composite electrolytes achieve a grain boundary conductivity of 1.06 × 10 −4 S/cm at room temperature, which is more than one order of magnitude higher than that for pure LLTO. The favorable conduction behaviors have been also discussed on the basis of microstructure and Li-ion distribution of the composites, as well as intrinsic attributes of LIB glass, such as isotropic homogeneity and existence of non-bridging oxygens.

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.

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...

Metal surface modification by electrolytic plasma processing (EPP) is an innovative treatment widely commonly applied to material processing and pretreatment process of coating and galvanization. EPP involves complex processes and a great deal of parameters, such as preset voltage, current, solution temperature and processing time. Several characterization methods are presented in this paper for evaluating the micro-structure surfaces of Ti45Al8Nb alloys: SEM, EDS, XRD and 3D topography. The results showed that the oxide scale and other contaminants on the surface of Ti45Al8Nb alloys can be effectively removed via EPP. The typical micro-crater structure of the surface of Ti45Al8Nb alloys were observed by 3D topography after EPP to find that the mean diameter of the surface structure and roughness value can be effectively controlled by altering the processing parameters. The mechanical properties of the surface according to nanomechanical probe testing exhibited slight decrease in microhardness and elastic modulus after EPP, but a dramatic increase in surface roughness, which is beneficial for further processing or coating.

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.

Nano-porous materials are spreading widely in various fields, such as electrochemistry, biochemistry, adsorption, and catalysis. In the present work, nano-porous γ-Al2O3 layers with 200 nm thickness have been fabricated on porous Ti-48Al-6Nb alloys by soaking in NaOH solution. Corrosion products of γ-Al2O3 with 200 nm thickness form after 48 h and begin to transform to nano-porous structures at 72 h in the areas with a smaller radius of curvature. Then the nano-pores shrink and reach the final size of 60 ± 10 nm within 120 h. Increasing the concentration of NaOH solution or soaking time accelerates the formation of nano-porous layers and the best situation is soaking for 120 h in a 2.0 mol/L NaOH solution. Meanwhile, the nano-porous γ-Al2O3 layers improve the electrochemical corrosion resistance of the porous alloys due to the more positive self-corrosion potential/current and the right-moved passive region. The NaOH corrosion method to fabricate nano-porous γ-Al2O3 layers is much easier than other methods and the porous alloys possess the better corrosion resistance and absorbability thanks to the nano-porous γ-Al2O3 layers and the micron-porous Ti-48Al-6Nb matrix. Keywords: Porous TiAl alloys, Nano-porous, γ-Al2O3, Total pore area, Polarization

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.

In order to study the hot deformation behavior and microstructure evolution of β-γ TiAl alloy containing high content of Nb, isothermal hot compression tests were conducted in the strain rate range of 0.001–1.0 s−1 and temperature range of 1273–1473 K. The Arrhenius-type constitutive equation was successfully established for expressing the non-linear relation among true stress, strain, strain rate and deformation temperature. The average absolute relative error and correlation coefficient are 6.009% and 0.9961, respectively, which reflects good predicted accuracy of developed constitutive equation. Conventional and 3D power dissipation maps based on the developed constitutive equations and dynamic material model were successfully established. Efficiency of power dissipation increases with higher deformation temperature, lower strain rate and higher strain, which indicates that more power is dissipated through changing microstructure. These deformed specimens with equal efficiency of power dissipation exhibit similar microstructures during hot deformation. The content of dynamic recrystallization (DRX) grains increases with the increase of efficiency of power dissipation (η). The microstructures with η ≥ 0.55 mainly consist of equiaxed DRX grains, corresponding to the temperature range of 1273–1473 K and the strain rate range of 0.001–0.01 s−1 at the strain of 0.5, which could be the optimum hot working window of alloy. Keywords: β-γ TiAl alloy containing high content of Nb, Constitutive equation, Power dissipation map, Microstructure evolution

γ-TiAl alloys, including two categories (the conventional TiAl and the high Nb-containing TiAl (high Nb–TiAl)), are technologically intriguing because of their applications at high temperatures. Specifically, the service temperature of the high Nb–TiAl alloys is 60–100 °C higher than that of conventional TiAl alloys. Recently developed TiAl alloys, for example TNB, TNM, β-γ alloys, belong to the high Nb–TiAl alloys, displaying similar behavior in phase transformation, strengthening, oxidation at high temperatures, and relationships between composition, microstructure, and mechanical properties. This work presents an in-depth review of the high Nb–TiAl alloys regarding the advances in phase diagram, formation mechanism of the new γ1 phase, microsegregation induced by adding a high content of alloying element Nb, and the mechanism of the B2/ω phase formation. Some challenges in developing the high Nb–TiAl alloys are also discussed.

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.

Fracture and deformation characteristics of the Ti-based metallic glass matrix composite have been studied by the tensile test and the in situ TEM tension test. Typically, the composite exhibits the high strength and considerable plasticity. Microscopically, it was found that shear deformation zone formed at the crack tip in glass phase, which can bring about quick propagation of shear bands. However, the plastic deformation zone nearby the crack tip in dendrites will postpone or retard the crack extension by dislocations. The attributions of micro-deformations to mechanical properties of composites were discussed.

Microstructural instability of fully lamellar Ti–45Al–8.5Nb–(W, B, Y) alloys during long-term thermal cycling (500 and 1000 cycles) between room temperature and 900 °C or 1000 °C was investigated. The coarsening of lamellar colony boundaries and precipitation of B2 particles are produced as a result of the microstructures degradation. The boundaries are coarsened by the precipitation of coarse γ grains. The width of the coarsened boundaries increases with increasing cycles and temperatures. Owing to the large interface energy, the coarsening of colony boundaries prefers to occur at boundaries with high misorientation angles. There is a coincidence-site lattice relationship between the precipitated γ grains and the lamellar colony at one side of the boundary. In addition, the precipitation of B2 phase occurs not only at lamellar colony boundaries by consuming (α2 + γ) lamellae but also at γ/γ interfaces via two possible mechanisms will be discussed in detail.

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.

The ordered ω phase in as-cast Ti-45Al-8.5Nb-0.2B alloy and its phase transformation during heat treatment are investigated. Ordered ω variants are observed to uniformly precipitate within the βo area in as-cast Ti-45Al-8.5Nb-0.2B alloy. After annealing at 850 °C for 500 h, the βo areas are replaced by large B82-ωo grains. Small γ precipitates are observed at the grain boundaries of the ωo phase and are thought to be transformed from the βo phase. Moreover, the ωo precipitates directly transformed from the parent α2 laths are found within the lamellar colonies. The orientation relationship between the ωo phase and the lamellar structure is 2 ¯ 0]α2; (111)γ//(11 2 ¯ 0)ωo//(0001)α2. The interfaces between the ωo and γ are semi-coherent. The ωo phase is an equilibrium phase at 850 °C in high Nb-containing TiAl alloys. When annealed at 1250 °C, the ordered ω is eliminated in a short time, and the βo phase is substituted by the coarsened α2 laths in the lamellar colonies after 12 h annealing.

Ti–48Al–6Nb porous alloys were synthesized by the powder metallurgy (PM) method, and the associated phase transformation and pore parameter were investigated in order to reveal the pore-formation mechanism. The present results indicate that the Nb–Al and Ti–Al phase transformations contribute to the pore-formation. It was found that the five-step phase transformations for the Ti–48Al–6Nb porous alloys occur as follows: (1) Ti + Al → TiAl3 at 600–700 °C; (2) Nb + Al → NbAl3 at 700–900 °C; (3) TiAl3 + Ti → TiAl at 900–1100 °C; (4) TiAl + Ti → Ti3Al/TiAl at 1100–1350 °C; (5) NbAl3 + Nb → Nb2Al and the Ti3Al turns to the major phase at 1350 °C. These phase transformations made the pore-diameter increasing continuously from 1.71 μm to 12.10 μm and also made the pore volume distributing widely. At the second step of 700–900 °C, the Nb–Al phase transformation leads to 5% more volume expansion compared to the Ti–Al based porous alloys. Meanwhile, the porosity and total pore area initially increase and then decrease at this step, but they increase intensely at the final step, which is needed as a catalytic carrier. Keywords: Porous materials, Phase transformation, Heat treatment, Powder metallurgy, Microstructure

Directional solidification (DS) experiments with thermal stabilization (TS) treatments were performed on Ti–46Al–5Nb (at.%) alloys in a Bridgman-type furnace using a quenching technology. Influence of the TS treatment on mushy zone and directional growth afterwards were investigated. The results show that the length of the mushy zone decreases but the β dendrite spacing in directional growth significantly increases with increasing TS time. During the DS process, β dendrite spacing is more homogeneous and its growth direction is more inclined to parallel to the axial direction with increase of the TS time. Al solute concentration in the mushy zone in a steady-state is always lower than that in original as-cast alloys. The mushy zone with the columnar β and α grains is easily produced after TS treatment on the alloys with microstructures of the directional dendrite segregation morphology before DS starting. TS treatment results in the redistribute of solute Al thus changes the phase constituent in the mushy zone. An appropriate TS is necessary to produce the L + β + α region in the mushy zone, which is of great benefit to control DS microstructures of TiAl peritectic alloys.

In this work, low-cost titanium was fabricated by gelcasting of titanium hydride powder. The effects of morphology and grain composition of powder raw material and solid loading on the rheological behavior of gelcasting slurry were studied. The degreasing, dehydriding and sintering behaviors of gelcasted green body were investigated by differential thermal analysis (DTA) and dilatometer. The results show that the solid loading of titanium hydride slurry reaches 50 vol%. Combination of dehydriding and sintering in one process accelerates the densification, and the relative sintered density of the final part achieves 96.5 %. In order to test the ability of gelcasting process for fabricating structural materials, a resin handle produced by 3D printing technology was used as a model and a titanium handle was successfully fabricated. Higher solid loading and better sinterability of titanium hydride powder promote manufacture of bulk titanium with high relative density, complex shape and well-defined microstructure.

High Nb containing TiAl porous alloys were synthesised by powder metallurgy (PM). In order to reveal reaction mechanism of Nb in preparation of the porous alloys, Nb–Al diffusion reaction was investigated using diffusion couples at relatively low temperatures of 600–800°C. The porous Nb–Al diffusion layer was identified as NbAl3 phase and the thickness of diffusion layer indicated that the Nb–Al diffusion mainly occurred at 800°C. In addition, the pore diameter distribution indicated that Nb–Al diffusion also contributed to the increase in pore diameter. According to these results, the diffusion reaction model was established for high Nb containing TiAl porous alloys.

There is widespread attention to surface profile and modification of 304 stainless steel for research development and application. Here, a successful electrolytic plasma processing (EPP) technique has been developed for both surface pretreatment and coating deposition of 304 stainless steel. Representative images confirm that the number of the pits increases and the ravines gradually disappear on the steel pretreated by EPP with the increase of processing time and applied voltage. Moreover, there is an obvious enhancement in surface roughness of 304 stainless steel after EPP pretreatment. In the case of coating deposition, the further EPP modification conducted on the pretreated sample offers a simple and effective technique for the production of zinc coatings having the features of full coverage and homogeneous distribution. The results show that a zinc coating with a thickness of approximately 0.5 μm can be obtained on the 304 stainless steel by means of EPP for only 60 s.

Microstructure instabilities of the fully lamellar Ti–45Al–8.5Nb–(W,B,Y) alloy were investigated by SEM and TEM after long-term thermal cycling (500 and 1000 thermal cycles) at 900 °C. Two major categories of microstructure instability were produced in the alloy after the thermal cycling: 1) The discontinuous coarsening implies that grain boundary migrations are inclined to occur in the Al-segregation region after the long-term thermal cycling, especially after 1000 thermal cycles. Al-segregation can be reduced during the process of long-term thermal cycling as a result of element diffusion; 2) The α 2 lamellae become thinner and are broken after 1000 thermal cycles caused by the dissolution of a 2 lamellae through phase transformation of α 2 →γ. The γ grains nucleate within the α 2 lamellae or (α 2 +γ) lamellae in a random direction.

Low-molecular mass organic gelcasting is utilized to fabricate titanium structural materials. The aim is to find a low-residual gel system for the fabrication of titanium with good ductility. The thermally reversible gel system is composed of 1,3:2,4-dibenzylidene sorbitol (gelator) and 1-butanol (solvent). Titanium hydride powders are used as raw material and the solid loading of gelcasting slurry reaches 48 vol%. Combination of dehydriding and sintering in one process accelerate densification and the relative sintered density reaches up to 97.5%. The contents of oxygen and carbon contamination are 0.31 and 0.08 wt%, respectively, which contribute to a high-level of mechanical property with tensile strength of 580 MPa and elongation of 8%. To achieve the near-net shape forming capability of low-molecular mass organic gelcasting, rosin is used to increase the green strength. Eventually, titanium parts with complex shape are successfully prepared.

The ordered intermetallic Mo5SiB2 displays a ceramic-like brittleness at the ambient temperature. The state density, charge distribution and elastic parameters were calculated by first-principles, based on the density functional method. The results indicated that the two different kinds of covalent bonds were intricately woven into the refractory phase. The improved Peierls–Nabarro stress which is caused by this kind of distribution mode makes dislocations move difficultly, resulting in intrinsic brittleness. The effects of substitutional alloying on the ductility of Mo5SiB2 were also assessed by the calculations on the elastic properties and dislocation line energy. It was shown that the metal (Nb, Tc) alloying was not to enhance effectively its toughness, but to improve the brittle-to-ductile transition temperature.

Nb and Sn are major alloying elements in Zr alloys. In this study, the microstructure, mechanical properties, corrosion behavior, cytocompatibility and magnetic resonance imaging (MRI) compatibility of Zr–2.5X (X = Nb, Sn) alloys for biomedical application are comparatively investigated. It is found that Zr–2.5Nb alloy has a duplex structure of α and β phase and Zr–2.5Sn alloy is composed of α phase. Both separate addition of Nb and Sn can strengthen Zr but Nb is more effective in strengthening Zr than Sn. The studied Zr–2.5X (X = Nb, Sn) alloys show improved corrosion resistance compared to pure Zr as indicted by the decreased corrosion current density. The alloying addition of Nb enhances the pitting resistance of Zr, whereas the addition of Sn decreases the pitting resistance of Zr. The extracts of Zr–2.5X alloys produce no significant deleterious effect on fibroblast cells (L-929) and osteoblast-like cells (MG 63), indicating good in vitro cytocompatibility. The Zr–2.5X (X = Nb, Sn) alloys show decreased magnetic susceptibility compared to pure Zr and their magnetic susceptibility is far lower than that of pure Ti and Ti–6Al–4V alloy. Based on these facts, Zr–2.5Nb alloy is more suitable for implant material than Zr–2.5Sn alloy. Sn is not suitable as individual alloying addition for Zr because Sn addition decreases the pitting resistance in physiological solution.

Ti–48Al–6Nb (at.%) porous alloys are fabricated by elemental powder metallurgy to study the pore formation and propagation mechanism. Reactive diffusion, pore formation process, and pore characteristics of the porous TiAl–Nb alloys are investigated at different temperatures. It is found that the porous alloys exhibit a uniform, maze-like network skeleton, viz., a typical α 2 -TiAl 3 /γ-TiAl fully lamellar microstructure. The reactive diffusivities between Ti and Al powders are dominant during the Ti–Al–Nb powder sintering. Gas release during sintering also plays an important role in the pore propagation and the compact expanding process. In addition, a pore-formation model is proposed to interpret the growth mechanism of pores and skeletons.

In situ Ti-based metallic glass matrix composites are fabricated by the Bridgman solidification, and the mechanical properties are investigated. The fine dendrites about 2 to 10 μm are uniformly distributed in the glass matrix. The compressive results show that the composites have high strength and large plasticity. The fracture strength for the composite at the withdrawal velocity of 1.6 mm/s is as high as 3000 MPa and the total fracture strain is up to 31.5 pct. Particularly, the dendrite size of the current composite would decrease with the increasing of the withdrawal velocity, which leads to the higher yield strength.

The resistivity and mechanical properties of Cu–Sn alloys with different compositions were explored by casting, normalising, cold work and subsequent annealing treatment. Results indicated that the Cu–Sn alloy had the characteristics of ultrahigh electrical conductivity, when the Sn content was ∼0·5 wt-%. Note that the resistivity of the as cast and annealed Cu–0·5 wt-%Sn alloys is 1·55 and 1·26 μΩ cm respectively.

Mo 5 SiB 2 (T2) was synthesized by sparking plasma sintering (SPS) under different heating rates and sintering temperatures. The powder mixture with a T2 composition (Mo–12.5Si–25B (at.%)) failed to produce the T2 single-phase alloy due to the volatilization of Si during SPS process. Extra 0.5 at.% Si added to this mixture offset the volatilization loss. It has been found that heating of the mixture at low heating rates favored the formation of binary phases in the solid state at medium temperatures. In this work, the T2 alloy with a fine-grained microstructure was obtained via a liquid–solid reaction when the mixture was heated fleetly to the temperatures above the silicon melting point at the rapid heating rate of 200 °C/min. The sintering temperature at 1500 °C for T2 synthesis is beneficial to enhance further densification, as well as to avoid abnormal grain growth at higher temperatures.

Porous Ti–xAl–8Nb (x = 45, 48, and 49 at%) alloys (i.e., intermetallic TiAl–Nb compounds) with cylindrical pores oriented along a single direction were prepared via unidirectional solidification in a hydrogen and helium gas atmosphere, while making use of the differences in the hydrogen solubilities of the liquid and solid phases of the alloys. The microstructures and pore morphologies of the alloys revealed that pore formation in the alloys was closely related to their solidification processes. In the case of the alloy with x = 45 at%, a primary solidified β (bcc) phase with high hydrogen solubility suppressed the formation of pores during solidification. On the other hand, in the case of the alloy with x = 49 at%, the formation of the primary β phase was suppressed, leading to the formation of elongated pores along the solidification direction. After being heat treated, the porous TiAl–Nb alloys exhibited a fully lamellar structure composed of α2-Ti3Al and γ-TiAl phases. These porous TiAl–Nb alloys with lamellar structures and directional pores exhibited superior compressive properties parallel to the direction of the pores.

The fracture behavior of the polycrystalline Mo5SiB2 alloy was investigated by three-point bending tests, in vacuum, in the range of 25–1400 °C at a crosshead speed of 10−3 mm/s. At room temperature (RT), the alloy exhibited a higher fracture toughness of 3.34 MPa m1/2 compared to single crystalline Mo5SiB2 with a value of ∼1.8 MPa m1/2. The improved toughness is attributed to crack overcoming the barrier effects of grain boundaries. With increasing temperature, the fracture mode underwent a conversion from transgranular cleavage at RT to a mix of transgranular cleavage and intergranular failure at elevated temperatures. At 1000 °C, the maximum increment (∼3.4 MPa m1/2) in toughness was found to be related to the brittle-to-ductile transition (BDT). At 1200 °C, the effects of extrinsic toughening mechanisms (microcracking, etc.) are limited, i.e. its toughness only increasing by ∼1.1 MPa m1/2. However, the cavities formed at triple junctions degraded toughness at 1400 °C.

The omega phase in as-cast high-Nb-containing TiAl alloy exhibits particles with sizes on the nano- or microscale within the B2 matrix. Omega particles are transformed from the B2 phase by heterogeneous nucleation on the B2/γ boundaries or homogeneous nucleation within the B2 phase. After annealing at 900 °C, the omega particles disappear and transform into B2 phase. The transformation is rapidly initiated, but is difficult to complete.

The objective of this study is to investigate the effect of alloying element indium on the microstructure, mechanical properties, corrosion behavior and in vitro cytotoxicity of Ti–In binary alloys, with the addition of 1, 5, 10 and 15 at.% indium. The phase constitution was studied by optical microscopic observation and X-ray diffraction measurements. The mechanical properties were characterized by tension and microhardness tests. Potentiodynamic polarization measurements were employed to investigate the corrosion behavior in artificial saliva solutions with and without fluoride. In vitro cytotoxicity was conducted by using L929 and NIH 3T3 mouse fibroblast cell lines, with commercially pure Ti (CP–Ti, ASTM grade 2) as negative control. All of the binary Ti–In alloys investigated in this work were found to have higher strength and microhardness than CP–Ti. Electrochemical results showed that Ti–In alloys exhibited the same order of magnitude of passivation current densities with CP–Ti in artificial saliva solutions. With the presence of NaF, Ti–10In and Ti–15In showed transpassive behavior and lower current densities at high potentials. All experimental Ti–In alloys showed good cytocompatibility, at the same level as CP–Ti. The addition of indium to titanium was effective on increasing the strength and microhardness, without impairing its good corrosion resistance and cytocompatibility.

t is well known that excellent soft magnetic properties are obtained in electrical steel with Si content of 6.5wt.%, such as high permeability, low coercive force, near zero magnetostriction, etc. However due to its poor ambient temperature ductility it is very hard to fabricate this material by conventional hot-cold rolling process. Meanwhile, high-density electropulsing treatment (EPT) attracts widespread applications as a new heat treatment approach. In this paper, heat treatment effect of high-density pulsing current on microstructure, micro-hardness and mechanical properties of as rolled Fe-6.5wt.%Si alloy sheet were investigated. The results showed that the microstructure and mechanical properties were closely related to the parameters of the pulses. Refined and uniform recrystallized grains were obtained by optimizing parameters of the high density pulses. Compared with the conventional heat treatment method, the time and temperature for heat treatment can be reduced dramatically. Therefore, it is considered that EPT is an efficient heat treatment approach for this rolled sheet. Mechanism of the effect of high-density pulsing current has been discussed also in this investigation.

The corrosion behavior of porous Nb-containing TiAl based alloys in HCl, NaOH and NH3H2O aqueous solutions with various concentrations has been studied simultaneously at room temperature. The results indicate that the porous alloys have good acidic resistance which is probably based on the combined work of Ti-Ti metallic bond and covalent bond. And pitting corrosion is the main corrosion behavior of Nb-free TiAl alloys during the first 40 hours, while intergranular corrosion is the main corrosion behavior later. Nb-containing alloys have better acid corrosion resistance due to inhibiting intergranular corrosion by Nb element. In addition, the porous TiAl based alloys are much more resistant to corrosion in alkaline solutions than in acidic solutions, owing to the different oxide films formed in alkalis solutions.

Compression and three-point bending tests were conducted at room temperature (RM) on the Mo5SiB2 (T2) alloy, which was prepared by sparking plasma sintering (SPS). It was found that almost no plastic deformation occured in the T2 alloy before failure but with a tremendous compressive strength of 2907 MPa. The fracture toughness determined from the single edge notch bend specimen is 3.34 MPa·m1/2, similar to the value of 3.5 MPa·m1/2 in ɑ-Al2O3. Once the cleavage crack initiated near the notch under continuous loads, it propagated on a certain plane (001) until the specimen completely fractured. In this work, the electronic structure was also calculated by the first-principles method, indicating that the contribution to RM brittleness is mainly caused by the covalent bonds which arrange alternately in the T2 phase.

High Nb containing TiAl alloys have been investigated traditionally as potential high temperature structural materials because of their high strength, good oxidation and creep resistance. However, the poor ductility and fracture toughness at room temperature limit their application, which could be improved by controlling microstructure to get refine and homogeneous fully lamellar structure. In this study, a high Nb containing TiAl alloy alloying Mn, B and Y with refined microstructure was produced. The solidification path was analyzed by DSC and SEM microstructure of the alloy was observed, after heating at a certain temperature for 1-24hrs and then quenching in water. The dissolution of β phase was also investigated. The results showed that the β phase could decompose only by heating in single β or near α phase field.

High Nb-containing TiAl porous and gradient porous alloys were synthesized by elemental powder metallurgy (EPM). The effects of powder size on pore structure and gas permeability were investigated. It is found that pores are interconnected and skeletons consist of typical γ-TiAl/α 2 -Ti 3 Al fully lamellar microstructure for Ti–48Al–6Nb porous alloys. With the fining of particle size of Ti, Al and Nb powders, the pore size and porosity of Ti–48Al–6Nb porous alloys decrease. Otherwise, the gas permeability increases almost linearly when the porosity increases from 17% to 52% and the pore size increases from 3.18 μm to 26.69 μm. A good agreement is shown between the simulations by capillary permeability model and the results of the actual gas permeability. In addition, gradient porous alloys were fabricated using different powder sizes by constrained sintering process, and a prediction of the relationship between permeability and coating thickness was discussed. Results from this study indicate the potential applications in microfiltration development by tailoring porosity of the coating.

nterdiffusion coefficients in α2-Ti3Al and γ-TiAl of conventional TiAl and TiAl-8Nb alloy were measured at the temperature ranging from 1273K to 1523K. Single-phase diffusion couples were employed, and the concentration profiles of Al after annealing were measured by an electron probe microanalyzer (EPMA), and the interdiffusion coefficients were calculated according to the Boltzmann-Matano method. The results showed that there was no significant concentration dependence of interdiffusion coefficients for all the alloys with various phases, and the values of interdiffusion coefficients covered three orders of magnitude (E-17-E-14) with the increase of temperature according to Arrhenius law. In α2-Ti3Al and γ-TiAl phase of conventional TiAl alloys, the pre-exponential factor and activation enthalpy were D0=3.95×10-5m2s-1,Q=276KJmol-1 ;D0=7.26×10-5m2s-1,Q=275KJmol-1 respectively. The pre-exponential factor and activation enthalpy were D0=4.54×10-6m2s-1, Q=244KJmol-1 in γ-TiAl phase of TiAl-8Nb alloys. However, the temperature dependence of interdiffusion coefficients in α2-Ti3Al of TiAl-8Nb alloys did not follow Arrhenius laws very well. With the addition of Nb, the interdiffusion coefficients increased significantly in α2-Ti3Al, but changed slightly in γ-TiAl at high temperature.

The isothermal static corrosion of Ti-45Al-8Nb and Ti-53Al-20Nb (at.%) in liquid Zn-55Al-1.6Si (wt.%) at 630°C was studied. Incubation periods without any corrosion occurred are exhibited in both alloys. After the incubation time, the corrosion rate increases noticeably which is controlled by the reaction-diffusion and dissolution mechanism. Thick transition layers were formed. The EDS results show that Si and Al diffused into the TiAl-Nb alloys preferentially, while Zn was hindered from diffusing and segregated in spots. All the transition layers are porous and composed mainly of Al3Ti (Nb), SiTi and AlNbZn2. The fracture of the transition layers of Ti-45Al-8Nb should be attributed to the defects of the lamellar colony boundaries. nanoIndenter was used to check the mechanical properties of the materials. The results show that the transition layers have higher micro-hardness and elastic modulus than the matrix.