•Melt hydrogenated Nb-Si based alloys were fabricated.•The morphology of primary Nb5Si3 changes with hydrogenation.•Hydrogenation affects the compression properties of Nb-20Si-6Mo alloy.Nb-20Si-6Mo (at.%) alloy was prepared by arc melting under Ar-H2 gas mixture atmosphere (Melt Hydrogenation Technology). It indicated that the microstructures of Nb-20Si-6Mo alloy change significantly after the melt hydrogenated treatment. The morphologies of primary Nb5Si3 phase change from polygon to dendrite shapes with the increase of H content. The room temperature compression at same strain rate indicates that the σmax decreases with the increase of hydrogen content. High temperature (1400 °C) compression at same strain rate indicates that H addition can cause the peak strain to take place in advance at high deforming temperature. The high temperature compression stress tends to firstly increase and then decrease due to the effect of H addition. The reason for the effects of hydrogenation on the high temperature compression stress of Nb-20Si-6Mo alloy has been discussed.

•The additions of Ti and Cr promote eutectic morphology anomalous.•Two kinds of Nbss phase were found after heat treatment.•Ti solid solution was found after heat treatment.•The formation mechanism of Ti solid solution is discussed.Nb-Mo-Si based alloys have been prepared by arc melting in a water-cooled copper crucible under an argon atmosphere. The effects of Al, Cr, Hf and Ti additions on the phase components and stability, microstructures and mechanical property of Nb-Mo-Si based alloys have been studied. The results indicated that the additions of Al, Cr and Ti elements do not change the phase components of Nb-20Si-6Mo alloys, which are composed of Nb solid solution (Nbss) and β-Nb5Si3. The phase component is α-Nb5Si3 and Nbss in Nb-20Si-6Mo-3Hf alloy. The additions of Cr and Ti element make the Nbss/Nb5Si3 eutectic morphology anomalous and coarsening. The element segregation is obvious found with the additions of Hf and Ti. The enrichments of Hf and Ti change the compositions of retained melt and promote the formation of fine eutectic structure. After heating treatment at 1873K for 10 h, β-Nb5Si3 phase translates into α-Nb5Si3 phase and γ-Nb5Si3 phase. The eutectic structures tend to anomalous and coarsening. The Ti solid solution (Tiss) phase was found in Nb-20Si-6Mo-20Ti alloy and the formation mechanism of Tiss phase was discussed. The high temperature (1523 K) compression strength of as cast Nb-Mo-Si based alloys increased with the additions of Al, Cr, Hf, and decreased with Ti addition.Download high-res image (399KB)Download full-size image

•Melt hydrogenation decreases flow stress of hot deformed Ti-6Al-4V alloy.•Hydrogen encourages dynamic recrystallization and improves hot workability.•Hydrogen increases the mobility and decreases the density of dislocations.The effect of hydrogen on hot deformation behavior of Ti-6Al-4V alloy was investigated. Ti-6Al-4V alloy was hydrogenated by melting alloy in gas mixture of hydrogen and argon (melt hydrogenation). Experimental results of hot compression at same strain rate showed hydrogen decreased flow stress at higher deforming temperature, which was attributed to hydrogen induced dislocation movement and dynamic recrystallization (DRX). At lower temperature, peak stress firstly decreased and then increased with increasing hydrogen content. Hydrogen decreased the peak stress and improved the hot workability of alloy deformed at same temperature and different strain rates. Microstructure observation of as deformed alloy indicated hydrogen promoted DRX on both α and β phase, and encouraged the decomposition of residual lamella. Electron back-scattered diffraction results indicated that hydrogen mainly encouraged discontinues DRX and decreased the dislocation density in α phase. Compared to unhydrogenated alloy, when hydrogen content was 5.31 × 10−2 wt.%, volume fraction of DRX increased from 42% to 53% and 41.8%–72.1% at strain rate of 0.01 and 0.001 s−1, respectively.Download high-res image (808KB)Download full-size image

•The α phase is primary phase of Ti-47Al-1W-0.5Si alloy during solidification.•After directional solidification process, the interlamellar space decrease.•The morphology of Ti5Si3 changes from short rod to strip along growth direction.•Fracture mode transforms from inter-granular to inter-lamellar after DS process.The as-cast Ti-47Al-1W-0.5Si alloys were prepared by induction skull melting (ISM) technology under argon atmosphere. Some bars were cut from as-cast ingot longitudinally and then were directionally solidified by Bridgman solidification technique. The microstructures and mechanical properties of the as-cast and directionally solidified Ti-47Al-1W-0.5Si alloy ingots with different lamellar orientations were investigated. After directional solidification, the grain boundaries were eliminated and the lamellar orientation was nearly parallel to the growth direction. The interlamellar spacing of directionally solidified samples decreased from average 1500 nm to less than 1200 nm and the interlamellar spacing was more homogeneous. In addition, the shape of Ti5Si3 transformed from short rod to strip along the growth direction, and the B2 phase disappears after directional solidification. At room temperature, the compressive yield strength of Ti-47Al-1W-0.5Si alloy increases from 573 in as-cast condition to 651 Mpa after directional solidification. The fracture mode transforms from inter-granular fracture to inter-lamellar fracture after directional solidification.

The hot deformation characteristics of dynamic recrystallization (DRX) of the MoNbHfZrTi refractory high-entropy alloy (HEA) was investigated using the isothermal compression tests in the strain rate of 0.001–0.1 s−1 at the temperature range of 800–1200 °C. Scanning electron microscope (SEM) with the electron backscatter diffraction (EBSD) technique was used to study the effect of deformation temperature and strain rate on the stress–strain behavior, microstructure evolution and dynamic recrystallization (DRX) during hot deformation. At 800 °C, the stress–strain curve exhibits a work-hardening stage until fracture at the strain rate of 0.1 s−1 and 0.01 s−1. Under other deformation conditions, the stress–strain curves exhibit the typical DRX characteristics. On the whole, the stress decreases with the increase of deformation temperature and decrease of the strain rate. The initial dendritic structure gradually disappears and more dynamic recrystallized grains form with the decrease of strain rate and the increase of the deformation temperature. The nucleation mechanism of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occurred simultaneously for the MoNbHfZrTi alloy during hot deformation. The effect of CDRX was weakened with the increase of deformation temperature and with the decrease of strain rate.

•A series of non-equiatomicMo–Nb–Hf–Zr–Ti alloys are synthesized by arc melting.•All non-equiatomicMo–Nb–Hf–Zr–Ti alloys are composed of single BCC phase.•The elemental concentration variation has no effects on phase constitute.•The elemental concentration variation has different effects on microsegregation.•The hardness and strength are reduced for allnon–equiatomicMo–Nb–Hf–Zr–Ti alloys.A series of non-equiatomic Mo–Nb–Hf–Zr–Ti alloys are synthesized to investigate the effects of the concentration variation of each composing elements on the microstructure and mechanical properties. It is found that all studied alloys form single body-centered-cubic (BCC) phase only with the variation of the lattice parameter, which indicates that the concentration variation of each composing elements has no effect on the phase constitutes. All studied alloys exhibit typically dendritic and interdendritic structure while the concentration variation of each composing elements has different effects on the microsegregation. The concentration variation of Zr leads to the most serious microsegregation. Elements with a higher melting point such as Mo and Nb solidify preferentially and thus are enriched in the dendrites. Both the increase and decrease of the concentration of each composing element reduce the hardness and strength of non-equiatomic Mo–Nb–Hf–Zr–Ti alloys compared with the equiatomic MoNbHfZrTi alloy.

•Carbon was added into Mo0.5NbHf0.5ZrTi alloy to prepare metal-matrix composite.•C0.1 and C0.3 alloys form one BCC phase and particulates-reinforced MC-carbide.•MC carbide is enriched with Zr and Hf and without any Mo.•The carbide formation increases compressive strength and improves plasticity.Refractory high-entropy Mo0.5NbHf0.5ZrTi alloy matrix composite with MC-carbide particulates-reinforced was prepared by arc melting. It is found that Mo0.5NbHf0.5ZrTiC0.1 and Mo0.5NbHf0.5ZrTiC0.3 alloys consist of one disordered body-centered cubic (BCC) solid solution phase as the matrix phase and MC carbide phase. MC carbide is enriched with Zr and Hf due to the higher binding strength and without any Mo. Noticeable strengthening from the carbide is not observed for C0.1 and C0.3 alloys while the compressive plasticity is improved slightly due to the decrease of solution strengthening for the matrix BCC disordered solid solution phase.

In this paper, we report a β seeding technique for the lamellar orientation controlling in TiAl alloy, which is a novelty and effective method for aligning the lamellar orientation of Ti–47Al–0.5W–0.5Si with primary α phase. The shorter composition transition zone and simpler process procedure can improve the deficiency of α seeding technique. The proper temperature gradient and normal growth rate are necessary for aligning the lamellar orientation in TiAl alloy with primary α phase using β seeding technique.

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb (at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated. The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 µm∙s-1 under the temperature gradients of 15-20 K∙mm-1, and the primary phase is transformed into an α phase at relatively higher growth rates (V >20 µm∙s-1). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 µm∙s-1 under a temperature gradient of 15 K∙mm-1. The increase of temperature gradient to 20 K∙mm-1 can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as \(L\xrightarrow{{L \to \beta }}L + \beta \xrightarrow{{L + \beta \to \alpha }}\alpha + {\beta _R}\xrightarrow{{\beta \to \alpha }}\alpha \xrightarrow{{\alpha \to \gamma }}\alpha + \gamma \xrightarrow{{\alpha \to {\alpha _2} + \gamma }}{\gamma _R} + \left( {{\alpha _2} + \gamma } \right)\), in which βR and γR mean the residual β and γ

•The relationship of the hydrogen content and H2 partial pressure was obtained.•There is a big temperature gradient between the top and bottom surface.•Hydrogen refines the microstructures of Ti600 alloy.•The flow stress and yield stress decrease with increasing the hydrogen content.In this paper, melt hydrogenation, which is a new hydrogen treatment method, was used to hydrogenate Ti600 alloy, the relationship between the hydrogen partial pressure and the hydrogen content was built by analyzing experimental data, and microstructure was observed and mechanical properties was tested. It was found that hydrogen addition made the thickness of the solidified shell thinner and a big temperature gradient existed from the top to bottom surface of the alloy melt. With increasing hydrogen partial pressure, the directional solidification structure can be formed in the Ti600 alloy ingots. Microstructure of Ti600 alloy was modified significantly and the amount of α′ and β phases after melt hydrogenation. When increasing the content of hydrogen to 7.2 at.%, γ hydride was obtained in Ti600 alloy. The flow stress and yield stress decrease with increasing the hydrogen content.

The amorphous alloys of Zr57Al10Cu15.4Ni12.6Nb5 and Zr55Cu30Ni5Al10 were melted and cast under the gaseous mixture of hydrogen and argon. The effects of hydrogen on glass plasticity of the two bulk metallic glasses were studied. Surprisingly, it was found that hydrogen addition can increase the glass plasticity significantly. The large plasticity of glass is attributed to the large amount of free volume induced by hydrogen addition, which could make this melt hydrogenation method to be a promising new way to improve plasticity of amorphous glass.Highlights► A simple and efficient method to synthesize BMGs with hydrogen was presented. ► Hydrogen addition can increase the glass plasticity significantly. ► The large plasticity of glass is attributed to the large amount of free volume.

Electromagnetic casting is a novel technology that combines the advantages of electromagnetic cold crucible and continuous casting. By controlling induction heating and strengthening bottom cooling, multicrystalline silicon ingot with 60 mm×60 mm in cross section was continuously and directionally solidified by this technology. The ingot shows smooth surface and consists of uniform columnar crystals with no precipitate except in the top part. The grain size of the top part is smaller due to higher impurity content and the impurities acting as nucleation sites for grain. The density of dislocations at the bottom and middle parts, about 1×106 cm−2, is much less than that at the top part.

The effects of technical parameters on the forming of surface defects of the silicon ingots prepared by electromagnetic continuous casting were investigated by the orthogonal experiment, and the corresponding defect formation mechanisms were discussed and established. The results indicate that the forming of the ripple and lap are mainly caused by the shrinkage of shell and controlled by the electromagnetic pressure, and the unmelted granules are those moving from the center to the edge on the liquid surface, which was directly cooled by the cold crucible wall and kept as unmelted state. The surface quality was shown to be significantly improved by increasing the input power, pulling velocity and the heat preservation time, and among them, the input power is the most important one.

Alloys of Ti–47Al were deoxidized via a simple method called hydrogen treatment (HT), which involves deoxidation with hydrogen in a melting process. Because of the increase in the partial pressure of hydrogen and the melting duration, the oxygen content of the alloys greatly decreased after HT. Activated hydrogen atoms dissociated at high temperatures, and the hydrogen molecules in the melting chamber seemed to affect the deoxidation reactions, which are represented by the following equations: O + 2H = H2O and O + H2 = H2O. Based on a comparison of the changes in Gibbs free energy, the hydrogen atoms were found to play a major role in deoxidation.

In this work, the hydrogen solubility in a titanium–aluminium (TiAl) binary alloy melt was investigated through a theoretical analysis and the results compared subsequently with values determined experimentally. Determination of the theoretical values of hydrogen solubility is based on a modified version of Sievert’s law, in which hydrogen solubility is related to the activity coefficient of the alloy melt and the hydrogen solubility in pure liquid metals. The activity coefficient is obtained in terms of the free volume theory, in which excess entropy is sufficiently taken into account. The experimental values of the hydrogen solubility in the two alloy melts, Ti45Al and Ti47Al, were determined to validate the calculated values. This was performed using hydrogen charging apparatus. The experimental values obtained were in good agreement with the calculated values.

In this paper we present a simple and effective method to reduce the oxygen content of titanium alloys by using the mixture of hydrogen (H2)/Ar gases as the reactive atmosphere during the remelting process of titanium alloys. The experimental results show that the decrease of oxygen content of Ti64 alloy is related to the hydrogen fraction of the mixture gas and the melting time. When the hydrogen fraction is 10%, the best deoxidation takes place. The oxygen contents, in the titanium alloy, can be effectively reduced, leading to the microstructure of titanium alloy and the micro-hardness can be refined and decreased, respectively. Additionally, hydrogen absorbed in our process can be easily removed by vacuum heat treatment.

Directionally solidified TiAl microstructures were investigated and it was found that there is competitive growth between the stable phase (β) and metastable phase (α) near the peritectic reaction L+β→α in Ti–Al binary system. The phase selection phenomena of Ti–Al system containing (44–50) at.% Al were theoretically studied based on the criterion of the highest interface temperature and with solidification interface response function model of single-phase alloys. Firstly, according to a thermodynamic model of Ti–Al binary system, a part of the phase diagram with the Al content of (44–50) at.% and above 1740 K was calculated. The peritectic reaction temperature is 1763 K, and the peritectic composition is Ti–47.3 at.% Al. The solidus and liquidus of α and β were described as polynomials. Using these polynomials, Co, Tm, me and ke were determined. Suppose that the solidification of α and β phases conforms to the solidification theory of single phase, the interface temperatures were calculated. For Ti–47 at.% Al, when temperature gradient (G) is 10 000 K/m, the critical growth rate of α phase from planar to columnar is about 3.6×10−6 mm/s. The critical growth rate linearly increases with increasing temperature gradient. With the same computational program, the interface temperatures of α and β phases with Al content from 44 to 50 at.% were calculated. Comparing the interface temperatures of α and β phases, and assuming the phase with the higher interface temperature to grow preferentially from the melt, the phase-selection map with the reference frame of the melt composition and the ratio of temperature gradient to growth velocity (G/V) was constructed. The theoretical results are in good agreement with experimental results.