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Abstract: A huge triple phase region of the nonfaceted–nonfaceted peritectic reaction within a large temperature region (around 8K) was obtained in directionally solidified Cu–Ge alloys, which provides a convincing experimental evidence for studying the morphology of peritectic reaction trijunction region. The huge non-isothermal triple phase region suggests that the peritectic reaction occurs in a composition range, which is different from the classic isothermal model with a fixed composition (Liquid+solid solution α(CuGe12) → solid solution ζ(CuGe12.8)). The geometrical morphology of triple junction region shows that the primary α phase remelted ahead of the growing peritectic phase and resolidified near the triple phase junction. And some new characteristics of the peritectic reaction were also observed and systematically analyzed. The results demonstrate that the peritectic reaction during directional solidification occurs in the style: Liquid+remelted α(CuGe(12∼12− x )) → ζ(CuGe(12.8–12.8+ y ))+resolidified α(CuGe12). The x and y depend on growth conditions and alloy composition. The remelting and resolidification phenomenon were discussed in detail, in which the effects of temperature gradient, mechanical equilibrium and solute diffusion were considered. Orientation relationship between the phases was studied by the electron back-scatter diffraction (EBSD) technology. [Copyright &y& Elsevier]

Abstract: Use of hydrogen as a temporary element in titanium alloys is an attractive approach for enhancing processability, and also for modification of microstructures and thereby improving final mechanical properties. In this article, microstructures, phases and phase transformations in Ti–6Al–4V alloy specimens containing 0.1, 0.3 and 0.5wt.% hydrogen has been investigated using optical microscopy, XRD and TEM. It was shown that there are fcc δ titanium hydride plates precipitated in the specimens containing hydrogen 0.302 and 0.490wt.%. Simultaneously, mass orthorhombic martensite α″ has been found in the specimens as well as δ hydride. One eutectoid transformation mechanism from βH to δ hydride and α based on diffusion transformation theory was presented. [Copyright &y& Elsevier]

Herein, the mechanical properties and high-temperature oxidation of (TiZr) 65-x-y Nb 15 Mo 20 Si x Al y refractory high-entropy alloys were systematically investigated. Results indicate that alloying with Si improves the strength but reduces the plasticity of the alloy. Maintaining a proper ratio of Al/Si is crucial to achieve the optimal performance of the alloy. The (TiZr) 50.5 Nb 15 Mo 20 Si 9.5 Al 5 has excellent high-temperature performance, with a yield strength of 815 MPa at 800°C and a compressive strain greater than 40 %. Moreover, the (TiZr) 50.5 Nb 15 Mo 20 Si 9.5 Al 5 exhibits a superior oxidation resistance compared to (TiZr) 55 Nb 15 Mo 20 Si 10 alloy at 1000℃. The introduction of Si enhances the oxidation resistance, decreases the diffusion of the Zr element at 1000 °C and facilitates internal oxidation and the creation of TiNb 2 O 7. However, it is important to note that the doping of Al in the higher Si alloys has negative impacts on the oxidation resistance and mechanical properties. • Alloy Si9.5AL5 has excellent high-temperature performance, with a yield strength of 815 MPa at 800°C and an elongation greater than 40 %. • The order of complex oxide generation is as follows: (Ti 0.8 Al 0.1 Nb 0.1)O 2 > Nb 2 Zr 6 O 17 > TiNb 2 O 7. • The addition of Si reduces the diffusion rate of Zr elements and promotes the formation the complex oxide TiNbO7. [ABSTRACT FROM AUTHOR]

Nb-Si superalloy is one of the most potential structure-materials of next generation aero engine. The influence of rare-earth element Gd on microstructure and mechanical properties of Nb-16Si-24Ti is explored in this paper. Results show that phase composition no change by doping of Gd. And the mass of Nbss/(Nb, X) 3 Si eutectic exists in the matrix instead of primary Nbss. The rare-earth element Gd reduces the Si contents in Nbss phases, and the content of O is also limited because of its high active. The deflection, bridging and secondary crack are easily found in coarsened Nbss, and the peak-value of fracture toughness is 8.37 MPa·m1/2 in NST-0.05Gd alloy. The phenomenon of solution strength is obvious, and the maximum value of RT compressive strength is 2214 MPa. The thermal compression test indicates that silicide phases play the major role for high-temperature performance, and the Nbss phases have the poor heat-resistance under HT condition. • The primary Nbss phases can be coarsened by the trace Gd element, and the Nbss/(Nb, X) 3 Si eutectic takes up all the matrix as the certain amount of Gd (0.1 at% or more). • The contents of Si and O elements in Nbss are limited, which is beneficial for the toughness of Nbss phases • The coarsened Nbss phases easily form the deflection, bridging and secondary crack, and the appearance of Nbss/(Nb, X) 3 Si eutectic has the huge stress triaxiality level, which is harmful for the toughness of Nb-Si alloys. • The rare element Gd has the great solution effect, and the silicide phases play the major role for the HT performance of Nb-Si alloys. [ABSTRACT FROM AUTHOR]

Sn–Ni peritectic alloys were directionally solidified using a Bridgman type directional solidification furnace. Microhardness ( Hv ) of both primary Ni 3 Sn 2 phase and peritectic Ni 3 Sn 4 phase and microstructure length scales including primary dendrite arm spacing λ 1 were measured as a function of growth rate. Dependency of microhardness on the solidification processing parameters and primary dendrite arm spacing ( λ 1 ) in the directionally solidified Sn–36 at.%Ni alloys were experimentally analyzed. The relationships between the solidification processing parameters (growth rate v and temperature gradient G ) and microhardness, and primary dendrite arm spacing and microhardness were obtained by linear regression analysis: for primary Ni 3 Sn 2 phase, Hv = 501.06 v 0.07 , Hv = 5896.3 λ 1 −0.42 ; for peritectic Ni 3 Sn 4 phase, Hv = 306.76 v 0.10 , Hv = 843.6 λ 1 −0.69 . It has been found that the values of λ 1 decreases with the increasing values of v while the values of microhardness ( Hv ) increase with growth rate under constant G . [ABSTRACT FROM AUTHOR]

Ultrasonic assisted solidification (UST), heat treatment and alloying are all effective methods to control the microstructure of materials and improve their properties. In this paper, the Nb-16Si-20Zr-2C- x W composites were prepared using ultrasonic assisted solidification technology of high temperature melt (≥1800℃) and heat treatment. The influence mechanism of ultrasonic, the W element, and the synergistic regulation mechanism of ultrasonic and heat treatment on the material were studied. Nb-16Si-20Zr-2C- x W(UST) alloys are mainly composed of Nbss phase and γNb 5 Si 3 phase, and their microstructures are mainly divided into strong active zone(SZ) and weak active zone(WZ). In the Nb-Si superalloy melt treated by ultrasonic wave, the area near the ultrasonic sound source is the strong active zone, while the area far away from the ultrasonic sound source is the weak active zone. The features of faceted primary phases and non-faceted primary phases in the SZ and WZ are explained by the equation of undercooling degree at different interfaces and effect of ultrasonic on component undercooling. As the W element content in the alloys (UST) increases, the content of the Nbss phase increases, the median diameter of the primary γNb 5 Si 3 phase decreases, and the room temperature fracture toughness and compressive strength show an upward trend. When the addition amount of W element reaches 2 at%, there are more crack deflections, crack bridging and secondary cracks in the crack propagation path. Through the special arrangement of primary γNb 5 Si 3 phase in the ultrasonic field, the energy density of ultrasonic waves in the melt is calculate to be 33.5 kW/m2. The microstructure of Nb-16Si-20Zr-2C- x W alloys(UST+HT) became more uniform and spheroidize. It is worth mentioning that there are plenty of long strip γNb 5 Si 3 phases with different orientations in the Nb-16Si-20Zr-2C-0.1W(UST+HT) alloy and the long strip γNb 5 Si 3 phases with the same orientation are arranged in a periodic manner. This special composite structure induces crack deflections and branching cracks when cracks propagation, so the room temperature fracture toughness of Nb-16Si-20Zr-2C-0.1 W (UST+HT) alloy reaches 15.66 MPa·m1/2. • Ultrasonic wave was introduced into ultra-high temperature melts. • Ultrasonic made the microstructure obviously divided into two zones. • The maximum melt acoustic streaming rate was calculated to be 1.82 m/s. • W has an inhibitory effect on the silicide phase under the action of ultrasonic. • Fracture toughness K Q value of Nb-16Si-20Zr-2C-0.1W(UST+HT) is 15.66 MPa·m1/2. [ABSTRACT FROM AUTHOR]

The refractory high-entropy alloy (RHEA) with a body-centered-cubic (BCC) solid-solution structure has excellent high-temperature softening resistance, which has attracted wide interest in the field of high-temperature alloys. However, its limited room-temperature plasticity has greatly hindered its engineering application. To obtain an excellent strength-plasticity matching relationship, in this reported study, the Ta content in the high-entropy alloy (HEA) was adjusted to rectify this shortcoming. A series of TiZrTa x NbMo (x = 1.0, 0.9, 0.8, 0.7, and 0.6 at. percent, at%) RHEAs were prepared using the vacuum arc-melting technique, and the microstructure and mechanical properties of these RHEA alloys were systematically investigated. The experimental results show that the TiZrTa x NbMo RHEAs are composed of main BCC1 and minor BCC2 phases, which exhibit a dendritic structure. By reducing the Ta content, the elemental segregation caused by the non-equilibrium solidification is reduced. In terms of mechanical properties, with the decrease of Ta content, the hardness and room-temperature yield strength of the alloy decreases slightly, but the room-temperature plasticity increases significantly. The Ta0.7 alloy has the highest plasticity (34.8 %), which is about twice that of the equimolar Ta 1.0 alloy, while the yield strength remained at 1297 MPa. The excellent mechanical properties of the alloys can be attributed to solid-solution strengthening and the formation of moderate amounts of interdendritic regions. The interaction between slip bands and dislocations formed during compression of the Ta0.7 alloy decreased its work hardening. Moreover, the theoretical model of solid-solution strengthening elucidates that the calculated values of the alloy's yield strength are consistent with that obtained experimentally. • The phase composition of the alloy was analyzed using the CALPHAD method. • The TiZrTa x NbMo alloys had excellent balance for strength and plasticity. • The Ta content of balance point between the strength and plasticity was 0.7 mol. • The calculated yield strength was consistent with that obtained experimentally. [ABSTRACT FROM AUTHOR]

Abstract: Bridgman type directional solidification experiments were conducted for Ti–46Al–8Nb alloy in a wide range of growth rates (1–70μm/s). The microstructure, microsegregation and the formation of B2 phase were investigated. Nearly planar and shallow cellular growths of primary β phase were observed at the growth rates of 1 and 2μm/s respectively, and a fully α 2/γ lamellar structure was formed finally. The growth rates of (2–5μm/s) and (⩾5μm/s) resulted in the transient cellular-dendritic and regular dendritic growth respectively, which were accompanied with sequent peritectic reaction resulting from the shift of L+ β → β transus line to lower aluminum content with increasing growth rate. Peritectic reaction promoted niobium enriched in the core of dendrites and the formation of B2 phase, which mainly was due to the stabilization of β phase during the β → α transformation and precipitated from α lamellae through the α + γ → α 2 + γ +B2 transformation. The final microstructure was composed of α 2/γ lamellar structure and B2 phase. [Copyright &y& Elsevier]

Abstract: TiAl-based alloy (Ti–43Al–3Si at.%) was directionally solidified at a constant temperature gradient (G =20K/mm) in a wide range of growth rates (5–60μm/s) by using a Bridgman type directional solidification furnace. The cellular spacing (λ), lamellar spacing (λ L ), volume fraction of Ti5Si3 phases (f V ) and microhardness (H V ) were measured. The values of λ, λ L and f V decrease with increasing growth rate (V). The value of H V increases with the increasing values of V and λ L −0.5, and decreases with the increasing values of λ, and f V . The dependence of H V on V, λ, λ L −0.5 and f V was determined by linear regression analysis. The fitted exponent values obtained in this work were compared with the previous experimental results for similar alloy systems. [Copyright &y& Elsevier]

Abstract: TiAl-based alloy (Ti–46Al–0.5W–0.5Si at.%) was directionally solidified at a constant temperature gradient (G =20K/mm) in a wide range of growth rates (2–100μm/s) by using a Bridgman type directional solidification furnace. The dendritic spacing (λ), lamellar spacing (λ L ) and microhardness (H V ) were measured. The value of H V increases with the increasing growth rate (V) and decreases with the increasing dendritic spacing and interlamellar spacing. The dependence of H V on V, λ and λ L was determined by linear regression analysis. The relationships between the V and H V , and λ and H V , and λ L and H V were given as H V =794.7 V 0.15, H V =398.1λ −0.31 and , respectively. The fitted exponent values obtained in this work were compared with the previous similar experimental results. [Copyright &y& Elsevier]

Microstructure and hydrogen permeation of Nb 56-x W x Ti 23 Co 21 (x = 0, 5, 10) alloys have been investigated. The alloying effect of W on the hydrogen diffusivity of Nb 56-x W x Ti 23 Co 21 alloys was discussed from a perspective of the chemical potential of hydrogen under non-dilute condition. All as-cast ingots were consisted of primary bcc-(Nb) and eutectic (bcc-(Nb)+B2-TiCo) structure. The relation between the hydrogen permeation flux J and the PCT factor f PCT is linear, from which the hydrogen diffusion coefficients were evaluated. The substituting Nb by W leads to improved intrinsic hydrogen diffusion coefficient D ∗. With increasing W content, the potential barrier for hydrogen atoms hopping between stable interstitial sites gradually decreases. The concentration-dependent diffusion coefficient D increases with increasing hydrogen concentration. • Hydrogen chemical potential was used to analyze the hydrogen diffusivity of dual-phase Nb-based alloys. • W Substituting Nb leads to enhanced intrinsic hydrogen diffusion coefficients D ∗. • W alloying contributes to decreased potential barrier for hydrogen diffusion. [ABSTRACT FROM AUTHOR]

Abstract In order to understand effect of solidification velocity on microstructure evolution and mechanical properties of high entropy alloys (HEAs), CoCrFeNi HEA samples were directionally solidified at different solidification velocities (5–50 μm/s). The results show that all samples were directionally solidified, but the solid-liquid interface and columnar grains are different. Microstructure of them is only composed of FCC solid solution phase. Morphology of solid-liquid interface transforms from cellular, to cellular-dendritic, and then to dendritic with increasing of solidification rate. The criterion of solidification rate for cellular/dendrite transition is calculated as about 26.1 μm/s based on experimental results. The width of columnar grains decreases from 200 μm to 100 μm with solidification rate increasing from 5 μm/s to 50μm/s. Meanwhile, secondary dendrite spacing decreases from 200 μm to 30 μm for dendritic solidified samples (v = 10–50 μm/s). Tensile testing show that the yield strength improves from 233 MPa to 383 MPa, and the ultimate tensile strength improves from 431 MPa to 596 MPa with increasing of solidification rate. Meanwhile, the elongation of all samples is more than 34.3%, but rupture morphology analysis shows that the rupture changes from quasi-cleavage fracture to ductility fracture. The improvement of tensile properties is resulted from grain boundary strengthening of the columnar grain and second dendrite. Highlights • The CoCrFeNi HEAscan be well directionally solidified by directional solidification process. • The microstructure of the directionally solidified CoCrFeNi HEAs exhibits a transition from cellular-dendritic growth. • The directionally solidified CoCrFeNi HEAsexhibits higher fracture strength and elongation. • The strengthening mechanism of the directionally solidified CoCrFeNi HEAs is grain boundary strengthening. [ABSTRACT FROM AUTHOR]

Abstract The mechanical parameters such as elastic moduli E and shear moduli G of second phases are critical inputs for modeling and optimizing mechanical properties of aluminum alloys. However, some of these mechanical parameters are lacking because these micron or even nano scale phases are hard to be tested. The aim of this paper is to present an efficient approach to test the mechanical properties of second phases in aluminum alloys accurately. The α-AlFeSi, β-AlFeSi and Mg 2 Si particles at the millimeter scale were prepared by directional solidification with the aid of calculation of phase diagrams (CALPHAD). Such a large size can effectively reduce the impact of particle penetration into matrix in nanoindentation and Vickers hardness experiments. The elastic moduli of α-AlFeSi, β-AlFeSi and Mg 2 Si were determined as 183.81 ± 8.64 GPa, 174.13 ± 6.81 GPa and 116.38 ± 4.06 GPa, respectively, and their Vickers hardness values were 883 ± 64, 765 ± 21 and 475 ± 22 HV, respectively. Besides, the continuous uniform plastic deformation (without pop-in phenomenon in load-displacement curve) of Mg 2 Si was observed and is attributed to the anti fluorite structure of Mg 2 Si. Further, by Vickers hardness and electron backscatter diffraction (EBSD) it was found that the hexagonal α-AlFeSi had greater hardness anisotropy than the near-tetragonal β-AlFeSi. Overall, the hardness values of α-AlFeSi particles in < u 0 u ¯ w > direction were higher than others, the hardness values of β-AlFeSi particles near <001> were lower than others. Highlights • Millimeter scale α-AlFeSi, β-AlFeSi and Mg 2 Si particles were prepared. • Mechanical properties of α-AlFeSi, β-AlFeSi and Mg 2 Si were determined. • Mg 2 Si shows continuous uniform plastic deformation under the nanoindentation. • The anisotropy of α-AlFeSi and β-AlFeSi were investigated. [ABSTRACT FROM AUTHOR]

Abstract To optimize the microstructure and mechanical properties of TiAl-based alloys, a novel high-throughput preparation method was developed using the electromagnetic cold crucible continuous casting (ECCCC) technique. Ti-48Al/Ti-44Al (at.%) and Ti-47Al-10Nb/Ti-47Al high-throughput gradient materials (HTGMs) were successfully fabricated using the ECCCC technique. Composition, microstructure and mechanical properties were systematically investigated. The results show that the HTGMs exhibit continuous and subtle variations in composition. The solute content at different pulling lengths was calculated, with the composition gradient controlled by the height of the molten pool. Using this method, the optimized composition of Ti-47Al-6Nb was determined by studying Ti-48Al/Ti-44Al and Ti-47Al-10Nb/Ti-47Al HTGMs. The ECCCC technique provides a new method to fabricate TiAl-based HTGMs; it is suitable for studying the effects of solute content on microstructure and mechanical properties. Highlights • A novel method to fabricate TiAl high-throughput graded materials was proposed. • TiAl high-throughput graded materials have subtle variation in composition. • Composition gradient is controlled by molten pool and adding alloy. • The optimized composition of Ti-47Al-6Nb alloy to yield is acquired. [ABSTRACT FROM AUTHOR]

To improve the mechanical properties of Nb-Si-Zr based superalloys at room temperature, the effect of a trace amount of Cr, C, Hf, Ta and Sc elements on the solidification processes, phase composition, mechanical properties and fracture behavior of Nb-Si-Zr based superalloys are studied by vacuum non-consumable arc-melting. The results show that the based alloy consists Nbss phase, γ-(Nb,X) 5 Si 3 phase and Zr-rich γ-(Nb,X) 5 Si 3 phase. However, there is no Zr-rich γ-(Nb,X) 5 Si 3 phase in 20Zr-2C and 20Zr-4Hf alloys, indicating that a trace amount of C or Hf elements accelerates the diffusion of Zr element, and improves the segregation of the Nb-Si-Zr based alloys for the later stage of the superalloy solidification. The Nbss fraction in the matrix alloy (20Zr alloy) is 60%. C element increases the Nbss phase fraction by 4%, and Cr and Sc elements reduce the Nbss phase fraction by 4% and 6%, Hf and Ta elements reduce the Nbss phase by 1% and 2% compare with the matrix alloy (20Zr alloy). C element is dissolved in Nbss phase and γ-(Nb,X) 5 Si 3 phase, while Cr element is more dissolved in Nbss phase and Sc element is more dissolved in Zr-rich γ-(Nb,X) 5 Si 3 phase. The toughness of 20Zr-2 C alloy is the highest, and its K Q value reaches 14.05 MPa·m1/2, compared with the matrix alloy (20Zr alloy) increased by 23.46%. The enhancement of Nbss phase fraction and petal shaped microstructure make the crack propagation more difficult, thus increasing the K Q value of the 20Zr-2C alloy. The compressive properties of 20Zr-4Cr alloy are the best, and its compressive strength reaches 1807.3 MPa, compared with the matrix alloy (20Zr alloy) increased by 17.09%. The reason is that a trace amount of Cr element increases the fraction of γ-(Nb,X) 5 Si 3 phase. • Nb-Si-Zr based superalloy was proposed. • The effects of Cr, C, Hf, Ta and Sc on Nb-Si-Zr based alloys were studied. • Adding 2 at% C greatly increases the content of Nbss phase most. • Fracture toughness K Q value of Nb-16Si-20Zr-2 C is 14.05 MPa·m1/2. • C and Hf accelerate the diffusion of Zr in the later stage of solidification. [ABSTRACT FROM AUTHOR]

An Al x (TiZrTa 0.7 NbMo) (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) series of refractory high-entropy alloys were prepared by a vacuum-arc melting process and annealed. The effects of aluminum (Al) addition on the microstructure, room temperature, and high temperature mechanical properties of refractory high-entropy alloys were systematically examined and the strengthening mechanism analyzed and discussed. The results showed that, the phase composition of the alloy was BCC 1 and BCC 2 phases in the absence of Al. Precipitated phase AlZr 3 formed in the alloy when Al was added. This series of refractory high entropy alloy has good strength at room temperature and high temperature. The compressive yield strength of Al 0.5 (TiZrTa 0.7 NbMo) alloy at room temperature was 1984 MPa and the compressive yield strength at 800 °C was 714 MPa. All alloys have plastic strain variables of 10.2–34.8 %, showing excellent room temperature plasticity. The increase in yield strength at room temperature primarily resulted from solution and precipitation strengthening. At high temperatures, precipitation strengthening became the dominant strengthening method. These research findings are expected to facilitate the understanding of the influence trends and mechanism of action of Al in refractory high-entropy alloys and serve as a reference for the design and development of Al-containing refractory high-entropy alloys. • The phase composition of the alloy was analyzed using the CALPHAD method. • The Al x (TiZrTa 0.7 NbMo) alloys had good balance for strength and plasticity. • The Al content of balance point between the strength and plasticity was 0.2 mol. • The solution and precipitation strengthening contribution value was calculated. [ABSTRACT FROM AUTHOR]

Metallic nanowires are widely employed as small-scale structural materials due to their characteristically small volume and high strength compared with their bulk counterparts. Nowadays, the mechanical properties of nanowires in pure metals are well understood with the help of experiments and simulations. However, the deformation of nanowires in metallic alloys remains elusive. In recent years, a new class of alloys called refractory multi-principal element alloys (RMPEAs) emerged. RMPEAs are alloys that form solid solution phases and consist of three or more principal elements, most of which are refractory metals. In this paper, we perform atomistic simulations to investigate the uniaxial deformation of nanowires in 16 body-centered cubic RMPEAs. For each RMPEA, three nanowires consisting of atoms randomly distributed in three different ways are used. The main finding is that dislocation slips on {110} planes and twinning on {112} planes, respectively, control the compressive and tensile plastic deformation of the nanowires. To provide references, we also study the deformation of nanowires in natural and A -atom potential-based artificial pure metals. Results show that the deformation of RMPEA nanowires cannot be predicted by simply extrapolating from those of pure metal nanowires, highlighting the significance of directly simulating RMPEAs using multiple random atomic structures. It is also found that RMPEAs possess a reduced tension-compression asymmetry compared with pure metals, regardless of the underlying plastic deformation mechanism. • Uniaxial deformation of nanowires in 16 RMPEAs are studied by atomistic simulations. • Compressive plastic deformation of the nanowires is controlled by dislocation slips. • Tensile plastic deformation of the nanowires is dominated by twinning. [ABSTRACT FROM AUTHOR]

The influence of the addition of V and B, Y on the microstructure, microsegregation and creep behaviour of the Ti–44Al–6Nb–1Cr (at.%) alloy is investigated. The creep tests performed at 800 °C and 150 MPa show that the Ti–44Al–6Nb–1Cr alloy with a small blocky microsegregation region and smooth colony boundary exhibits the poorest creep resistance. The addition of 2.0 V at.% promotes the formation of a γ phase in the microsegregation region and leads to a large blocky microsegregation area in the matrix. The large blocky microsegregation region with a low specific surface area provides colony boundary strengthening and improves creep resistance. The addition of B and Y affords an obvious refinement in the lamellar colony, provides an increasing opportunity for cavity nucleation at the colony boundary, and thus reduces the creep resistance. The creep failure mechanisms of the three alloys are compared and analysed. [ABSTRACT FROM AUTHOR]

Nb-20Si– x Mo ( x = 2,4,6)-based ultrahigh temperature alloys were prepared by high-vacuum non-consumable arc melting in a water-cooled copper crucible. The morphological evolution of the primary β-Nb 5 Si 3 phase in the Nb-Mo-Si-based alloys was discussed in detail based on the data of X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. The results indicate that the phase constituents in the studied alloys consist of Nb solid-solution (Nbss) phase and β-Nb 5 Si 3 phase, with Nb 3 Si phases also appearing in the Nb-20Si-2Mo alloy. The Mo addition does not change the morphology of the primary β-Nb 5 Si 3 phase in the Nb-20Si alloy. The primary β-Nb 5 Si 3 phase in the investigated alloys displays various shapes, such as dendritic, L-like, hollow square and square ones. The morphological evolution of the primary β-Nb 5 Si 3 phase follows two main paths in the Nb-Mo-Si alloys. (I) At a lower cooling rate, the primary β-Nb 5 Si 3 morphology undergoes a transformation from L-like shapes to hollow square shapes before finally forming square shapes, and (II) at a higher cooling rate, the primary β-Nb 5 Si 3 morphology transforms from Y-like shapes to hollow square shapes and ends with forming square shapes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to reveal the effect of solidification rate on microstructure and improve room and high temperature tensile properties, Ni 36 Co 30 Fe 11 Cr 11 Al 6 Ti 6 high entropy alloys (HEAs) were prepared by directional solidification at different solidification rates (5, 10, 20, 50 and100 μm/s). Results show that two kinds of dendrite form in the directionally solidified process. One is primary solidified dendrite, which is composed of FCC 1 phase enriched in Co, Cr and Fe. Another is secondary solidified dendrite, forming between the primary solidified dendrites and consisting of the FCC 2 phase enriched in Ni, Al and Ti. As the solidification rate increases, the primary solidified dendrite spacing decreases, the solute enrichment layer overlaps, and the constitutional undercooling in the inter-dendrite region decreases, increasing the difficulty of the secondary solidified dendrite formation. Tensile results show that the strength of Ni 36 Co 30 Fe 11 Cr 11 Al 6 Ti 6 HEAs gradually increases with the increase of solidification rate. The alloy with a solidification rate of 100 µm/s exhibits excellent comprehensive room temperature properties, the yield strength, fracture strength and tensile strain are 732 MPa, 1020 MPa and 34.9 %, respectively. Furthermore, the directionally solidified Ni 36 Co 30 Fe 11 Cr 11 Al 6 Ti 6 HEA still remains high yield strength of 533 MPa at 900 ℃. Fine grain strengthening, solid solution strengthening, second phase strengthening and hysteresis diffusion effects play an important role in enhancing the mechanical properties. By comparing the mechanical properties of other HEAs at room and high temperature, the proposed directionally solidified Ni 36 Co 30 Fe 11 Cr 11 Al 6 Ti 6 HEAs with excellent mechanical properties show great potential in industrial applications. • Directionally solidified Ni 36 Co 30 Fe 11 Cr 11 Al 6 Ti 6 high entropy alloys contain two forms of dendrites. • The effect of solidification rate on dendrite growth is clarified. • The relationship between microstructure and mechanical properties is revealed. [ABSTRACT FROM AUTHOR]

For microstructure modification and performances enhancement, ultrasound treatment was applied in the solidification of Ti44Al6Nb1Cr alloy. The effects of ultrasound treatment on the microstructure characteristic and mechanical performances were studied. The results demonstrate that ultrasound treatment could alleviate the shrinkage defects and improve the quality of casting ingot. The coarse dendritic morphology is modified into small grain structure and the large lamellar colony is refined from 465 μm to 38 μm. The essential elements (Ti, Al) are uniformly redistributed in the lamellar colony, while the alloying elements (Nb, Cr) are aggregated at the lamellar colony boundary leading to micro-segregation. After ultrasound treatment, the microhardness of TiAl alloy is increased from 377 HV to 461 HV and the yield strength is enhanced from 409 MPa to 985 MPa. The ultrasound cavitation enhances the heterogeneous nucleation and the acoustic stream promotes the redistribution of crystal nucleuses, both contributing to grain refinement. [ABSTRACT FROM AUTHOR]

The high-temperature hydrogen charging was carried out to reveal the effect of hydrogen on the microstructure and hot deformability of TiAl alloys. The Ti–44Al (at.%) alloy and Ti–48Al alloy were hydrogenated at temperature range from 1373 K to 1693 K under hydrogen pressure of 0.1 MPa, and effects of hydrogen on the hot deformation behavior were tested on Gleeble-1500D at temperature of 1373–1523 K under strain rate of 0.01 s −1 . Results show that hydrogen content increases with increasing of temperature and the volume fraction of α 2 phase, and hydrogen absorption reaches to equilibrium within 30 min at high temperature. There is a linear relationship between ln C H and 1/ T , the heat of solution of hydrogen is calculated as 86.0 kJ/mol and 76.2 kJ/mol respectively, suggesting that hydrogen absorption in TiAl alloys is an endothermic reaction. Hydrogen promotes the formation of γ phases because hydrogen promotes the transition of L(α 2 /γ)→γ and nucleation of γ phase. The presence of hydrogen promotes the dynamic recrystallization and dislocation movement during hot deformation, resulting in decrease of flow stress. The peak stresses of Ti–44Al alloy and Ti–48Al alloy are decreased by 29% and 38% respectively after hydrogenation, which corresponds to the decrease of deformation temperature by about 50 K. [ABSTRACT FROM AUTHOR]

The bending and nanomechanical behaviors of a Zr 55 Cu 30 Ni 5 Al 10 bulk metallic glass were investigated in the present work via melt hydrogenation. The room temperature bending plasticity of samples after hydrogen addition is observed improving gradually with the increase of addition amount. A softening phenomenon is appeared in the results of nanoindentation, concomitantly with a decrease in the elastic modulus of the samples. A simple “spacer” model of hydrogen atoms can be used to describe for the softening behaviors of the Zr 55 Cu 30 Ni 5 Al 10 samples prepared by melt hydrogenation in this work. The “spacer” can enhance the distance of the metal atoms and decrease the corresponding atomic cohesion which is represented as the elastic modulus. The local changes of the atomic configuration give rise to the emergence of large void structures which will facilitate the local atomic displacement. Consequently, the “spacer” effect promotes the shear band proliferation and weakens the local shear deformation, and this in turns, leads to the large plastic deformation in the amorphous alloy. The results may have some implications for improving the plasticity of bulk metallic glass and understanding its hydrogen-induced softening. [ABSTRACT FROM AUTHOR]

In order to improve the poor hot workability of titanium matrix composites (TMCs), an advanced melt hydrogenation method was introduced in this study. The (TiB+TiC)/Ti-6Al-4 V composites were fabricated by melt hydrogenation which was directly melting alloys in gas mixture of H 2 and Ar. Microstructure of as-cast TMCs indicated that melt hydrogenation increased the length of TiB whiskers and aggravated the clustering of reinforcements at primary β grain boundaries, which was due to the increased overheat on melt surface. Hot compression results indicated melt hydrogenation improved the hot workability of TMCs in (α + β) phase region, and the peak stress was reduced from 371 to 271 MPa at 800 °C/1 s−1 and from 119 to 60 MPa at 900 °C/0.01 s−1, respectively, which expanded the optimal hot processing window. Microstructure after hot deformation showed that the proportion of DRX grains was increased from ∼56% to ∼81%, which was mainly attributed to the accelerated migration of DRX grain boundary and the decreased density of dislocations, which was due to the dislocation consumption by DRX formation and the improved mobility of dislocations. Therefore, the improvement of hot workability resulted from the formation of more DRX grains and enhanced mobility of dislocations. • Hydrogen increases the length of TiB whiskers in as-cast composites. • Hydrogen improves the hot workability of TMCs in (α+β) phase region. • Peak stress decreases from 371 to 271 MPa at 800°C/1s-1 and 119 to 60 MPa at 900°C/0.01s-1. • Hydrogen accelerating DRX (from ~56% to ~81%) contributes to the superior hot workability. [ABSTRACT FROM AUTHOR]

Nb-silicide in situ composites have been considered as alternatives applied for nickel-based superalloys in aerospace field. The synergistic effects of Fe and C on microstructural evolution, properties and hot deformation behavior of Nb-24Ti-16Si-2Al-3 V- x Fe- y C alloys are investigated. Results show that the microstructure of 4Fe-1 C alloy consists of Nbss/β-(Nb, X) 5 Si 3 eutectics and γ-(Nb, X) 5 Si 3 blocks, while the TiC particles precipitate with the increase of C content. For 6Fe- y C alloys, a new silicide phase Nb 4 FeSi is formed around the (Nb, X) 5 Si 3 phase. Element Fe is mainly distributed in Nb 4 FeSi and part of Fe-rich (Nb, X) 5 Si 3 , while C is mainly dissolved in TiC particles, followed by Nbss. The orientation relationship of Nbss and TiC in accordance with {110} Nbss //{111} TiC. The room temperature fracture toughness and compressive strength exhibit a trend of enhancing first and then reducing with the co-addition of Fe and C. The K Q value of the 4Fe-3 C alloy reaches the highest with a mean of 14.49 MPa·m1/2. Element C can effectively promote the high temperature compressive strength, and Fe can improve the hot deformability. The initial eutectic coupled structure has evolved into a continuous Nbss matrix with dispersed silicides during hot deformation. Furthermore, the splitting mechanism of silicides during hot deformation is determined. [Display omitted] • A novel compositions Nb-24Ti-16Si-2Al-3 V- x Fe- y C (x = 4, 6; y = 1, 3, 5) are designed. • C can effectively improve the hot compressive strength, while Fe can promote the hot deformability of the alloy. • The orientation relationship of TiC and Nbss is defined, which conforms to (111) TiC // (110) Nbss. • After hot deformation, the eutectic colonies have evolved into Nbss matrix containing dispersed (Nb, X) 5 Si 3 phase. • The preferential softening of the Fe enrichment region induces the fracture of the (Nb, X) 5 Si 3 blocks. [ABSTRACT FROM AUTHOR]

The relationship between shear transformation zones (STZs) and serrated flow dynamics of Co-, Fe-, Zr-, La- and Al-based metallic glasses (MGs) with different plasticity were analyzed by nanoindentation. The serrated flow dynamic behaviors were analyzed using statistical analysis of shear stress drops, Δ τ. The STZ size was estimated via the nanoindentation creep method based on the cooperative shear model. Examination of two distinct types of serrated flow dynamics were performed. For MGs (e.g., Al- or Zr-based) where STZs could be easily activated or at low loading rates, shear stress exhibited a decreasing trend and followed a power-law distribution, which was an indicator of a self-organized critical (SOC) state in the dynamics. In contrast, a Gaussian law distribution was found for STZs in hard MGs (e.g., La-, Co-, and Fe-based) or at high loading rate. In the case of Gaussian distribution case, serrated flow dynamics were in a chaotic state, which indicated that STZ aggregation reduced the stresses required for plastic deformation. [Display omitted] • Serration and STZ of 5 MGs were studied by nanoindentation at various loading rate. • For the soft MGs with more STZs, self-organized critical state in the dynamics. • For less-ductile MGs with smaller STZs, a chaotic dynamics state occurs. • The cut-off of the shear stress drop truncated with STZ volumes increased. [ABSTRACT FROM AUTHOR]

In order to further optimize the mechanical properties of TiAl composites, Ti 2 AlN/Ti48Al0.5W composites with 1 at% Zr/Hf addition were prepared by arc melting, which were termed as Base alloy, Zr-containing alloy and Hf-containing alloy. The grain size, element distribution, phase content and compressive properties were investigated and compared in this work. With 1 at% Zr/Hf addition, the lamellae colony size decreased from 33 µm to 25/23 µm, and Zr atoms almost dissolved into the single γ phase in the form of solid solution atoms, while Hf atoms distributed in both α 2 /γ lamellae and single γ phase, and part Hf atoms also existed in the form of B2 phase, which illustrated the higher solid solution of Zr atoms in TiAl composites comparing with Hf atoms. The γ phase content increased and the α 2 phase content decreased, especially with 1 at% Zr addition, which acted as γ-phase stabilizing element. With 1 at% Hf addition, the compressive strength and strain increased from 2010 to 2221 MPa and from 23.9 % to 26.2 %, respectively, which was attribute to the solid solution strengthening of Hf atoms by substituting Ti atoms, refinement of α 2 /γ lamellae and increased γ phase content. Compared with 1 at% Hf addition, 1 at% Zr addition led to higher compressive strength and strain, which were 2344 MPa and 26.8 %, respectively, and this result was due to the higher concentration of Zr atoms in γ phase and higher content of γ phase. On account of density, cost and performance, Zr atoms are more suitable as an alloying element to further optimize the microstructure and mechanical properties of TiAl composites. • The strengthening effects of Zr and Hf on Ti 2 AlN/Ti48Al0.5W composites were compared in this study. • 1 at% Zr addition increased the compressive strength and strain from 2010 to 2344 MPa and 23.9% to 26.8%, respectively. • Zr was more suitable to be the alloying element in TiAl composites compared with Hf. [ABSTRACT FROM AUTHOR]

In order to further optimize the mechanical properties of TiAl composites, Ti 2 AlN/Ti48Al0.5W composites with 1 at% Zr/Hf addition were prepared by arc melting, which were termed as Base alloy, Zr-containing alloy and Hf-containing alloy. The grain size, element distribution, phase content and compressive properties were investigated and compared in this work. With 1 at% Zr/Hf addition, the lamellae colony size decreased from 33 µm to 25/23 µm, and Zr atoms almost dissolved into the single γ phase in the form of solid solution atoms, while Hf atoms distributed in both α 2 /γ lamellae and single γ phase, and part Hf atoms also existed in the form of B2 phase, which illustrated the higher solid solution of Zr atoms in TiAl composites comparing with Hf atoms. The γ phase content increased and the α 2 phase content decreased, especially with 1 at% Zr addition, which acted as γ-phase stabilizing element. With 1 at% Hf addition, the compressive strength and strain increased from 2010 to 2221 MPa and from 23.9 % to 26.2 %, respectively, which was attribute to the solid solution strengthening of Hf atoms by substituting Ti atoms, refinement of α 2 /γ lamellae and increased γ phase content. Compared with 1 at% Hf addition, 1 at% Zr addition led to higher compressive strength and strain, which were 2344 MPa and 26.8 %, respectively, and this result was due to the higher concentration of Zr atoms in γ phase and higher content of γ phase. On account of density, cost and performance, Zr atoms are more suitable as an alloying element to further optimize the microstructure and mechanical properties of TiAl composites. • The strengthening effects of Zr and Hf on Ti 2 AlN/Ti48Al0.5W composites were compared in this study. • 1 at% Zr addition increased the compressive strength and strain from 2010 to 2344 MPa and 23.9% to 26.8%, respectively. • Zr was more suitable to be the alloying element in TiAl composites compared with Hf. [ABSTRACT FROM AUTHOR]

To meet the requirements of the new generation of aeroengines, Nb-Si-based alloys have been widely studied as candidate materials for the new generation of blade materials; however, the lower ambient fracture toughness limits their applications. In this study, the effects of various contents (0, 0.5, 1, 3 at%) of TiB 2 on the microstructure and ambient fracture toughness of the hypoeutectic composite of Nb-16Si-20Ti-2ZrC (base) composite were investigated. Ingots were manufactured via arc melting in an Ar atmosphere. Results demonstrate that the ingots were composed of Niobium based solid solution (Nbss) and γ-(Nb,X) 5 Si 3 , and TiC phases precipitate with a TiB 2 content of 3 at%. A moderate amount of TiB 2 (1 % TiB 2 and below) markedly refined the microstructure, and the average phase sizes of Nbss and γ-(Nb,X) 5 Si 3 decreased to 12.5 and 9.8 µm in the Nb-16Si-20Ti-2ZrC-1TiB 2 composite, respectively. TiB 2 promotes the movement of the eutectic point to the low Si side in the Nb-Si-based composite, transforming from a hypoeutectic (base) alloy to a eutectic alloy and a hypereutectic alloy. The ambient fracture toughness reached 18.6 MPa·m1/2, increased by 22 %. Many microcracks near the primary crack generated by the fine eutectic colony passivate the primary crack, and the deformation ability of the small-sized Nbss phase is enhanced, resulting in many dimples. The ambient fracture toughness of the Nb-16Si-20Ti-2ZrC-1TiB 2 composite in this study is the highest among the reported as-cast Nb-Si-based composites, which can promote Nb-Si-based composites to meet the blade material requirements. • TiB 2 promotes the movement of the eutectic point to the low Si side. • Fracture toughness was increased to 18.6 MPa·m1/2 by addition of 1 % TiB 2. • An appropriate amount (less than 1 %) of TiB 2 refined the microstructure. [ABSTRACT FROM AUTHOR]

To reveal the phase formation mechanism and the strengthening mechanism, the Al x Co 4 Cr 1.5 Fe 1.5 Ni 5 alloys (x = 1.0, 1.2, 1.4, 1.6, 1.8, 2.0) were prepared by arc melting. Phase formation and prediction, tensile properties, strengthening mechanism and deformation mechanism were researched in detail. Results show that the microstructure of Al x Co 4 Cr 1.5 Fe 1.5 Ni 5 alloys changes from single FCC phases (x ≤ 1.4)) to FCC and BCC phases (x＞1.4), which was in good agreement with the calculated phase diagram (CALPHAD) prediction. The lower mixing enthalpy of Ni-Al leads to the segregation of Al element at the inter-dendritic region, which promotes the formation of BCC and FCC lamellar structure in hypoeutectic Al 2 Co 4 Cr 1.5 Fe 1.5 Ni 5 alloy. The ultimate tensile strength of Al 2 Co 4 Cr 1.5 Fe 1.5 Ni 5 alloy is 835 MPa, meanwhile the fracture strain is 31.8 %. The tensile strength of Al 2 Co 4 Cr 1.5 Fe 1.5 Ni 5 alloy is attributed to the interface strengthening and the solid solution strengthening. The massive cross slip of dislocations in primary FCC phases and the dense dislocations blocked by the FCC-BCC interface increase the tensile strength. The interface barrier strength of the BCC-FCC interface in Al 2 Co 4 Cr 1.5 Fe 1.5 Ni 5 alloy was estimated to be 3.9 GPa, which shows the higher barrier strength than conventional BCC-FCC interfaces. Coupled effects of the primary FCC and the lamellar structure of FCC and BCC phases will maintain good mechanical properties. • The phase transformation mechanism of Al x Co 4 Cr 1.5 Fe 1.5 Ni 5 HEAs was revealed. • The interface strengthening and the solid solution strengthening is attributed to the tensile strength of HA2 alloy. • The interface barrier strength in HA2 alloy is higher than conventional BCC-FCC interfaces. • The massive cross slip of dislocations and the dense dislocations blocked by the FCC-BCC interface were observed. • The hypoeutectic Al 2 Co 4 Cr 1.5 Fe 1.5 Ni 5 showed a combination of ductility and strength. [ABSTRACT FROM AUTHOR]

Nickel can enhance the elevated temperature property of Al alloys, but blocky and aggregated Ni-rich intermetallic induced by high Ni content can seriously reduce the properties at room and elevated temperatures. Accordingly, finding ways to optimize Ni-rich intermetallic and performance at room and elevated temperatures is of great significance. In this regard, we couple travelling magnetic fields (TMF) with sequential solidification to alter the content, morphology, distribution and growth of Ni-rich intermetallic in Al-Cu-based alloys with high Ni content, further to increase the related performance. Moreover, simulations are combined with experiments to comprehensively reveal the mechanisms on these optimizations during each stage of solidification process. Current findings indicate that Ni-rich intermetallic and matrix phase α -Al are effectively refined, modified and diffusely distributed, and growing in random directions by TMF process. Hence, the performance at both room and elevated temperatures are greatly improve. Specifically, Down-TMF increases the UTS, YS, elongation and hardness at 298 K from 147.8 MPa, 90.8 MPa, 6.0 % and 75.6 kg·mm−2 without TMF to 186.7 MPa, 102.7 MPa, 10.4 % and 85.2 kg·mm−2. While, Up-TMF raises them to 179.2 MPa, 95.1 MPa, 8.5 % and 83.2 kg·mm−2. Additionally, Dwon-TMF improves the UTS, YS and elongation at 473 K from 99.0 MPa, 77.6 MPa and 17.3 % without TMF to 153.7 MPa, 86.1 MPa and 18.2 %, and from 74.7 MPa, 65.6 MPa and 18.5 % at 573 K to 84.4 MPa, 75.8 MPa and 25.7 %. While, Up-TMF promotes their elevated performance to 162.9 MPa, 88.9 MPa and 17.6 % at 473 K, and to 87.2 MPa, 77.0 MPa and 22.5 % at 573 K, respectively. Noteworthily, Down-TMF generates more overall positive effects than Up-TMF. • Travelling magnetic fields coupled with sequential solidification effectively optimizes the Ni-rich intermetallic. • TMF greatly improves mechanical performance at room and elevated temperatures for Al-Cu-based alloys with high Ni content. • Down-TMF can generate more overall positive effects to alloy castings than Up-TMF. [ABSTRACT FROM AUTHOR]

To reveal homologous substitution and refined mechanisms, and improve mechanical properties, Ti42Al6Nb 2.5 C x Ta alloys were prepared by arc melting. Element distribution, lamellar colony size, length-diameter radio of Ti 2 AlC phase, tensile properties, and the corresponding influence mechanism were studied. Results show that the relative contents of γ and Ti 2 AlC phase increase, the content of α 2 phase decreases, when Ta content increases from 0 to 2.0 at%. The relative content of B2 phase increases among the lamellar colonies and the length-diameter ratio of Ti 2 AlC particle decreases. Lamellar colony size decreases from 24.9 to 10.6 µm. With the increase of Ta content, the supercooling at the front of the solid-liquid interface increases and the β phase is refined. The increase of the precipitated phase content leads to the increase of the number of α phase heterogeneous nucleation particles, which will refine the lamellar colony. Tensile results show that the strength increases from 238 to 513 MPa and strain increases from 2.7 % to 11.1 % of Ti42Al6Nb 2.5 C1.6Ta alloy when the tested temperature increases from 750 to 950 ℃. The high temperature tensile properties of the alloy with more Ta content are relatively higher. Ta forms large lattice distortion after solid solution into the matrix due to its large atomic size, and Ta makes more Nb dissolve into the collective to increase the effect of solid solution strengthening. Another reason is the refinement of microstructure. • Alloys with finer microstructure and better high temperature tensile properties were prepared by adding different contents of Ta. • The lamellar colony and the Ti2AlC precipitates are refined, the size of lamellar colony decreases from 24.9 to 10.6 µm. • The tensile strength and elongation of alloy increase with the increase of Ta content. • Tensile results show that the strength increases from 238 to 513 MPa and strain increase from 2.7 % to 11.1 % of Ti42Al6Nb2.5C1.6Ta alloy when the tested temperature increase from 750 to 950 ℃. [ABSTRACT FROM AUTHOR]

In order to accelerate the hydrogen absorption and desorption rates of hyper-eutectic Mg-Ni alloy, Mg 85 Ni 15- x Ag x (x = 0, 0.2, 0.4, 0.6) alloys were prepared and the results show that fishbone eutectic was formed without change of phase composition. It was found that Ag atoms were dissolved in fishbone eutectic Mg 2 Ni and the average cell parameters of Mg 2 Ni increase with the increase of Ag content. Owing to adjusting lattice parameters, the hydrogen absorption and desorption rates of Mg 85 Ni 14.8 Ag 0.2 alloy are significantly improved. About 4.47 wt% hydrogen is absorbed at 275 ℃ only in 1 min, with the maximum capacity of 5.84 wt% in 60 min. The Mg 85 Ni 15- x Ag x (x = 0.2, 0.4, 0.6) hydrides can be completely dehydrogenated within 10 min at 275 ℃. The corresponding activation energies for dehydrogenation are determined to be 80.4, 97.1 and 97.8 kJ/mol, respectively, which are lower than 117.8 kJ/mol of Mg 85 Ni 15. The onset dehydrogenation temperature of Mg 85 Ni 14.8 Ag 0.2 hydride is reduced to 201 ℃, which is much lower than 220 ℃ of the Mg 85 Ni 15 hydride. The dissolving of Ag in Mg 2 Ni leads to a lattice expansion, which facilitates the diffusion for hydrogen atoms and decrease the energy barrier required for nucleation of Mg/Mg 2 Ni. As a result, the hydrogen de-/absorption rates were obvious accelerated and the onset dehydrogenation temperature was decreased. • The lattice size of Mg2Ni was regulated by doping Ag. • Hydrogen storage capacity of Mg85Ni14.8Ag0.2 alloy reaches 5.84 wt% at 275 ℃. • The Ea value for decomposition of Mg85Ni14.8Ag0.2 hydride decreases to 80.4 kJ/mol. • Addition of Ag reduces the initial desorption temperature of Mg85Ni15 hydrides. [ABSTRACT FROM AUTHOR]

Microstructural characteristics and mechanical properties of Ti–49Al (at.%) alloy was investigated by directional solidification technology under different solidification parameters in a Bridgman-type furnace. Variations of primary dendritic arm spacing, interlamellar spacing and mechanical property with solidification parameters were investigated. The values of primary dendritic arm spacing and interlamellar spacing decrease with increasing cooling rate. Values of microhardness and tensile strength increased with the increase of cooling rates, and decreased with the increase of primary dendritic arm spacing and interlamellar spacing. The relationships between the primary dendritic arm spacing, interlamellar spacing, microhardness, tensile strength and cooling rate are proposed. The tensile strength of directionally solidified Ti–49Al (at.%) alloy increases with the increase of microhardness. There is a linear correlation between microhardness and tensile strength of the directionally solidified TiAl alloy. The experimental correlation between tensile strength and microhardness was established. The results were further compared with previous similar experimental results. [ABSTRACT FROM AUTHOR]

Ti44Al6Nb1.0Cr2.0V alloy was directionally solidified (DS) with cold crucible under different input powers (40 kW, 45 kW, 48 kW and 50 kW; corresponding to temperature gradient of 15.2 K/mm, 21.0 K/mm, 22.9 K/mm, 25.5 K/mm) and the same pulling velocity (8.33 μm/s). The results show that all the DS samples have the β-solidification characteristics and when the power is 45 kW or 48 kW, they have the well-DS macrostructure. After DS, the lamellar width becomes smaller and more homogeneous from the as-cast 1650 ± 1200 nm to less than 710 ± 376 nm. The higher the power is, the smaller the lamellar width it has. The original casting microcracks can be eliminated after DS and the percentage of small angle lamellas is higher for the well-DS samples. For the DS samples, the nanoindentation hardness in lamella region and the lamellar width yield the relationships of H N = 16.12 d − 0.127 and r 2 = 0.992 . When the as-cast and heat treatment (HT) conditions are considered, the relationships are changed as H N ' = 16.09 d ' − 0.127 and r ' 2 = 0.971 . The nanoindentation hardness of B 2 phase and block γ phase keep almost the same respectively under the different conditions. RT tensile property is improved to average UTS 589.4 MPa and average elongation 1.16% after DS (P = 45 kW) comparing with the original as-cast 498.8 MPa and 0.53%. Trans-granular and trans-lamella fracture is the main fracture mode for the DS samples. After annealing ( at 1250 °C, 1300 °C, 1350 °C), lamellar width is coarsen to more than 2002 ± 1286 nm; B 2 phase segregation is reduced obviously, but some new microcracks form; HT samples exhibits a poor RT tensile property, in which the HT2 sample has a relatively higher property and it yields the average UTS 440.6 MPa and average elongation 0.45%. [ABSTRACT FROM AUTHOR]

Bridgman-type directional solidification experiments were conducted for Ti–50Al–4Nb alloy. The continued growth of the non-preferred primary phase with the effect of the growth rate was investigated for the parallel lamellar structure. The planar–cellular–dendritic growth of primary α phase is observed with increasing growth rate and α 2 /γ lamellar structure is obtained finally. The growth of the non-preferred crystals is blocked by the nucleated preferred crystals at low growth rates (<3 μm/s) due to the external particles, as well as that at high growth rates (⩾15 μm/s) due to the tilting crystals and the high undercooling. While the continued growth of the non-preferred crystals is achieved at proper middle growth rates, which always takes on a slope form owning the developed lateral dendrites. Finally the parallel columnar crystals with the parallel lamellar structure are obtained at selected growth rates (5–7 μm/s) by controlling the continued growth of the preliminary non-preferred crystals at the initial interface. [ABSTRACT FROM AUTHOR]

The long period stacking ordered (LPSO) structure in Mg-based alloy presents a high content of Mg and evenly-dispersed transition metal (TM) and rare earth (RE) elements, which shows great potential in hydrogen storage. In this work, Mg 96 Y 2 Zn 2 alloy with high proportion of LPSO structure was prepared, forming fine α-Mg dendrite and the interdendrite region with homogeneous Zn and Y distributions. Results indicate that Mg 96 Y 2 Zn 2 alloy decomposes into YH 2 + Mg(Zn) composite after hydrogen ab/desorption, of which nanosized YH 2 is in-situ formed from the irreversible decomposition of LPSO and eutectic phase and Mg(Zn) is generated from the dissolution of Zn atoms in Mg lattice. This composite can absorb 5.36–5.79 wt% hydrogen and desorb hydrogen at a lower temperature than pure MgH 2 , which ascribes to the "hydrogen pump" effect of YH 3 and the catalyst of MgZn 2. The hydrogenation process of Mg 96 Y 2 Zn 2 alloy is dominated by one-dimension diffusion rate of hydrogen atoms, while the dehydrogenation process is dominated by two-dimension interface movement of the metal/hydride phases. The in-situ formed YH 2 nanophase and Mg(Zn) originated from LPSO structure are stable and evenly dispersed, thus the Mg 96 Y 2 Zn 2 alloy shows enhanced hydrogenation capacity and kinetics. • High proportion LPSO structure is obtained in Mg 96 Y 2 Zn 2 alloy. • Dispersed YH 3 and MgZn 2 nanophases are formed from H-induced reaction of LPSO phase. • Faster hydrogenation kinetics are achieved owing to YH 2 and MgZn 2 nano-catalysts. • In-situ formed YH 2 presents more superior catalytic effect than external YH 2. [ABSTRACT FROM AUTHOR]

Quasicrystal (QC) phases are often observed in Al-based alloys with Al content< 85 at%, and their particle size is typically greater than 0.3 µm. In this study, an in situ formed QC phase was observed in Al-4.1Mn-0.98Mg-0.8Sc-0.5Zr alloy (Al ≈ 96 at%) when processed using Selective Laser Melting (SLM). The particle size of the QC phase processed by SLM is 55–85% smaller than when processed by other methods, which is likely due to the lower Mn concentration and higher cooling rate. Moreover, the shape of the QC phase is spherical near the bottom of the melt pool and becomes a strip in the center of melt pool with the variation of solidification conditions. At higher cooling rates, it can be inferred that the formation of QCs is promoted while the growth of QC phase is effectively inhibited. According to calculations and electron spectroscopy (EDS) results, the possible chemical composition of the QC phase is considered as Al 80 Cu 2.83 Mn 17.16 (at%) with an e/a of 1.9849 and R av of ~1.4152 Å. During aging, a portion of the QC phase could transform into Al 6 Mn through a peritectoid reaction. The homogeneously dispersed QC transforms the Al 6 Mn into a finer and more dispersed phase. Consequently, the mechanical properties are cooperatively improved both by Al 6 Mn and the residual QC phase. After aging, the ultimate strength is 548.67 ± 4.66 MPa with an elongation of 12.2 ± 1.09%. These results indicate that SLM expands the formation range of QC and is an ideal processing route to fabricate in situ QC-strengthened Al-based alloys. [ABSTRACT FROM AUTHOR]

• Successfully adjusted the solid-liquid phase region to increase the effective time of ultrasound in the primary nucleation and growth process. • TiC completely forms Ti 2 AlC phase with the increase of niobium content under the action of ultrasound. • The Ti 2 AlC and lamellar colony are refined because niobium causes component undercooling and further increases heterogeneous nucleation. • The compression performance of TiAl composites is improved, the maximum strength is 2474.2 MPa and the maximum strain is 28.0% with 4 at% Nb. To refine microstructure, adjust the ratio of γ/α 2 and improve the mechanical properties, Ti46Al 2.6 C- x Nb (at. %) alloys were prepared by arc melting under the action of ultrasound. The phase composition, lamellar colony size, elements distribution, the kinds and distribution of carbides and compressive properties were investigated. Results show that as the Nb content increases, the relative content of α 2 phase and Ti 2 AlC phase increases, and the relative content of γ phase decreases. TiC phase completely reacts to form Ti 2 AlC phase and its shape changes from coarse long rod to short rod. The microstructure was refined with the addition of Nb under ultrasound treatment. Lamellar colony size decreases from 36.9 to 21.5 µm, lamellar spacing decreases from 1.6 to 0.8 µm, and the length-diameter ratio of Ti 2 AlC decreases from 5.4 to 4.0 when the Nb content increases from 0 to 8.0 at. %. During solidification process, not only the component supercooling and heterogeneous nucleation of Nb are increased, but also the crystal grains are broken and the nucleation barrier is reduced under the action of ultrasound to promote the nucleation. The microstructure is refined and the uniformity of the element distribution is improved. Compressive results show that the strength increases from 2074.1 to 2474.2 MPa and the strain increases from 23.4% to 28.0% when the Nb content increases from 0 to 4 at. %. The refinement of lamellar colony and Ti 2 AlC, the ratio of γ/α 2 and solid-solution of Nb are the main reasons for improving compressive properties. [ABSTRACT FROM AUTHOR]

The on-board hydrogen storage needs light, compact, and affordable system to replace the compressed hydrogen tanks. MgH 2 is regarded as one of the most promising candidates for solid-state hydrogen storage. Due to the thermodynamically stable Mg-H bond, the poor dissociation ability of H 2 molecules and recombination ability of H atoms on Mg surface, and the low diffusion coefficient of H atoms in Mg, especially in MgH 2 , there remains a challenge to design a material capable of absorbing and desorbing hydrogen rapidly at moderate temperatures. Herein, common strategies to improve the hydrogen storage properties of Mg-based materials are introduced. Some recent studies for overcoming the thermodynamic and kinetic barriers are reviewed, especially the approaches to fabricate nanostructure in Mg-based materials. The nanometric crystals or particles are synthesized by mechanical deformation, reduction in solution, physical/chemical vapor deposition, interface confinement. The nano-catalyst can be decorated by external doping or in-situ synthesis. In particular, the thermodynamic/kinetic parameters for hydrogenation and dehydrogenation are provided, and the mechanisms for the enhanced properties are summarized. The principles of structural design and catalytic decoration are discussed, and challenges and perspectives of fabricating nanostructures in Mg-based hydrogen storage materials are proposed. [ABSTRACT FROM AUTHOR]

• The critical ultrasonic cavitation intensity decreases with overheating increasing, and the increasing acoustic cavitation effect refined the lamellar colony and Ti 2 AlC in weak acoustic active zone. • In strong acoustic active zone, the stronger acoustic pressure and acoustic streaming effect increase the nucleation rate, promoting the lamellar colony refinement and element distribution uniformly. • Microstructure refinement, enhanced size decreasing and element segregation reduction improve the Vickers hardness of composite in ultrasound treatment. To reveal the coupling mechanism between melt superheat and ultrasonic action, Ti48Al2Cr2Nb2.5C alloy was prepared by arc melting with ultrasound treatment under different temperature gradients. And numerical simulation, microstructure evolution and Vickers hardness were researched in detail. Simulation results indicate that there are weak acoustic active zone (WZ) and the strong acoustic active zone (SZ) defined by acoustic pressure distribution. The acoustic pressure in SZ is over 10 MPa with an intense acoustic cavitation effect, and the acoustic streaming effect is significant in UC 350 due to the higher acoustic pressure and fluid velocity of 0.19 m/s. Experiment results reveal that the lamellar colony size increases from 39.9 to 67.3 µm and then decreases to 50.9 µm, and the aspect ratio of Ti 2 AlC decreases from 6.5 to 3.3 with overheating increasing in WZ. However, Al segregation is severe. In SZ, the lamellar colony size decreases from 80.3 to 43.7 µm and then increases to 56.1 µm. The aspect ratio of Ti 2 AlC decreases from 3.9 to 2.5. From UC 150 to UC 250 in WZ, temperature gradient increasing suppresses the ultrasonic refinement effect. With overheating increasing, the critical ultrasonic cavitation intensity decreases, and the acoustic cavitation effect increases, refining the lamellar colony and Ti 2 AlC. The stronger acoustic pressure and acoustic streaming effect increase the nucleation rate to refine the lamellar colony and element distribution uniformly in SZ. In WZ, Vickers hardness increases from 191.4 to 223 HV and then decreases to 211.2 HV. Vickers hardness increases from 211.3 to 227.7 HV but decreases to 208.4 HV in UC 300 and then decreases to 208.8 HV in SZ. The highest Vickers hardness in WZ and SZ are in UC 350. Lamellar colony refinement, the aspect ratio of Ti 2 AlC reduction and Al segregation attenuation improve the Vickers hardness. [ABSTRACT FROM AUTHOR]

• Sc element refines primary γ-(Nb,X) 5 Si 3 and Nbss. • Fracture toughness was increased to 16.4 MPa·m1/2 by addition of 0.05% Sc. • The degree of restraint of Nbss phase decreases with the increase of Sc. The effect of Sc addition on the phase composition, microstructure, and room-temperature fracture properties of arc melted Nb-Si-Ti-ZrC-based alloys was systematically investigated. The constituent phases of all alloys are Nbss, γ-(Nb,X) 5 Si 3 and TiC. Sc refines the microstructure, while the grain sizes for the primary γ-(Nb,X) 5 Si 3 and Nbss phases drop from 213 µm and 22.3–23 µm and 5.7 µm, respectively. The content of Nbss and TiC decreases with increasing Sc. The Sc dissolves and then softens Nbss phase. The degree of restraint of Nbss phase decreases with increasing Sc content, therefore causing formation of dimples in the fracture and interface decohesion. Because of the refinement of the primary γ-Nb 5 Si 3 phase, the dimples and interface decohesion created by the low degree of constraint of Nbss phase, the K Q value of Sc3 alloy becomes 16.4 MPa·m1/2. The average grain size of Nbss phase becomes the lowest (5.7 µm), when 0.5 at% Sc is added. The small grain size of Nbss phase deteriorates room-temperature fracture toughness. [ABSTRACT FROM AUTHOR]

• A novel metastable Co-rich high entropy alloy is prepared by adjusting Cr content and deformation mechanism is revealed. • The deformation induced phase transition is found in the alloy with high stacking fault probability at room temperature. • Co 40 Fe 20 Ni 20 Cr 20 high entropy alloy has higher deformation ability than other alloy and strain hardening exponent is 0.95. [Display omitted] To understand the phase transformation and its effect on mechanical properties of high entropy alloys (HEAs) during deformation, Co 40 Fe 20 Ni 40- x Cr x (x = 10, 20, 30 at%, hereinafter simplified as HCr10, HCr20 and HCr30, respectively) HEAs were prepared, and the microstructure and tensile properties were investigated. The results show that HEAs are mainly composed of face-centered cubic (FCC) phase, and the σ phase forms in HCr30 due to the valence electron concentration (VEC) decreasing from 8.8 to 8.0 with increasing Cr content. For HCr10, the phase transforms from FCC to hexagonal closed-packed (HCP), but for HCr20 and HCr30, the phase transforms from FCC to HCP and σ phases during tensile deformation. The volume fraction of HCP phase firstly increases from 4.29 to 15.40 vol%, then decreases to 7.67 vol%, and σ phase increases from 0 to 28.89 vol% with the increase of Cr content. The stacking faults (SFs) probability increases with the increase of Cr content, contributing to the occurrence of the phase transformations. Tensile testing shows that the yield strength increases by 50% in HCr30 compared with HCr10, and the ultimate strength and elongation reach the maximum values (718 MPa and 59.05%) in HCr20. HCr20 shows a high strain hardening exponent (n = 0.95) after tensile deformation. The phase transformation induced by tensile deformation mainly accounts for the strength-ductility improvement, whereas an increase in σ phases will reduce the ductility. [ABSTRACT FROM AUTHOR]

• Zr addition shifts the eutectic point to lower Si content. • Zr addition decreases elemental segregation and increases Nbss content. • Fracture toughness K Q value of 8.2Zr containing alloy is 16.29 MPa m1/2. To improve microstructure and fracture toughness of MASC alloy, the effects of element substitution of Zr for Hf on the microstructure and room-temperature fracture toughness in Nb-24.7Ti-16Si- x Hf- y Zr-2.0Cr-1.9Al (x + y = 8.2) alloys have been investigated in this study. The results show that the substitution of Zr for Hf in MASC alloy promotes the eutectic reaction of (Nb, X) 3 Si phase → Nbss/γ(Nb, X) 5 Si 3 eutectic, and shifts the eutectic point to higher Si content, which leads the microstructure transformation from hypereutectic structure to nearly eutectic structure. The primary γ(Nb, X) 5 Si 3 phase content decreases and Nbss phase content increases with the increase of Zr content. The room-temperature fracture toughness increases with the increase of Zr content, and the room-temperature fracture toughness values of 4Zr-4.2Hf, 6Zr-2.2Hf and 8.2Zr-0Hf alloys are 16.03–16.29 MPa m1/2. The room-temperature fracture toughness value of 16.29 MPa m1/2 is the highest value in the nondirectionally solidified Nb-Si based alloys. [ABSTRACT FROM AUTHOR]

In this study, the microstructure and mechanical properties of the as-cast and as-annealed CoCrFeNiW x (x = 0, 0.2, 0.4 and 0.6, x value in molar ratio) high-entropy alloys were investigated. Both W-addition and annealing treatment were conducive to the formation of the μ phase in CoCrFeNiW x alloys. With the increase of x value, the microstructures of the as-cast HEAs evolved from single-phase solid solution (x = 0, 0.2) to hypoeutectic structure (x = 0.4), then to hypereutectic structure (x = 0.6). The as-cast CoCrFeNiW 0.4 alloy exhibited a tensile strength of 690.7 MPa with a considerable elongation of 33.1%, which resulted from solid solution strengthening and second phase strengthening. After annealing, the as-annealed CoCrFeNiW 0.4 alloy showed an increased tensile strength of 970.9 MPa, owing to the precipitation of μ-phase particles. Deformed microstructures indicated that cracks generated from μ-phase precipitates of different sizes, and suppressed by the ductile FCC solid solution. Moreover, the high-density dislocation tangles around the μ-phase precipitates lead to a high strain hardening rate, which improved the strength significantly. Image 1 • Microstructure evolves from hypoeutectic to hypereutectic with W addition. • Annealing treatment contributes to the μ phase precipitation. • As-annealed CoCrFeNiW 0.4 alloy possesses relatively high tensile strength. [ABSTRACT FROM AUTHOR]

Directional composite microstructures have successfully been prepared with directional columnar crystal forms at the center and equiaxed crystals with Ti 2 AlC particles at the edge of sample. Microstructure evolution, influenced by different solidification rates, is discussed and forming mechanisms of this composite microstructure have been revealed. Results show that the primary phase of α is observed from 60° of the primary dendritic arms, and β is from 90°. When the drawing velocity increases from 0.2 to 1.1 mm/min, the width of the columnar crystal decreases from 1.95 to 0.61 mm, and the distance between growth sites of Ti 2 AlC and most edge of the sample decreases from 1.04 to 0 mm. Obviously, the solid-liquid interface forms with low solidification rate. Forming the Ti 2 AlC phases consumes Ti the in local area, and the α primary phase forms with more than 49.4 at. % Al. The change of solidification rate has no effect on the concave shape of the solid-liquid interface because the influence of the drawing rate cannot change the influence of the heating power on the thermal balance of the molten pool. Ti 2 AlC particles move to the skin layer with the melt flow during directional solidification. This leads to a decrease in the heterogeneous nucleated particles near the centre of molten pool, to guaranteeing the growth conditions necessary for large columnar crystals. • Designed microstructure have been prepared successfully with columnar crystal at center and equiaxed crystal at edge. • Under low solidification rate Ti 2 AlC particles have enough time to move to the skin layer with melt flow. • A novel method to form in-situ surface by directionally solidified technology has been proposed. [ABSTRACT FROM AUTHOR]