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Abstract The morphological evolution and coarsening kinetics of γ′ precipitates in the γ'+γ area of a Ni 3 Al-based alloy during isothermal aging at temperature range of 800 °C to 1000 °C have been investigated. For the aging temperature above 900 °C, the average size of γ′ precipitates increases and the morphology of γ′ precipitates gradually changes from cuboidal to strip-like and L-shaped with increasing aging time up to 50 h. The temporal evolution of the average size of γ′ precipitates is evaluated by the experimentally measured data and it suggests that the cube rate law fits better for experimental data at the early stage coarsening, while the square rate law fits better for the later stage coarsening. Moreover, using these determined values of coarsening rate constants which determined by using a coarsening rate equation based on the LSEM model, the activation energy of the early stage and later stage coarsening have been calculated as 174.2 kJ/mol and 186.3 kJ/mol, respectively. Highlights • The morphology evolution of γ′ precipitate in a Ni 3 Al-based alloy was studied. • The coarsening process of γ′ precipitates occurs in two stages. • The cube law rate exhibits a better fit for the early stage coarsening. • The square law rate fits better for the later stage coarsening. • The activation energy of the early and later stage coarsening was calculated. [ABSTRACT FROM AUTHOR]

To study the hot deformation behavior of Ni 3 Al-based alloy, hot compression tests were conducted in the temperature range of 1050–1250 °C with the strain rates from 0.01 to 10 s −1 . With the increase in deformation temperature and the decrease in strain rate, flow stress of the Ni 3 Al-based alloy would be decreased. Based on the obtained constitutive equation, the calculated values of peak flow stress are in good agreement with the experimental ones, and the activation deformation energy is determined as 802.71 kJ/mol. Moreover, by dynamic material model (DMM), processing maps of the hot-deformed Ni 3 Al-based alloys are established. It is indicated that the optimum processing parameters for the studied alloy correspond to deformation temperature of 1250 °C and strain rates from 0.01 to 0.1 s −1 . Specimens deformed under the optimum processing conditions exhibit fine and uniform grains, which is a typical dynamic recrystallization (DRX) microstructure. The DRX degree could be effectively enhanced with the increase of deformation temperature and the decrease of strain rate. [ABSTRACT FROM AUTHOR]

Internal Mg diffusion (IMD) process can produce MgB 2 superconducting wires with engineering critical current density several times higher than that of traditional powder in tube processed wires, which makes it an attractive and promising method for mass producing practical MgB 2 wires. However, the MgB 2 layer growth stops shortly after the onset of the heat treatment and unreacted B always remained in the MgB 2 layer within MgB 2 wires, negatively affecting the J e performance. Thus it is of great importance to have an in-depth understanding of the mechanism of reaction between Mg rod and B powder forming the MgB 2 layer within IMD wires during heat treatment, and identify the critical factor that controls the formation rate of MgB 2 layer. In present work, the kinetics mechanism of reaction between Mg and B forming MgB 2 layer in the internal Mg diffusion (IMD) processed MgB 2 wires were systemically studied in present work. It was found that the reaction between Mg and B forming MgB 2 layer during the heating treatment is controlled by varied mechanisms. At initial stage, the formation of MgB 2 layer is mainly determined by the rate of chemical reaction between liquid Mg and B powder. As the reaction processes and the thickness of synthesized MgB 2 layer increases, the slow Mg diffusion-limited mechanism gradually becomes dominant at final stage. On the basis, herein Mg rod with a thin Cu coating is proposed to replace normal Mg rod in IMD procedure to accelerate the formation of MgB 2 layer within wires. As a result, Cu coating can change the kinetics mechanism of MgB 2 layer formation and enable the formation of MgB 2 layer to get rid of the restriction of slow Mg diffusion. Complete dense MgB 2 layer without B-rich or unreacted B regions was successfully synthesized within Cu coated IMD wires with larger diameter (1.03 mm) at temperature as low as 600 °C (below Mg melting point). The kinetics mechanism of MgB 2 layer formation determined in our work can provide a valuable guide for optimizing processing parameters of IMD MgB 2 wires. Moreover, the Cu coating technique proposed here opens a promising way to fabricate practical high performance IMD wires at low heating temperature. [ABSTRACT FROM AUTHOR]

The un-doped Mg 11 B 2 and Cu-doped Mg 11 B 2 bulks using 11 B as a boron precursor were fabricated by solid-state reaction and sintered at different temperature in present work. By analyzing the sintering process, it was found that 11 B original powder is more reactive and can react with Mg severely even at low temperature before Mg melting, which leads to the formation of refined Mg 11 B 2 grains. Consequently, the critical current density of Mg 11 B 2 sample prepared in this work is higher than that of natural MgB 2 . Furthermore, it was found that proper Cu addition could obviously accelerate the reaction between Mg and 11 B and promote the formation of Mg 11 B 2 due to the appearance of Mg–Cu liquid at low sintering temperature before Mg melting. On the other hand, Cu addition also introduced more MgCu 2 impurity, leading to a relatively lower critical current density on the whole. Interestingly, the critical current density of the Cu doped sample sintered at 800 °C is surprisingly enhanced at the low field, compared with that of the corresponding un-doped one. This is due to the peritectic reaction which generated small-sized MgCu 2 occurred that at 800 °C, providing MgCu 2 pinning centers well-distributed in the MgB 2 matrix. [ABSTRACT FROM AUTHOR]

Isochronal DSC measurements of various degrees cold-rolled Inconel 718 alloy samples were implemented to investigate the effect of cold rolling on the precipitation kinetics of intermediate phases in Inconel 718 alloy. The result shows that the values of the onset temperature ( T O n s e t 1 ) , the peak temperature ( T P e a k 1 ) and the end temperature ( T E n d 1 ) of γ″-phase precipitation decrease as the degree of cold rolling increases. In contrast, all values of the onset temperature ( T O n s e t 2 ) , the peak temperature ( T P e a k 2 ) and the end temperature ( T E n d 2 ) of δ-phase precipitation first increase and then decrease with the degree of cold rolling increasing as a consequence of a combined effect of lack of Nb atoms and recrystallization. Two sets of isothermal aging experiments were also carried out to study the effect of cold rolling on the morphology evolution of γ″-phase and δ-phase in Inconel 718 alloy. It is found that cold rolling promotes γ″→δ transformation and the morphology of γ″-phase turns more globular as the degree of cold rolling increases for the cold-rolled Inconel 718 alloy samples with isothermal aging at 800 °C for 8 h (procedure A). Moreover, for the cold-rolled Inconel 718 alloy samples with isothermal aging at 950 °C for 6 h (procedure B), cold rolling facilitates δ-phase precipitation and the δ-phase particles gradually sphericalized with increase in the degree of cold rolling. [ABSTRACT FROM AUTHOR]

Sn added bulk FeSe 0.5 Te 0.5 samples are synthesized through two-step solid state reaction route. It is found that Sn addition can significantly enhance the transformation from the hexagonal phase to the superconducting tetragonal phase during the sintering process of FeSe 0.5 Te 0.5 samples due to the appearance of liquid Sn at low temperature. With increase of superconducting phase proportion as well as improvement of its crystallinity, 5 wt% Sn addition dramatically enhances the zero resistivity temperature ( T c 0 ) by 3 K accompanying with a bit improvement in the onset temperature of superconducting transition ( T c on ). From this view, our work suggests a new prospective that how to search proper dopant to further improving superconductivity in 11 type Fe based superconductors. [ABSTRACT FROM AUTHOR]

Highlights: [•] Glycine doped MgB2 can be made at low temperature by Cu-activated sintering. [•] We obtained excellent J c in glycine-doped MgB2 sintered at 600°C for 15h. [•] Cu and glycine have synergistic effects on improvement of J c. [•] We illustrated the precipitation morphologies of secondary phase MgCu2 and MgO phases. [ABSTRACT FROM AUTHOR]

Abstract: The present study examines the effect of solidification rate on the type and morphology of intermetallic compounds (IMCs) of Al-based alloy with high Fe and Si contents in a wide cooling rate range of 1–100K/s. It was found that the final microstructure of Al–14wt.% Fe–2wt.% Si alloy mostly depends upon the solidification rate, which suggests that different solidification rates produced dissimilar microstructures. The increase of cooling rate significantly affects the phase selection sequence, amount and morphology of the IMCs in Al–14wt.% Fe–2wt.% Si. [Copyright &y& Elsevier]

Abstract: The minor Cu addition (<3at%) could apparently reduce the amount of MgO impurity in the in situ sintered MgB2 samples, accompanying with the improvement in the critical transition temperature (T c) from 36.4K for the undoped sample to 38.6K for the samples with 3at% Cu addition. The decrease of MgO impurity within Cu-doped MgB2 samples can be attributed to the following factors: (i) the formation of local Mg–Cu liquid at about 485°C wrapped the neighboring Mg particles and thus protected them from the oxidation before the melting point of Mg; (ii) when the sintering temperature increased above the melting point of Mg, the minor Cu addition could also decrease the vapor pressure of Mg liquid and then reduce the volatilization of Mg liquid, which can depress the reaction between gaseous Mg and O2 impurity in the protective Ar gas forming MgO impurity. Our results provide a new route to govern the oxidation of Mg during the in situ sintering of pure or doped MgB2 samples by altering the minor amount of Cu addition. [Copyright &y& Elsevier]

Abstract: MgB2 nanoparticles were synthesized by a one-step reactive sintering method. The sample was heated from room temperature to 994K, and then directly cooled down at a rate of 40K/min. The results of X-ray diffraction indicate that MgB2 superconductors with high quality were successfully prepared, and only a few impurities were detected on it. The sample consisted of two distinguishable structures by the TEM observation: nanoparticles and single crystals, which is a result of nonequilibrium conversion under the direct cooling condition. The nanoparticles with a diameter of about 10–20nm are considered to be at the onset for the formation of MgB2 phase. Transition temperature (T c) and critical current density (J c) of the compound structures were determined to be 38.5K and 1.8×105 Acm−2 by means of SQUID measurement, which indicate good superconducting properties. The inter-diffusion and dislocation incorporation mechanisms for the formation and growth of MgB2 nanoparticles are also proposed, and the further growth will be accelerated due to an adequate holding time at 994K. [Copyright &y& Elsevier]

Abstract: The effect of small additions of In, up to 1wt.%, on the microstructure of the eutectic Sn–3.7wt.%Ag–0.9wt.%Zn solder was investigated. As observed by microstructural analysis, the increase of In content made β-Sn easy to form but suppressed the formation of the AgZn phase in the Sn–3.7Ag–0.9Zn solder. After annealing at 473K for 20 and 50h, the microstructure varied a lot in the morphology of the investigated Sn–Ag–Zn–In solder. The β-Sn dendrites grew coarser but dimmer accompanied with the segregation of the intermetallic compounds (IMCs) along their boundaries. Furthermore, the suppressed Ag–Zn IMCs formed in the Sn–3.7Ag–0.9Zn–1In solder. And the coarsening of the β-Sn dendrites and the growth of IMCs particles in the microstructure of the samples brought a significant softening during annealing of the investigated Sn–Ag–Zn–In alloys. [Copyright &y& Elsevier]

Abstract: The structural stability of the rapidly solidified eutectic Sn-3.7wt.%Ag-0.9wt.%Zn solder was explored by aging at room temperature (298K). After aging for 2 weeks, the microstructure varied a lot in the morphology of intermetallic compounds (IMCs) in the investigated Sn–Ag–Zn solder. Dot-like Ag3Sn IMCs in the rapidly solidified solder congregated and formed bulk ones in order to minimize the system energy, which is contrary to a stable microstructure in the slowly cooled specimen. It seems that the coarsening of the IMCs in the microstructure of the rapidly solidified specimen brought a significant softening during aging of the explored Sn–Ag–Zn alloy. Furthermore, with the addition of zinc, the solder system becomes harder than the eutectic Sn–Ag system and the Ag–Zn phase dispersed in the eutectic Sn–Ag–Zn solder could strengthen the system as a reinforcing secondary phase. [Copyright &y& Elsevier]

Abstract: The effect of addition of In, up to 1wt.%, on the formation of intermetallic compounds (IMCs) in the solidified Sn–3.7%Ag–0.9%Zn lead-free solder was investigated. As observed by microstructural analysis, the typical structure of Sn–Ag–Zn solder is composed of β-Sn phase and mixed granules of Ag3Sn and AgZn IMCs. After alloying with In, it evolves into a mixture of randomly distributed rods and granules of Ag3Sn and AgZn. Clearly, the addition of In into the explored Sn–Ag–Zn solder promotes the formation of rod-like IMCs for the reason that the growth competition of the Ag3Sn and AgZn IMCs was destroyed by the selective adsorption of In atoms on a certain preferable crystalline planes of the separated IMCs. The change in the morphology of the formed IMCs leads to a great difference in the mechanical performances, for example, the measured microhardness of the investigated solders evolves from 16.95HV to 21.35HV with the increase of In content. [Copyright &y& Elsevier]

γ′-strengthened CoNi-based superalloys containing Ti and Ta are studied to investigate the γ′ stability after long-term aging at 900 °C. Both thermodynamic predictions and experimental observations suggest that there is no formation of CoAl or Co 3 W phases, indicating that the γ′ phase remains stable in the investigated CoNi-based superalloys. Typical γ/γ′ structure with a relatively high amount of γ′ phase remains in the microstructures of explored CoNi-based superalloys containing Ti and Ta after aging for 3000 h. The secondary MC carbides separate out from γ matrix during aging and different orientation relationships between MC and matrix exist for the explored CoNi-based superalloys: (100) γ // (100) MC , [ 001 ] γ // [ 001 ] MC for the alloy with Ti; (11 1 ¯ ) γ // ( 1 ¯ 1 1 ¯ ) MC , [ 011 ] γ // [ 011 ] MC for the alloy with Ta. Nanoindentation was performed to evaluate the mechanical properties of individual γ channel and γ′ precipitate. It is found that indentation modulus of γ′ phase in the explored CoNi-based superalloys locate between those of Ni 3 Al and Co 3 (Al, W) alloys, and Ta exhibits stronger strengthening effects as compared to that of Ti. • γ′ phases are stable in CoNi-base alloys after long-term aging at 900 °C. • Young's modulus of γ′ in CoNi-base alloys are between those of Ni 3 Al and Co 3 (Al,W). • Ta has a stronger strengthening effect on γ′ than Ti. [ABSTRACT FROM AUTHOR]

In order to better understand the thermal stability and mechanical properties of the nominal Ti–22Al–25Nb alloy with different initial microstructures, microstructural evolution after long-term aging, and mechanical properties at room temperature and 650 °C of four differently heat-treated Ti–22Al–25Nb alloy were investigated. The investigation results indicated that α 2 platelets formed by aging at (α 2 +B2) region were not as stable as previous research claimed and spontaneously decomposed into fine (O+β/B2) mixture during the annealing at 650 °C. As a result, the alloy's steady-state creep strain rate dropped dramatically at the secondary creep stage and tensile strength grew. On the contrary, long time aging exposure resulted in the connection of original dispersed lenticular O precipitation and thus degraded the ductility of all specimens. Besides, the discontinuous precipitation B2→β+O caused by supersaturation of solutes in the matrix also harmed the mechanical properties. Under the condition of 650 °C/250 MPa, the steady-state creep strain rates of the Ti–22Al–25Nb specimens show a significant dependence on their grain size. Crept samples showed that most dislocation motion was restrained in the β channels under a small strain and then extended into O phase when creep strain grew over 10%. Finally, by comparing the room temperature tensile property and creep resistance of four samples, supertransus solution and furnace cooling ensured the alloy most balanced mechanical properties among lamellar samples. The supertransus then furnace-cooled sample did not only have an excellent creep resistance but also kept a moderate room temperature elongation to failure of nearly 6%. • Thermal stability and mechanical properties of four different microstructures were investigated and compared. • Metastability of α 2 phase resulted in its decomposition and promoted the creep resistance of the alloy. • Grain size including equiaxed precipitation dominates creep resistance in intermediate stress regime. • Discontinuous precipitation and piling up of dislocations promoted the interfacial cracks. [ABSTRACT FROM AUTHOR]

In this paper, the effects of hafnium (Hf) addition on the thermal stability of the microstructure and tensile properties of Al-containing 16Cr-ODS steels were investigated. High temperature aging tests of the ODS steels with and without Hf were conducted at 750 °C, 950 °C and 1150 °C for 100 h respectively. And the microstructure evolution after thermal aging was investigated using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The results indicated that Hf addition could improve the coarsening resistance of the oxide particles and the grains during aging treatment due to the formation of stable Y 2 Hf 2 O 7 particles. Tensile tests on the aged-steels showed that the ultimate tensile strength and yield strength of 16Cr-Hf-ODS steel were higher than those of 16Cr-ODS steel. It was considered that the excellent thermal stability and tensile properties of 16Cr-Hf-ODS steel were related with the refined microstructure. [ABSTRACT FROM AUTHOR]

A deformation&isothermal treatment is applied to investigate the effect of hot deformation on precipitation of γ″ and δ phase in Inconel 718 alloy. The results show the precipitation behaviors are tailored by the deformation temperatures that γ″ phase precipitation is accelerated at 800 °C in place of δ phase as for 900 °C. Deformation at 800 °C and 900 °C makes the precipitation peaks for γ″ phase shift towards lower temperatures to be 742 °C and 735 °C, while the precipitation peaks for δ phase moving towards higher temperatures to be 941 °C and 943 °C, respectively. When subjected to the deformation&isothermal treatment at 800 °C, the thus formed structure is composed of ellipsoidal, disc and irregular-shaped γ″ precipitates with sizes varying from 80 nm to 440 nm in the interior of broadened GBs, as well as dispersively ultrafine intragranular γ″ particles. As for the specimen deformed at 900 °C, the δ-phase precipitation is promoted under the effect of static recrystallization occurring at the initial aging stage, that granular, disc and needle-shaped δ precipitates with 7.5 vol % are generated in the limited duration of 1 h. [ABSTRACT FROM AUTHOR]

To investigate the effects of hafnium (Hf) addition on the microstructural and mechanical performance of an aluminum (Al) added 16Cr ODS steel, the microstructures of the 16Cr ODS steel with and without Hf were investigated by EBSD and TEM, and tensile tests were conducted at high temperature to evaluate the strength and ductility changes at different temperatures. The Hf addition is beneficial to the grain refinement of the 16Cr ODS steel. The average grain size decreases from 1.795 μm to 1.027 μm with Hf addition. In addition to the Y Al O nanoparticles, Y Hf O complex oxides with a finer size are also observed in the sample with the Hf addition, which reduces the mean size of oxides within the steel and increases the number density of the oxide particles. The yield strength and tensile strength of the 16Cr ODS steel are further improved by Hf addition due to its optimized microstructures with further refined grains. [ABSTRACT FROM AUTHOR]

After solution treatment at 520 °C for 6 h and subsequent aging at 200 °C for 6 h, eutectic Mg 2 Si phase in Al–10%Mg 2 Si alloy transforms from long rod to short fiber-like and spherical morphologies, and a great number of nano-sized β″ particles precipitate in the Al matrix. The fine eutectic Mg 2 Si phase combined with nano-sized precipitates gives rise to enhanced hardness and high tensile strength of Al–10%Mg 2 Si alloy (increasing from 186 MPa to 234.6 MPa). [ABSTRACT FROM AUTHOR]

Highlights: [•] Sn expedites the transformation from Fe7Se8 to β-FeSe. [•] β-FeSe textured grains and their better connection with each other were observed. [•] The Tc of the Sn doped FeSe0.93 samples is not depressed compared to undoped one. [ABSTRACT FROM AUTHOR]

To improve the surface activity of molybdenum (Mo), a method combining anodizing and deoxidizing annealing in a H 2 atmosphere has been proposed to prepare nanocone-structured active Mo foils (NCSAMFs) in this paper. The morphology, composition and catalytic properties of the as-prepared NCSAMF were characterized by field-emission scanning electron microscopy (FESEM), energy dispersive spectrometry (EDS) and electrochemical measurements. Nanoconical structures were generated under a voltage of 20 V for 15 min in the optimized electrolyte, and all the oxygen atoms in the nanoconical structure layer were removed under deoxidation at 650 °C for 3 h in a H 2 atmosphere while retaining the nanoconical structure and activity. Compared with the Mo foils treated under different conditions, the NCSAMFs exhibit superior hydrogen evolution reaction (HER) activity with a low onset overpotential of 123 mV and a Tafel slope of 96 mV dec−1, indicating that the NCSAMFs possess high activity and outstanding long-term stability in acidic media. Therefore, the NCSAMFs prepared in this paper are promising transition metal HER electrocatalysts and serve as active matrix materials for Mo-based materials. In addition, the surface energies of the NCSAMF and the Mo foils without nanotreatment were calculated at the atomic and mesoscopic scales, respectively, to provide more insights into the origin of the studied process, and the calculation results demonstrate that the high activity of NCSAMFs is mainly derived from the increase in Mo crystal surface area with high surface energy caused by the nanotreatment and the corresponding increase in the amount of active sites. Image 1 • Molybdenum foils were anodized in NH 4 F electrolytes to prepare a nanocones on the surface. • A deoxiding annealing process is carried out to obtain nanocone-structured active Mo foils (NCSAMFs). • The electrochemical catalytic activity is tested. • The surface energies are calculated at atomic and mesoscopic scales, respectively. [ABSTRACT FROM AUTHOR]

In order to prepare high performance oxide-dispersion-strengthened tungsten based alloys, the W-Y 2 O 3 composite powder precursors were synthesized by novel freeze-drying method. The average W grain size of prepared W-Y 2 O 3 composite powder is 14.2 nm, and corresponding grain size distribution is extremely narrow. The low-temperature sintered W-Y 2 O 3 alloys possess an ultrafine W grain size of about 410 nm while maintaining a comparatively high sintering density of 98.1%. The Y 2 O 3 particles (20–50 nm) with a W-Y-O diffusion layer and Y 2 WO 6 particles (<20 nm) distribute uniformly within W grains and especially at W grain boundaries. These W-Y-O phases adsorb free oxygen impurity, resulting in the purification and strengthening of W matrix. What's more, coherent or semi-coherent interfaces are observed between Y 2 WO 6 particles and W matrix, reinforcing the bonding strength of phase boundary. The combined action of the factors mentioned above leads to the hardness of sintered W-Y 2 O 3 alloys in our work as high as 690.2 ± 32.0 HV 0.2. These results indicate that the freeze-drying method and subsequent low temperature sintering is a promising method for preparing high performance W-Y 2 O 3 alloys with ultrafine grains. • The W-Y 2 O 3 powders of 14.1 nm were prepared by novel freeze-drying method. • The low-temperature sintered W-Y 2 O 3 alloys possess W grains of 400 nm meanwhile its density is 98.1%. • Nano oxide particles of 10∼30 nm uniformly distribute in W matrix. • Combining the interfacial relationship and the oxygen distribution, the mechanism of boundary purification and enhancement is proposed. [ABSTRACT FROM AUTHOR]

The present work investigates the change in the microstructure and the improvement in the microhardness in the Mo-modified Ti 2 AlNb alloys induced by solution treatment. Typical Widmanstätten O + B2 structure was observed in the alloys directly aged in the O + B2 phase region, with a lower hardness value compared to that of the alloy without Mo addition. Well-arranged O + B2 colonies were observed in the specimen solution treated in single B2 phase region, and it exhibited a favorable hardness performance of up to 620 HV. Besides the precipitation hardening of the O phase, the solution of Mo in B2 phase should be responsible for the enhancement of hardness. Further microstructural observation proved that the substitution of Mo for Nb in the lattice of B2 phase induced the lattice distortion mainly the (200) and (211) planes, and the induced entanglement of dislocations also contributed to the enhancement of hardness. Image 1 • Well-arranged O + B2 structure is obtained in Ti 2 AlNb alloys by Mo addition. • Solution treatment and ageing enhanced hardness of Mo-modified Ti2AlNb alloys. • Lattice distortion of B2, especially along (211) face, is induced by adding Mo. • Orientation relationship of (211) B2 //(26 1 ¯ ) O is identified in the aged Ti 2 AlNb alloy. [ABSTRACT FROM AUTHOR]

Abstract The microstructure and phase evolution in Nb-Al system during MA are systemically studied. The existence of Nb-Al amorphous layer surrounding Nb nano-grains after short-time ball milling was confirmed by the HRTEM observation as well as the crystallization behavior in the annealing experiment. It was also found that after short time milling, only very little Al can enter the crystal lattice of Nb and substitute for Nb, whereas most of Al mainly concentrate in this amorphous layer surrounding Nb nano-grains. What's more, interface thermodynamic analysis and calculation were employed to investigate the phase formation and evolution of this amorphous layer in Nb-Al binary system during MA process. Accordingly, positive driving force (≡-ΔG) definitely exists for the formation of the amorphous layer {NbAl} surrounding Nb nano-grains. Thus, these amorphous layers can form and exist stably in the short-time milled Nb-Al system. With the milling time prolonging, this amorphous layer becomes unstable and can crystallize into Nb(Al) ss solid solution when its thickness increases beyond a critical thickness. The critical thickness of this amorphous layer is also predicted by the interface thermodynamics calculation and the corresponding calculated value (approximately 1.9 nm) is consistent with the observed thickness (about 2.2 nm) by the HRTEM within the error range. More importantly, the solubility of Al in the lattice of Nb can be significantly increased resulting from the crystallization of this amorphous layer {NbAl}. Our work provides a new and rational perspective to well understand the complicated microstructure and phase evolution in various nano-sized systems during MA process. Graphical abstract Image 1 Highlights • Nb-Al amorphous layer was found surrounding Nb nano-grains. • The amorphous layers crystallized into solid solution after long-time milling. • The solubility of Al in Nb increased resulting from the crystallization. • The observation well agreed with the interface thermodynamics calculation. [ABSTRACT FROM AUTHOR]

Abstract It has been proved that for an immiscible W-Cu system with positive heat of reaction, the immiscibility can be overcome and direct alloying can be realized between the constituent elements at a critical temperature range closes to the melting point of copper (T mCu) in our earlier researches. However, the thermodynamic mechanism of direct alloying between W and Cu is not yet clear. In this paper, a thermodynamic model has been established for direct alloying between W and Cu based on Miedema's theory. Combining the differential scanning calorimetry (DSC) tests for the storage energy in W and Cu, the analysis results based on the model show that the initial energies of W-Cu system including the surface energy, the storage energy and energy caused by external pressure are able to overcome the positive reaction heat and serve as the driving forces for the direct alloying. In other words, the direct alloying between immiscible W and Cu is thermodynamically feasible. Additionally, the thermodynamic analysis results also show that the final alloying microstructure of W-Cu system depends on whether the initial energy is larger than the Gibbs free energy change of W/Cu amorphous phase formation and the cooling rates during the alloying. These thermodynamic calculation results agree well with the experimental results. The amorphous phase is more likely formed during powder metallurgical sintering than during diffusion bonding of W and Cu rods. Lastly, the analysis results explain well why the key to realize direct alloying between W and Cu is to control the alloying temperature at a critical temperature range closes to T mCu. Graphical abstract Image 1 Highlights • Direct alloying between W and Cu is thermodynamically feasible. • E initial including E stor , E surf and E p are the driving forces for direct alloying. • The final alloying microstructure depends on E initial and the cooling rates. • The key to realize alloying to control the alloying temperature closes to T mCu. • The amorphous phase is more likely formed during powder metallurgical sintering. [ABSTRACT FROM AUTHOR]

Abstract The bimodal distribution of tungsten grain size, which is also called bimodal structure, is a universal phenomenon in synthesizing tungsten based nanopowders by wet chemical method, and could significantly deteriorate further sintering characteristics of the powders. In this paper, W-Y 2 O 3 composite nanopowders with small grain size and no bimodal structure were synthesized by an improved wet chemical method. It was found that reaction conditions, especially the hydrogen ion concentration(ρ(H+)) could significantly influence the microstructure and morphology of the prepared composite nanopowders and has an optimum value of approximately 0.16 mol L−1. W-Y 2 O 3 composite nanopowders synthesized under this optimum value got not only the minimum average grain size, which is about 17 nm, but also the best grain size uniformity with no bimodal structure. The microstructure of co-deposited precursors and prepared powders was analyzed in detail to figure out the specific influence mechanism of ρ(H+), and it was indicated that ρ(H+) could influence the microstructure and the Y 2 O 3 content of the reduced powders through affecting the fraction of precursor that has little yttrium content, which may lead to the bimodal distribution of tungsten grain size of the prepared powders. Furthermore, the formation mechanism of bimodal structure was systematically analyzed and a feasible method was proposed to eliminate the bimodal structure. It is suggested that the bimodal distribution of tungsten grain size is attributed to the distribution of Y 2 O 3 during hydrogen reduction process and can be well avoided by controlling ρ(H+). Highlights • We optimized the wet chemical method used to prepare W-Y 2 O 3 nanopowders. • The existence of yttrium plays a vital role in the formation of bimodal structure. • A proper formation mechanism of bimodal structures was proposed. [ABSTRACT FROM AUTHOR]

The influence of Cu addition on the microstructure and superconducting performance of ex-situ sintered MgB 2 is systemically studied. It is found that the critical current density ( J c ) of the Cu-doped sample is improved compared to the un-doped samples. In particular, the J c of the Cu-doped sample sintered at 900 °C for only 10 min is the best value at low fields. The reason is that Cu addition can obviously promote the decomposition of MgB 2 : MgB 2 ↔ MgB 4 + Mg , which produces more Mg that can then react with Cu, forming local Mg-Cu liquid at high temperature. The presence of this local Mg-Cu liquid can significantly enhance the migration and self-sintering of MgB 2 , leading to the significant improvement in the grain connectivity and J c . Our results indicate that Cu addition is a promising method to fabricate high-performance ex-situ MgB 2 bulks and wires. [ABSTRACT FROM AUTHOR]

Cu/Nb joints are promising to be used in resistance to radiation damage because of their outstanding properties. Due to the immiscibility in the Cu-Nb system, metallurgical interface is difficult to construct directly between Cu and Nb. In this paper, a direct bonding method was used to construct the metallurgical interface directly between Cu and Nb rods and prepare Cu/Nb joints. The key point of the method is to anneal Cu-Nb assemblies at the temperature between 92% and 98% of the melting point of Cu (T mCu = 1356 K) for 3 h under a pressure of 106 MPa. The maximum tensile strength and bending strength of the obtained Cu/Nb joints are 222 MPa and 47 MPa, respectively. The Micro-test results show that a diffusion layer with a thickness of about 36 nm forms between Cu and Nb, indicating that a metallurgical interface is successfully constructed between Cu and Nb. The high temperature structure of Cu at the temperature close to the T mCu provides diffusion paths probably. In addition, a thermodynamic model was established for the direct bonding of Cu and Nb. Through the thermodynamic calculations and differential scanning calorimetry (DSC) tests, the storage energies in the Cu and Nb rods are proved to serve as the thermodynamic driving force for the diffusion between Cu and Nb. [ABSTRACT FROM AUTHOR]

The Y 2 O 3 doped tungsten composite powder was synthesized via an improved chemical method where the polymeric surfactant Polyvinyl Pyrrolidone (PVP, K17) addition and ultrasonic treatment were both innovatively employed. Consequently, the average tungsten grain size of 10.7 nm was obtained for the composite powder precursor. To suppress the grain growth, two-step spark plasma sintering (SPS) was employed for the consolidation of powder precursor. It was found that in the sintered W-Y 2 O 3 alloys, nano (2–10 nm) and submicron (100–300 nm) oxide particles were uniformly dispersed within tungsten grains and at tungsten grain boundaries, respectively. The average tungsten grain size of W-Y 2 O 3 alloys whose final sintering temperature are 1400 and 1600 °C are 450 and 550 nm, respectively. Both of them are much smaller than that reported in previous studies. Their corresponding Vickers microhardness is 543.2 and 729.1 Hv, respectively, also better than the values obtained in previous works. These results indicate that the improved wet chemical method combined with ultrasonic treatment and PVP addition developed in our work is a promising way to fabricate high performance oxide-dispersion-strengthened tungsten based alloys with ultra-fine grain. [ABSTRACT FROM AUTHOR]

Self-sintered ex-situ MgB 2 polycrystalline bulks have experienced a two-step sintering process, initially at 900 °C for 0–20 min and then at 650 °C for 1 h. MgB 2 was decomposed to MgB 4 and Mg at 900 °C and composed again at 650 °C from MgB 4 and Mg. The reversible reaction promotes the material migration, and thus eliminates pores and enhances the connectivity between the grains. The critical current density ( J c ) is significantly improved due to both improved grain connectivity and the additive pinning centers such as MgB 4 and new-born boundaries. This two-step sintering process can be a promising method to fabricate high-performance ex-situ MgB 2 bulks and wires. [ABSTRACT FROM AUTHOR]

• The major highlight of this paper is how to improve the thermal stability of γ I ′ phase by investigating its directional coarsening behavior. • The second highlight is clarifying the kinetics of γ I ′ directional coarsening and its connection mode. • The third highlight is clarifying the influence of initial radius, number, volume fraction and γ channel distance on the kinetics of γ I ′ directional coarsening. • The forth highlight is clarifying the influence of lattice misfit, elemental diffusion and interfacial composition width on γ I ′ directional coarsening. Polycrystalline Ni 3 Al-based superalloy with up to 70% γ ′ phase owns excellent strength at around 1100 °C. It has been successfully used as a substitute for turbine blade crown, which is one of the aircraft components working in the severest environment (i.e., sometimes nearly overheating). However, few studies concentrate on the coarsening resistance and phase thermal stability in this newly designed polycrystalline superalloy at the working temperature, hence, directional coarsening behavior of primary γ ′ (γ I ′) has remained restricted. This paper investigated directional coarsening behavior of γ I ′ during aging treatment (1100 °C) after overheating at 1270 °C with furnace cooling (SFA), air cooling (SAA) and water cooling (SWA) respectively. It clearly showed that γ I ′ grew into lamellar and gradually became irregular during the directional coarsening process. Moreover, its morphological combination mode was illustrated. Since parallel short lamellar γ I ′ connected each other, long lamellar formed. When perpendicular long lamellar γ I ′ connected each other, it became L -type or T-type, i.e., precursor of irregular γ I ′. The kinetics of directional coarsening regime of γ I ′ was in line with the matrix-diffusion - controlled (MDC) theory according to the evaluation of γ I ′ size. This was caused by higher migration of γ I ′ interfaces than the diffusion of solute elements. It was also confirmed that driving force of directional coarsening mainly depended on the competition of elemental diffusion, lattice misfit distribution and γ channel distance. [ABSTRACT FROM AUTHOR]

The effect of primary O phase on thermal compressive deformation behavior of a Ti 2 AlNb-based alloy are investigated at temperatures from 990 °C to 1020 °C and strain rate range from 0.001 s−1 to 1 s−1. The results indicated that the primary O phase has a significant effect on the hot deformation behavior of the studied alloy. The peak stress and the degree of the stress descent of the specimen with primary O lath and Widmanstätten O + B2 structures are higher than those of the specimen with Widmanstätten O + B2 structures under higher strain rate. Besides, both the critical stress and critical strain for the onset of dynamic recrystallization (DRX) of the studied alloy upsurge with increasing the strain rate or decreasing the deformation temperature. The specimen with primary O lath and Widmanstätten O + B2 structures exhibits greater critical stress and critical strain than the specimen with Widmanstätten O + B2 structures. The strain rate sensitivity exponents of the investigated alloy with primary O lath and Widmanstätten O + B2 structures is higher than that of the alloy with Widmanstätten O + B2 structures at low temperature (990 °C), while the reverse is true at high temperature (1020 °C). Beyond that, the DRX mechanisms of the explored alloys with different morphologies of O phase were also investigated by EBSD and TEM analysis. • Thermal deformation behavior of Ti 2 AlNb-based alloy with two morphologies of O phase was studied. • The existence of primary O lath results in more significant effect of flow softening. • Critical conditions for the onset of DRX is sensitive to morphology of O phase. • The studied Ti 2 AlNb-based alloys with different morphologies of O phase undergo DDRX in all deformation conditions. [ABSTRACT FROM AUTHOR]

Oxygen evolution reaction (OER) is crucial for electrochemical water splitting. However, corrosive issues in the alkaline environment impede its applications. Here, we explore the corrosion-resistant multiphase Ni 3 Al-based intermetallic alloy (Ni 3 Al alloy) as self-supported OER catalysts for the first time. A two-step treatment, annealing at high-temperature followed by acid oxidation is performed to improve the catalytic activity. Further investigations demonstrate that the annealing process can tune the size of γ′ precipitates, which will modify the strain effects caused by γ/γ′ lattice mismatch to boost the OER activity. The oxidation of Ni during the acid pretreatment can facilitate the formation of active high-valent Ni sites during the OER process, and further enhance the OER performance. The optimized electrode exhibits a low overpotential of 280 mV at 10 mA cm−2, a small Tafel slope of 73.5 mV dec−1 and long-term stability of 180 h in 1 M KOH solution. The findings in this paper will inspire the exploration of high stability OER catalysts and the functional application of Ni 3 Al alloy. • Ni 3 Al high-temperature alloy shows high OER activity and stability. • The boosted OER activity was ascribed to modified strain effect at γ/γ′ interface. • The formation of active high-valent Ni sites also promotes the catalytic activity. • Ni 3 Al alloy treated by acid oxidation has better catalytic performance for OER. [ABSTRACT FROM AUTHOR]

In this work, hot compression deformation behavior and processing maps of ATI 718Plus superalloy were studied by isothermal compression test on Gleeble-3800 thermal simulation testing machine. The results showed that under the present conditions (with deformation temperature range of 1030–1180 °C, strain rate of 0.01–10 s−1 and deformation amount of 50%), flow stress decreased with the increasing of deformation temperature and decreasing of strain rates. Meanwhile, simplified and strain-compensated classical Arrhenius constitutive equation models were established respectively, and strain-compensated constitutive equation improves the accuracy of flow stress prediction. In addition, processing maps showed that the optimal hot processing parameter ranges were 1140–1180 °C with strain rate of 0.01–0.1 s−1 respectively. The effects of strain rate and deformation temperature on microstructure evolution was investigated and low strain rate and high deformation temperature are conductive to DRX. • The strain compensation constitutive equation for 718Plus was established and accuracy was compared with simplified equation. • The optimal parameter range of hot working is determined by the establishment of hot processing maps. • Combining the processing maps and microstructure observation the evolution behavior of DRX structure is described. [ABSTRACT FROM AUTHOR]

With the aim of preparing high performance oxide-dispersion-strengthened tungsten based alloys by low temperature sintering, W–Y 2 O 3 composite nanopowders were prepared by novel hydrothermal synthesis method. The average size of W grain in these composite nanopowders is only 15.1 nm and corresponding grain size distribution is quite narrow. Studies on synthetic mechanism of powder precursors show that the Keggin structure of raw materials is destroyed during hydrothermal synthesis process, resulting in the formation of many W–OH bonds. Then W–OH bonds react with Y3+ to form W–O–Y bonds, which is a process of co-deposition in which elements are uniformly doped. The co-deposition mechanism also shed light on the approach to exploit other high quality powder systems. Then the low temperature sintering characteristics of these powders were investigated in details. It was found that the sintered W–Y 2 O 3 alloys possess refined W grains of about 210 nm while maintaining a relatively higher density of 98.0%. What's more, nano Y 2 O 3 particles (<50 nm) distribute uniformly within W grains and at W grain boundaries. The combined effect of the above factors leads to the hardness of sintered W–Y 2 O 3 alloys in our work as high as 726 ± 25 HV 0.2. Based on our results, the hydrothermal synthesis method is a promising way to prepare composite powder precursors for the sintering of high performance oxide-dispersion-strengthened tungsten based alloys with ultrafine grains and uniform oxide particles distribution. Image 1 • The W–Y 2 O 3 powders of 15.1 nm were prepared by novel hydrothermal synthesis method. • The new W–O–Y bonds are synthesized during hydrothermal process to achieve co-deposition. • The low-temperature sintered W–Y 2 O 3 alloys possess fine W grains of 210 nm meanwhile its density is 98.0%. • Nano oxide particles (<50 nm) uniformly distribute within W grains and at W grain boundaries. • The hardness of W–Y 2 O 3 alloys is as high as 725.6 ± 25.0 HV 0.2. [ABSTRACT FROM AUTHOR]

This paper reports on the precipitate evolution and its influence on the high temperature creep resistance of 9% Cr martensite ferritic steel FSW joint. The creep rupture time of the FSW joints welded by 400 rpm and 200 rpm are 859.29 h and 1173.8 h respectively, while that of BM is 3398 h. The creep fracture occurs in ICHAZ where a pronounced increase of coarse Laves phase is detected. Most of the Laves phase particles are large in size and mainly nucleate and grow rapidly at the micrograin boundaries adjoining the M 23 C 6 particles where W, and other elements segregate. The large Laves phase particles will deteriorate the pinning effect of M 23 C 6 , weaken the solid solution strengthening effect and attribute to the increased cavity nucleation. In addition the size of Laves phase in ICHAZ is bigger than that in stir zone (SZ). These reasons explain why creep fracture occurs in the ICHAZ and the time to rupture will be shorter than base metal. • The creep rupture time of RAFM FSW joints welded by 400 rpm and 200 rpm are 859 h and 1173 h respectively. • The precipitate in different regions of crept specimens were analyzed by SEM and TEM. • Laves phase nucleates near the M 23 C 6 carbides located at grain boundary where P and Si segregate. • The creep fracture of FSW joint occurs in ICHAZ containing a lot of Laves phase and M 23 C 6 carbides. [ABSTRACT FROM AUTHOR]