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In this work, silica‐based ceramic cores with alumina as a mineralizer were prepared via an injection molding method, and the effects of alumina on the microstructural evolution and properties at 1450°C (simulating the process of equiaxed castings) and 1550°C (simulating the process of columnar/single crystal castings) were investigated. It was found that alumina promoted the cristobalite crystallization of fused silica refractory during sintering but inhibited the devitrification rate in the subsequent heating. The flexural strength of silica‐based ceramic cores at an ambient temperature and 1450°C improved with an increasing alumina content, whereas the opposite trend appeared at 1550°C. The creep resistances of silica‐based cores were improved significantly and then slightly deteriorated with an increasing alumina content from 5% to 20%, depending on the competition effects of alumina hindering the viscous flow of liquid silica (favorable), but suppressing the devitrification rate (unfavorable). The results of this work show that silica‐based cores need to follow different compositional design principles for equiaxed and columnar/single‐crystal turbine blade castings. [ABSTRACT FROM AUTHOR]

Skutterudites based on CoSb 3 are considered potential candidates in thermoelectric applications in the field of mid-temperature power generation. The annealing process necessitates more than a week to complete the peritectic phase transition for traditional melting-quenching-annealing-sintering methods. Herein, a new strategy combining the temperature gradient zone melting process with hot pressing (TGZM-HP) was successfully employed to synthesize YbFe-doped p-type CoSb 3 in a shorter time. The resulting samples show enhanced thermoelectric properties compared to TGZM-grown ones, especially in the case of 1 at.% YbFe-doped CoSb 3. This compound exhibits a maximal Seebeck coefficient of 166.3 μVK−1 and a power factor of 1.8 mWm−1K−2 at 723 K, showing an increase in these values of 25% and 51%, respectively. Moreover, phonon scattering appears to be enhanced due to the significant reduction in the grain size of the TGZM-HP samples, resulting in a minimum thermal conductivity of 2.71 Wm−1K−1. A dimensionless figure of merit (ZT) of 0.48 was obtained for 1 at.% YbFe-doped CoSb 3. This value is 1.6 times larger than that measured for TGZM-grown CoSb 3. This work provides an alternative strategy to fabricate skutterudite materials and foster their development. [ABSTRACT FROM AUTHOR]

Cu3N microcrystals were synthesized by a supercritical MeOH-NH3 technique with copper oxide and NH3 as copper and nitrogen source, respectively. The influence of reaction temperature on the fabrication of Cu3N particles was studied. The microstructures of products were characterized by XRD, SEM, FTIR, TG-DTA and XPS. Results show that product compositions depend greatly on the reaction temperatures and highly pure phase of Cu3N can be obtained at 250 °C. The as-prepared Cu3N displays a unique pyramidal morphology and the average edge length is 3–5 μm. Furthermore, the synthetic mechanism of Cu3N is also proposed. [ABSTRACT FROM AUTHOR]

Sn1−xMnxTe (x = 0, 0.09, 0.15, 0.20) bulk materials were prepared by melt spinning combined with spark plasma sintering process. Nanoscale grains were obtained, and the solid solubility of Mn was much enhanced by the ultrafast-cooling synthesis technique. The maximum of Seebeck coefficient and power factor are 242 µVK−1 and 19.97 µW cm−1K−2 at 873 K with the doping concentration of 15 at% Mn. A large amount of grain boundaries and doped atoms improve the scattering of heat-carrying phonons in a wide range of frequencies, and the scattering mechanisms are also explained by theoretical calculation. As a result, the minimum of lattice thermal conductivity is 0.66 µVK−1 at 873 K, the corresponding figure of merit is 1.26 for Sn0.85Mn0.15Te sample. This value is improved by 35% comparing with previously reported result. Our work indicates that melt spinning process is effective to develop SnTe related thermoelectric materials with excellent thermoelectric properties, which has the widespread commercial value and the prospects for development. [ABSTRACT FROM AUTHOR]

Twinned dendrites and regular dendrites have been respectively produced in A356 alloy by Bridgman solidification. The primary trunks of twinned dendrites grew along <110> direction and were split by a coherent (111) twin plane were characterized by electron backscattered diffraction (EBSD), which is different from the <100> regular dendrites. The growth of twinned dendrites was found to have a rotation relationship around a common <110> direction. With respect to the specimens with regular dendrites, the mean tensile strength, elongation and toughness of the specimens with twinned dendrites were increased from 230 MPa to 239 MPa, 16.69–18.99%, 34.45 J/m 3 to 40.88 J/m 3 , respectively. Meanwhile, the coefficient of thermal expansion (CTE) of the specimen with twinned dendrites increase monotonically up to 2.28 × 10 − 5 K − 1 at temperature of 473 K. This CTE was successfully lowered than that of specimen with regular dendrites at the temperature above 323 K, and reaches the maximum decrease around 4.2% at 473 K. The fractures of two types of specimens were both originated from the Si-rich regions where dimples formed. In addition, detailed analyses on coherent twin boundary (CTB) of twinned dendrites by transmission electron microscope (TEM) and high-resolution TEM (HRTEM) indicate that the mechanisms of twin and dislocation-CTB interactions can enhance the strength and ductility of the A356 alloy with twinned dendrites. [ABSTRACT FROM AUTHOR]

Un-doped single crystals of Mg2Si1−xSnx (x = 0.55, 0.65 and 0.75) were successfully prepared by high temperature gradient directional solidification (HGDS). In the Mg2Si0.45Sn0.55 crystal, Mg2Si precipitates were observed in the solidified microstructure, and no precipitates in the single crystals of Mg2Si0.35Sn0.65 and Mg2Si0.25Sn0.75. By measuring the electronic transport properties of these three single crystals, the Mg2Si0.35Sn0.65 has a largest PF value, about 2.5 times more than that of the nanocrystalline prepared by solid-state reaction methods. The corresponding ZT values of Mg2Si0.35Sn0.65 single crystal are greatly improved. It indicates that, the Mg2Si1−xSnx crystals prepared by HGDS can not only have a uniform microstructure, but also optimize the TE performance of the crystal. In addition, the first-principles calculation has been conducted to examine the intrinsic properties of Mg2Si1−xSnx single crystals, and the calculated data agree well with the experimental results. [ABSTRACT FROM AUTHOR]

Based on the twin planes formed at 3000 μm/s, the growth stability of twinned dendrites in Al-4.5 wt% Cu alloy was studied in detail at a low growth rate during Bridgman solidification. Experimental results show that the twinned dendrite growth could still keep stable even after decreasing the rate to 20 μm/s. This is far below the usual rate threshold for the formation of twinned dendrites (∼1000 μm/s). The dominated twinned structures at 20 μm/s have their (1 1 1) twin plane and 〈1 1 0〉 dendritic trunk closest to thermal gradient direction. Importantly, distinct stacking fault (SF) structures directly connecting two adjacent twin boundaries provide a direct experimental evidence for the close correlation between the SFs and the twinned dendrite growth in aluminum alloys. [ABSTRACT FROM AUTHOR]

Solid-state cooling based on elastocaloric effect possesses potential for refrigeration. The improvement of elastocaloric effect and thermal cycle stability is beneficial to enhance the refrigeration efficiency and service life of refrigeration equipment. Here, single-crystal Ni 50 Mn 31.6 Ti 18.4 shape-memory alloys were successfully prepared using directional solidification technique. The microstructure, martensitic transformation, isothermal stress-strain responses and elastocaloric effect of the single crystals with different solidification rates have been systematically studied. The single crystal solidified at 10 µm s−1 exhibits lower stress hysteresis Δ σ hy and much more superior adiabatic temperature |Δ T ad | than those of alloys solidified at 100 µm s−1 and 500 µm s−1, with the values of 34 MPa and 24.8 K. In addition, the single crystal at low solidification rate also performs excellent cyclic stability. All these results show that the single crystal with larger austenite primary dendrite arm spacing (PDAS) can effectively enhance the elastocaloric effect and thermal cycling stability in Ni 50 Mn 31.6 Ti 18.4 alloys. This work sheds a bright light on the relationship between the elastocaloric performance and microstructure for shape-memory alloys. • Single-crystal Ni 50 Mn 31.6 Ti 18.4 alloys were successfully prepared by directional solidification. • The alloy with large primary dendrite spacing has the excellent adiabatic temperature of 24.8 K. • Single-crystal prepared with larger primary dendrite spacing can effectively enhance the thermal cycling stability. [ABSTRACT FROM AUTHOR]

Phase selection under far-from-equilibrium condition has been investigated in three Cr 2 Nb-20/30/40Ti (at. %) alloys processed by laser surface melting (LSM) with scanning speeds of 50–200 mm/s. Phase identification was performed by micro-area XRD technique combined with SEM plus EDS methods. The results show that, at all scanning speeds of 50–200 mm/s, the microstructure of Cr 2 Nb-20Ti alloy consisted of single-phase Cr 2 Nb, whereas it evolved to monolithic β-Ti in the Cr 2 Nb-40Ti alloy. Meanwhile, the microstructure of Cr 2 Nb-30Ti alloy solidified with eutectic β-Ti/Cr 2 Nb at 50 mm/s, which transited to monolithic β-Ti as scanning speeds increased to 100–200 mm/s. Microstructures evolution was interpreted based on Trivedi-Magnin-Kurz (TMK) and Kurz-Giovanola-Trivedi (KGT) models incorporating the effect of non-equilibrium solidification. After the process of LSM, the micro-hardness of Cr 2 Nb-20Ti alloy was improved to a level of 9.0 GPa, and the fracture toughness was reduced to about 3.1 MPa m 1/2 ; while for the Cr 2 Nb-30/40Ti alloys, the micro-hardness decreased within a range of 5.5–6.6 GPa, but with a great increase of facture toughness of 21–23 MPa m 1/2 . [ABSTRACT FROM AUTHOR]

• Finding plate-like and hyperbranched seaweed in 3D conditions of metallic alloy. • Constructing a non-regular dendritic pattern selection map. • Well understanding effect of interface energy anisotropy on the dendritic pattern. Dendritic morphology and pattern selection are fundamentally important issues in solidification cycles. Here, a well-designed experiment was employed to investigate the solid–liquid interface energy anisotropy on the directionally solidified microstructures of Al-3 wt% Mg single crystals. Various dendritic morphologies, including regular dendrite, plate-like seaweed and hyperbranched seaweed, were found along the <100>, <110> and <111> orientations, respectively. The primary dendritic arm spacing (PDAS) of the seaweed pattern is closely related to the interface energy anisotropy and deviates from the current PDAS model. A non-regular dendritic pattern selection map is constructed to interpret the different dendrites occurring in alloy solidification. [ABSTRACT FROM AUTHOR]

By combining the techniques of directional solidification and coating Y2O3, a Cr–20Nb–40Ti alloy was manufactured successfully with various growth rates. The revolution of microstructures and corresponding mechanical properties was discussed to develop the Cr2Nb based alloys with good combination of mechanical properties. The results show that the favorable growth dynamics of plane (220) of Laves phase Cr2Nb was observed with the increase of growth rate. Phase selection took place in microstructures evolved from the primary Cr2Nb, via the dendrite-like eutectic Cr2Nb/β-Ti, and finally to the primary β-Ti, with increasing the growth rate from 5 to 200 μm/s. Based on the coupled zone of eutectic, the competitive growth of solidified phases in the directionally solidified Cr–20Nb–40Ti alloy was elucidated. In addition, the mechanical properties of alloy depended on the growth rate, and the fracture toughness of the alloy reached 16.50 MPa m1/2 at 200 μm/s, much larger than 1.40 MPa m1/2 for single-phase Cr2Nb. [ABSTRACT FROM PUBLISHER]

Abstract: Al–40% Cu hypereutectic alloy samples were successfully directionally solidified at a growth rate of 10μm/s in different sizes (4mm, 1.8mm, and 0.45mm thickness in transverse section). Using the serial sectioning technique, the three-dimensional (3D) microstructures of the primary intermetallic Al2Cu phase of the alloy can be observed with various growth patterns, L-shape, E-shape, and regular rectangular shape with respect to growth orientations of the (110) and (310) plane. The L-shape and regular rectangular shape of Al2Cu phase are bounded by {110} facets. When the sample size was reduced from 4mm to 0.45mm, the solidified microstructures changed from multi-layer dendrites to single-layer dendrite along the growth direction, and then the orientation texture was at the plane (310). The growth mechanism for the regular faceted intermetallic Al2Cu at different sample sizes was interpreted by the oriented attachment mechanism (OA). The experimental results showed that the directionally solidified Al–40% Cu alloy sample in a much smaller size can achieve a well-aligned morphology with a specific growth texture. [Copyright &y& Elsevier]

The microstructures and orientations of Al-40%Cu hypereutectic alloy prepared in a 0.45 mm high-purity alumina crucible at different directional solidification rates (5, 10 and 100 μm/s) were investigated. With solidification rate increasing, the primary phase Al2Cu of the alloy transits from complex dendrite morphologies to regular faceted square shapes. The surfaces of regular square morphology about Al2Cu phase are {110} facets. The orientation text results by XRD showed that at a growth rate of 5 μm/s, the phase displayed texture orientations at (310), (110), and (220). At 100 μm/s, the phase possessed a very strong preferred orientation at (310). The experimental results showed that growth in small-size samples at a higher growth rate can achieve a regular microstructure with preferred orientation. The growth morphologies are deduced from attachment energies descripted the microstructure evolution of the primary phase. [ABSTRACT FROM AUTHOR]

• A two-step method was utilized to synthesize multi-element doped Ce 0.3 Ni 1.5 Co 2.5 Sb 12 film efficiently. • Well-crystallized CoSb 3 and CeSb coexisted, leading to a positive effect on electrical properties. • The maximum value of S and PF reach −250 μV/K and 5275 μW/(mK2) for Ce 0.3 Ni 1.5 Co 2.5 Sb 12 film at 673 K. The skutterudite CoSb 3 has been widely concerned, as one of the most studied candidate materials for thermoelectric applications. Here, n-type bulk skutterudite Ce 0.3 Ni 1.5 Co 2.5 Sb 12 was successfully prepared under TGZM effect and its thin film deposited by PLD on single crystal Si substrate, to further understand the effect of microstructure and the electrical transport mechanism on the thermoelectric properties from bulk to thin film. It is found that two phases of Sb and CoSb 3 coexist in bulk sample, while abundant nano-granular structures with well-crystallized CoSb 3 and little of CeSb generate in the film. As a result, the maximum value of S and PF reach −250 μV/K and 5275 μW/(mK2) for film sample at 673 K. The PF value of film is four times higher than that of the bulk sample. This provides a novel avenue to increase the thermoelectric properties of skutterudite. [ABSTRACT FROM AUTHOR]

Abstract: Microstructure and mechanical properties of a binary Cr2Nb-based alloy containing the Laves phase Cr2Nb and Cr phase have been studied in the arc-melted condition. Microstructure consisting of fully coupled eutectic was firstly observed in this hypereutectic alloy. The Cr2Nb primary phase experienced a faceted to nonfaceted transition as the solidification distance increased. Based on the maximum interface growth temperature criteria, the microstructure evolutions in this alloy were explained successfully by means of the competitive growth between the β-Cr2Nb phase and eutectic. Meanwhile, the yield strength and fracture toughness of the samples taken from the bottom of the ingot attained 1986MPa and 4.3MPa⋅m1/2 respectively, which were considerably enhanced over the monolithic Cr2Nb. The enhancements were explained in terms of the second Cr phase toughening and grain boundary toughening mechanisms. Furthermore, the variation of mechanical properties in different position of arc-melted ingot was proven to result in different eutectic fracture mechanisms. [Copyright &y& Elsevier]

Abstract: Bridgman type directional solidification experiments were carried out in Nd14Fe79B7 alloy. Microstructure evolutions along with growth velocities were investigated. At low velocities from 1 to 200μm/s, the microstructures consisted of leading α-Fe dendrites and peritectic Nd2Fe14B phase in the interdendritic region. At medium velocities of 425 and 500μm/s, an incomplete banded structure formed, containing circles of α-Fe/Nd2Fe14B–Nd2Fe14B dendrites. At high velocities of 1, 3 and 5mm/s, a transition from γ-Fe dendrites to Nd2Fe14B dendrites occurred. Phase selection phenomena were theoretically studied using the extremum criterion which assumes growth-controlled phase selection. The theoretical results are in agreement with experimental results at low velocities of 1–200μm/s and high velocities of 3 and 5mm/s. The incomplete dendritic bands at 425 and 500μm/s were explained by nucleation of Nd2Fe14B phase at the γ-Fe dendritic interface, and growth competition between γ-Fe and the nucleated Nd2Fe14B phase. Phase transition from γ-Fe to Nd2Fe14B at 1mm/s was interpreted by abundant nucleation and quick growth of Nd2Fe14B phase at the γ-Fe dendritic interface. [Copyright &y& Elsevier]

Abstract: Microstructure evolution of Ni–24.19wt%Nb hypereutectic alloys was investigated to develop a method to increase the productivity of in situ composite in directional solidification. The results show that at the constant growth velocity of 10μm/s, coarse priamry β-Ni3Nb plate dendrites were firstly precipitated from the liquid. However, at the growth velocity changing abruptly from 1μm/s to 10μm/s, only coupled growth eutectics were observed in the solidification microstructure. The coupled eutectic growth stable at 10μm/s is possibly ascribed to the interface growth temperature of coupled eutecitc exceeding that of β-Ni3Nb single phase. Another probable factor to permit the coupled eutectic growth stable at 10μm/s is the difficult nucleation of β-Ni3Nb phase in front of the eutectic growth interface. By changing solidification process, the productivity for the formation of in situ composite of Ni–24.19wt%Nb hypereutectic alloy is improved by a factor of 5. [Copyright &y& Elsevier]

SnTe based materials have attracted considerable attention owing to their eco-friendly nature. Here, we employed a melt spinning method combined with spark plasma sintering (MS-SPS) technique to optimize the electrical and thermal transport performance of SnTe based alloys. Doping with Sb optimizes carrier concentration efficiently, leading to an enhanced Seebeck coefficient and power factor. Unlike the original phonon dispersion relation, the introduction of Sb heightens the phonon-phonon interaction and flattens the phonon dispersion, resulting in a significant decline in the phonon group velocity. Combined with the enhanced scattering from lattice distortion, nanoprecipitates and large amounts of grain boundaries, an ultralow lattice thermal conductivity of 0.57 Wm−1K−1 was obtained for Sn 0.91 Sb 0.18 Te 1.18 MS-SPS alloy. An improved ZT value of 0.95 can be realized, which is 2.2 times higher than that of pure SnTe alloy. The enhanced phonon scattering and reinforced phonon-phonon interaction give valuable access to develop high-performance SnTe based thermoelectric materials. • Melt spinning followed by spark plasma sintering technique was employed to prepare nanostructured materials. • Introduction of element Sb changes the phonon dispersion relations of SnTe based materials. • An ultralow lattice thermal conductivity of 0.57 Wm−1K−1 was obtained for Sn 0.91 Sb 0.18 Te 1.18 alloy. • Nanostructuring is effective to enhance the thermoelectric performance. [ABSTRACT FROM AUTHOR]

A quinary Cr–12Nb–5Ti–5Mo–5Si (at.%) alloy was prepared by arc melting in this study to improve the strength and toughness of Laves phase Cr 2 Nb alloy. The quinary alloy was solidified with full eutectic Cr 2 Nb/Cr, in which the Cr 2 Nb phase exceeded 55 vol % and existed as matrix. The eutectic was developed with an orientation relationship of {001}Cr 2 Nb//{001}Cr, leading to a low lattice mismatch of 15.57% at the eutectic interface. The yield strength and fracture toughness of the quinary alloy reached 1950–1980 MPa and 14.0–14.8 MPa⋅m1/2, respectively, much higher than most reported Cr 2 Nb alloys. The reason for the superior properties is the synergistic effects of solid solution strengthening, alloying toughening, and eutectic strengthening-toughening. • The alloy Cr–12Nb–5Ti–5Mo–5Si was solidified with full eutectic Cr 2 Nb/Cr. • The orientation relationship of the full eutectic was {001}Cr 2 Nb//{001}Cr. • The improved strength and toughness was achieved in the quinary alloy. • Relevant strengthening and toughening mechanisms were discussed. [ABSTRACT FROM AUTHOR]

Three Cr–8Ta-xSi (x = 0, 6, 10) (at.%) alloys were fabricated by vacuum non-consumable arc melting in this study to investigate the microstructure evolutions and mechanical properties of the Si-modified Cr 2 Ta/Cr in-situ composites. The element Si was incorporated preferentially into Cr 2 Ta rather than phase Cr, thus the microstructure transited from the dendrite Cr plus the eutectic Cr 2 Ta/Cr in the Cr–8Ta alloy, to the full eutectic Cr 2 Ta/Cr in the Cr–8Ta–6Si alloy, and finally to the dendrite Cr 2 Ta plus the eutectic Cr 2 Ta/Cr in the Cr–8Ta–10Si alloy. The Cr–8Ta–6Si alloy was developed with a good combination of strength and toughness, and the yield strength and fracture toughness of this alloy reached up to 960 ± 20 MPa and 15 ± 0.3 MPa m1/2, respectively. The good combination of strength and toughness in the Cr–8Ta–6Si alloy resulted from the synergistic effects of solid solution strengthening, alloying toughening, interface strengthening and second phase toughening. • The Si-modified Cr 2 Ta/Cr in-situ composites were fabricated in this study. • A good combination of yield strength and fracture toughness was achieved. • Relevant strengthening and toughening effects were discussed in details. [ABSTRACT FROM AUTHOR]

At different directional solidification rates (10, 20 and 100 μm/s), microstructures and growth orientation variations of Al 2 Cu dendrite in Al-40%Cu alloy were characterized. When solidification rates were ranged from 10 to 100 μm/s, three-dimensional microstructure of Al 2 Cu dendrite changed from faceted L-shaped patterns to non-faceted complex dendrite morphology in transverse section. By the macro and micro orientation analysis characterize methods, [001] growth direction of Al 2 Cu dendrites with different morphologies was determined. The deviation angle between [001] direction and the heat flow direction was increased with solidified rate increasing. The experimental results showed that the regular solidified microstructure and growth orientation along the heat flow direction could be well controlled under lower directional solidification rate. [ABSTRACT FROM AUTHOR]

In this study, the laser repairing (LR) method is applied to 12Cr12Mo stainless steel, and the corresponding microstructural evolution, texture characteristic, and hardness variation are investigated. A 50/50 uniaxial tensile-test is conducted, and the mechanical failure mechanism is investigated using scanning electron microscope and transmission electron microscopy. The results show that the microstructure consists mainly of martensite and tiny M 23 C 6 carbide particles in the deposited zone (DZ), while the substrate contains tempered martensite and coarse M 23 C 6 carbide. The high laser-power generates a cubic texture in the DZ, while the substrate maintains a texture-free matrix. From the substrate to the DZ, the hardness increases sharply at the fusion line and then decreases gradually along the building direction. The tensile-test result indicates that the overall mechanical properties of LRed 12Cr12Mo stainless steel are comparable to wrought parts. Moreover, the microvoid-coalescence fracture indicates good plasticity of the LRed samples. Using this approach, we successfully repaired a 17th-stage stator cascade of a gas turbine, which indicates that LR can be used for the type of steel investigated in this study. • A defect-free metallurgical bonding could be obtained under both argon and air atmosphere during laser repairing. • The mechanical properties of laser repaired samples were comparable with those of the wrought substrate. • The 17th stage stator cascade of a gas turbine made from 12Cr12Mo steel was successfully repaired by laser repairing. [ABSTRACT FROM AUTHOR]