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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]

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]

Abstract: The influence of temperature and pressure on the mechanical and thermodynamic properties of single crystal Bismuth Telluride material was investigated by the first-principle calculation and quasi-harmonic Debye model. The computation results indicated that the lattice constant in the c axis increases nonlinearly and the elastic constants Cij and bulk module B 0 increase linearly with increasing the pressure. The Young modulus of the Bi2Te3 crystal in the x and y axes are 28.6% larger than that in the z axis. The Poisson’s ratios along the three axes range from 0.2752 to 0.3853. When the temperature is less than 150K, the lattice specific heat capacity of Bi2Te3 depends on the temperature. The thermal expansion coefficients of Bi2Te3 vary remarkably with the temperature and pressure. The chemical balance bonding of Bi2Te3 is predicated to be based on the density functional theory, revealing the evident natural bonding interaction between Bi–Te and Te–Te. Those calculations are in good agreement with previous experimental data. [Copyright &y& Elsevier]

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]

Bulk NbCr 2 samples with ultra-fine grains have been fabricated by mechanical milling and subsequent spark plasma sintering. The room-temperature fracture toughness of these bulk samples achieves as high as 11.3 ± 0.5 MPa m1/2, which is remarkably improved compared to the as-cast monolithic NbCr 2 Laves phase (1.2 MPa m1/2). The microstructural analysis indicates the presence of abundant non-equilibrium Nb and Cr solid solutions among the grain boundaries. The formation of Nb and Cr solid solutions is attributed to the applied pulsed electric current during spark plasma sintering, which can decrease the critical nucleation undercooling, accelerate the atomic diffusion and enhance the nucleation rate. Based on the microstructural characterization, the enhancement in fracture toughness is ascribed to formation of non-equilibrium Nb and Cr solid solutions toughening and grain boundary toughening mechanisms. Furthermore, strong texture components have been detected in these bulk samples. For the sample sintered with the powder milled for 30 h, the <100> direction of NbCr 2 Laves phase is perpendicular to the compressive axis while the Nb and Cr solid solutions show random orientation. When sintered with the powder milled for 60 h and 90 h, the texture component in NbCr 2 Laves phase transforms into <110> fiber texture, and the Cr solid solution also show strong <100> and <110> texture components perpendicular to the compressive axis. The evolution of texture component is discussed by considering the crystallization behaviour of the milled powders. • Ultra-fine NbCr 2 bulks have been produced by mechanical alloying and spark plasma sintering. • The fracture toughness of the bulks attains 11.3 ± 0.5 MPa m1/2, which is 9.3 times of NbCr 2. • Non-equilibrium Nb and Cr solid solutions are detected among the NbCr 2 grain boundaries. • The NbCr 2 Laves phase and non-equilibrium Cr solid solutions show strong texture. • The texture in NbCr 2 Laves phase transformed from <100> to <110> with the increase of milling time. [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]

Solid-state refrigeration is considered a promising alternative to traditional vapor compression refrigeration technology. Researchers demonstrated that Ni–Mn–Ti alloys have a remarkable elastocaloric effect. However, the Ni–Mn–Ti alloy can only operate under a uniaxial stress field due to its antiferromagnetic austenite, which narrows its range of refrigeration temperatures. Here, we activated the magnetism of the alloy by replacing some Ni atoms with Co atoms. The Ni–Co–Mn–Ti–B alloy demonstrates a reversible maximum magnetic entropy change(Δ S m) of 24.9 J kg−1 K−1 and achieves an adiabatic temperature change of 7.8 K under a 7 T magnetic field. Additionally, we demonstrate that adding boron (0.2%) can improve the mechanical properties and cyclic stability in Ni–Mn–Ti alloys. The elastocaloric effect of 21.3 K with high cycle stability (1013 cycles) were successfully achieved in the directionally solidified (Ni 35 Co 15 Mn 35 Ti 15) 99.8 B 0.2 alloy. Furthermore, we have demonstrated that the magnetoelastic coupling effect can effectively extend the refrigeration temperature range. The alloys achieved large caloric effects in the temperature range of 240 K–340 K. This provides a new strategy for designing high-performance materials for wide-temperature-domain refrigeration. • The addition of Co activates the magnetic structure coupling of Ni–Mn–Ti alloy. The alloy exhibits a reversible maximum magnetic entropy change Δ S m of 24.9 Jkg −1 K −1 and achieves an adiabatic temperature change of 7.8 K under a 7 T magnetic field. • The addition of B can effectively improve the strength and ductility of the alloy. The material shows a highly stable (1013 cycles) elastocaloric effect of 21.3K. • The material can achieve a continuous refrigerable operating temperature range from 240 K to 340 K by combining elastocaloric, magnetocaloric and magnetoelastic coupling effects. • The stress-assisted method can not only improve the reversibility of caloric effect , but also broaden the temperature range of reversible caloric effect and improve the refrigeration performance of the active refrigerator. [ABSTRACT FROM AUTHOR]

In this study, the effects of solution treatment (ST) on the microstructure and anisotropic mechanical properties of Hastelloy X fabricated by laser directed energy deposition (L-DED) were investigated. The microstructure of the as-deposited (AD) sample presented coarse granular grains aligned with the deposition direction. Due to the segregation of Mo and Cr elements, a special cellular-structure was formed in the inter-dendritic region, along with the strip-like carbides consisting of both M 23 C 6 and M 6 C carbides. The cellular-structure homogenized rapidly after the ST. In detail, as the ST temperature increased, recrystallization generally occurred. Meanwhile, the strip carbides began to gradually dissolve and dissociate into dispersed particles which led to a drop in microhardness from 220 HV to 161 HV. The room temperature tensile results showed that the yield strength in the transversal direction (TD) and longitudinal direction (LD) decreased with the increase of ST temperature, reaching a minimum value of 285 MPa after the ST at 1175 °C. In addition, the ultimate tensile strength of the LD and TD samples reached maximum values of 693 MPa and 703 MPa at 1150 °C, respectively. It should be noted that the elongation of the low ductile TD sample can be improved significantly from 30% to 75% after the ST at 1175 °C. Therefore, it can be concluded that the anisotropy within the TD and LD samples can be effectively eliminated by controlling the cellular-structure, grain-structure, and carbide morphology during ST. • Cellular-structure is the major reason for anisotropic yield strength of as-deposited Hastelloy X. • Recrystallization during solution treatment could improve the anisotropic mechanical properties. • The dissolution of strip-like carbides benefits for enhancing the elongation of as-deposited Hastelloy X. [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]