•Elastic properties of titanium borides are calculated by first principles calculation.•Thermodynamical stability of titanium borides is analyzed.•Heat capacity and thermal expansion coefficient for ...titanium borides are calculated and compared.•Grüneisen parameters of titanium borides are calculated.
The anisotropic elastic and thermal expansions of the titanium borides (TiB2, Ti3B4, TiB_Pnma and TiB_Fm3¯m) are calculated from first-principles using density functional theory. All borides show different anisotropic elastic properties; the bulk, shear and Young’s moduli are consistent with those determined experimentally. The temperature dependence of thermal expansions is mainly caused by the restoration of thermal energy due to phonon excitations at low temperature. When the temperature is higher than 500K, the volumetric coefficient is increased linearly by increasing temperature. Meanwhile, the heat capacities of titanium borides are obtained based on the knowledge of thermal expansion coefficient and the elasticity, the calculations are in good agreement with the experiments.
NiTi//Al2O3 composites have many advantages such as large elasticity, superior hardness and wear resistance. But there is little information available about the novel adhesion strength, fracture ...mechanism and interfacial bonding of NiTi//Al2O3 composites at the atomic scale. Therefore, the work of adhesion (Wad), interfacial energy (γ) and electronic structure of NiTi(111)//α-Al2O3(0001) interfaces have been calculated using first-principles calculations. For the models with the same stacking site, O-terminated interfaces have larger Wad and smaller γ than Al-terminated interfaces. For the models with same termination, the stability of Ni(Ti)-Al interfaces decreases with the order of HCP > MT > OT, while the stability of Ni(Ti)-O interfaces decreases as MT > HCP > OT. The Ti-O-MT interface belongs to the most stable interfacial configuration with the smallest interface energy among all studied interfaces. Using Griffith's theory, it is predicted that the mechanical failure of NiTi//α-Al2O3 interfaces are inclined to initiate in the interior of NiTi bulk or at the interface rather than Al2O3 side in most cases. Furthermore, density of states and electron density difference analysis indicate that the dominant interfacial adhesion mechanism for the NiAl interfaces is the formation of mainly metallic TiAl and NiAl bonds, while the TiO interface exhibits mixed covalent/ionic character with higher interfacial binding strength.
The interface energies of Ni-Al2-HCP2 and Ti-Al2-HCP2 configurations at 298 K are increased with the increasing of the partial pressure of oxygen, indicating their thermodynamic stabilities are decreased in the oxygen rich condition. Meanwhile, the Ni-O-MT and Ti-O-MT interfaces are stabilized by increasing the partial pressure of oxygen. Display omitted
•NiTi//α-Al2O3 interfaces were studied to reveal their adhesion strength, fracture mechanism and interfacial bonding.•Ti-O-MT interface is the most stable interfacial configuration with the smallest interface energy of 2.07 J/m2.•The mechanical failure are inclined to initiate in the interior of NiTi bulk rather than the interface.
In this study, hot deformation behavior of in-situ nanosized TiB2/AZ91 composite is investigated by analyzing the constitutive equation, hot processing maps and microstructure evolutions. Hot ...compression tests are conducted in different temperatures and strain rates range of 523–673 K and 0.001–1 s−1 with a constant strain of 0.69. The results show that deformation temperatures and strain rates have a strong influence on the flow behavior of the composite, exhibiting typical work hardening, softening and steady stages. The constitutive equation is established through determining material constants, which can predict the flow stress precisely. In the meanwhile, the stress exponent (n) is calculated as 5.4, suggesting the hot deformation mechanism of TiB2/AZ91 composite is dominated by the dislocation climb. And the calculated apparent activation energy (Q) is 168.8 kJ/mol, which is higher than that of unreinforced AZ91 alloy due to the addition of nanosized TiB2 particles. Furthermore, the hot processing maps of TiB2/AZ91 composite are developed based on dynamic materials model, presenting three domains: one instability region in the range of 523–623 K & 0.01–1 s−1, and two safe regions in the range of 548–600 K & 0.001–0.005 s−1 and 648–673 K & 0.1–1.0 s−1 with the peak efficiency value of 0.36, respectively. By observing microstructures, full dynamic recrystallization (DRX) occurs in the safe regions, while the mechanism of instability region is dominated by mechanical twining and high density dislocation.
•The deformation mechanism of in-situ nanosized TiB2/AZ91 is clarified.•Constitutive equation of the composite is established.•Hot processing maps of TiB2/AZ91 composite at different strains are developed.•Two optimum processing windows of the composite are identified.
Electro‐reforming of renewable biomass resources is an alternative technology for sustainable pure H2 production. Herein, we discovered an unconventional cation effect on the concurrent formate and ...H2 production via glycerol electro‐reforming. In stark contrast to the cation effect via forming double layers in cathodic reactions, residual cations at the anode were discovered to interact with the glycerol oxidation intermediates to steer its product selectivity. Through a combination of product analysis, transient kinetics, crown ether trapping experiments, in situ IRRAS and DFT calculations, the aldehyde intermediates were discovered to be stabilized by the Li+ cations to favor the non‐oxidative C−C cleavage for formate production. The maximal formate efficiency could reach 81.3 % under ≈60 mA cm−2 in LiOH. This work emphasizes the significance of engineering the microenvironment at the electrode–electrolyte interface for efficient electrolytic processes.
Glycerol oxidation selectivity can be efficiently steered via cation–intermediate interactions, resulting in highly selective glycerol electro‐reforming into hydrogen and formate. This work emphasizes the significance of engineering the microenvironment at the electrode–electrolyte interface for efficient electrochemical processes.
The microstructure and crystallography of M7C3 carbide in 20 wt.% Cr hypereutectic cast iron have been investigated. The results show that primary M7C3 carbide subjected to fast cooling displays an ...irregular hollow hexagonal structure. Rapid solidification has remarkable effects on the morphology and orientation of primary M7C3 carbide. Transmission electron microscopy (TEM) results indicate that M7C3 carbide has hexagonal structure and crystallizes on {011−0} planes with some stacking faults generating along 0001 orientation. The twins in M7C3 are formed on (011−0) twin plane accompanying self-perpetuating sub-steps which can impellingly promote M7C3 carbide growth. The chemical formulas of primary and eutectic M7C3 carbides are determined to be Cr3.78Fe3.22C2.99 and Cr3.75Fe3.25C2.99, respectively. High-resolution transmission electron microscopy results reveal that primary M7C3 carbide emerges outcrop and promptly grows after nucleating in liquid phase. The existences of sub-steps and stacking faults show that crystalline defects can act as an important role in M7C3 carbide growth. Moreover, an outflanking growth model of primary M7C3 carbide is discussed based on the crystalline defects and growth condition, which results in the formation of hexagonal carbides.
•Directionally solidified M7C3 displays strong textured structures.•The chemical formulas of Cr-rich M7C3 are determined.•Crystal planes of primary M7C3 are {011−0} and its growth is discussed.•Crystalline defects impel the growth of primary M7C3 carbide.
The ground state properties of W–C binary compounds (h-WC, c-WC, α-W
2C, β-W
2C, γ-W
2C, ɛ-W
2C) are studied in this paper by using first-principles calculations. Formation enthalpy and cohesive ...energy for each phase are calculated. The calculated elastic constants satisfy the Born–Huang's stability criterion, indicating all studied compounds are mechanically stable. All W–C compounds studied in this paper exhibit larger bulk modulus values than many other binary types of carbide such as Fe
3C, Cr
7C
3, Cr
3C, and TiC. Using a theoretical method based on the works of Šimůnek, the hardness of the crystal is estimated. The electronic structures of these compounds are calculated and discussed. Stoner's polarization theory for itinerant magnetism is applied to explain the observed paramagnetic behavior of the compounds. Moreover, the heat capacity is also calculated for each compound based on the knowledge of the elasticity and Debye temperature.
► This paper investigates the six Cr-C binary compounds (h-CrC, c-CrC, Cr3C, Cr3C2, Cr7C3, and Cr23C6) by using DFT; all of the carbides show a mixed character of metallic, covalent and ionic bonds; ...the thermodynamic stability decreases in the sequence of Cr3C2> Cr7C3> Cr3C≈ Cr23C6> h-CrC> c-CrC, and the positive value of formation enthalpy for c-CrC can explain its metastable nature. ► The stability of h-CrC can be explained based on the electronic structures of Cr sub-lattices in cubic and hexagonal phases. ► By using Šimůnek's theory, the calculated theoretical values of hardness are consistent with the current experimental results for Cr3C2, Cr7C3 and Cr23C6. The hardnesses of the two CrC phases are notably higher than other phases; they could be used as the wear resistance coatings. ► The calculated elastic constants indicate that h-CrC has higher elastic moduli; but this phase is more brittle than other carbides because of smallest B/G ratio. The values of GV/GR and BV/BR for all these Cr-C carbides are distributed around one (GV/GR≈1, BV/BR≈1), indicating that they are locally isotropic crystals not matter with their crystal class.
In the present study, the ground state properties of chromium carbides (h-CrC, c-CrC, Cr
3C, Cr
3C
2, Cr
7C
3, and Cr
23C
6) are calculated by means of the first-principles pseudopotential method using the CASTEP code. The equilibrium crystal structures and thermodynamical stability of the six chromium carbide phases are discussed. Moreover, the chemical bonding in these carbides are interpreted by calculating the density of states, electron density distribution and Mulliken analysis; all the six chromium carbides have a combination of metallic, ionic and covalent bonding characteristic, while Cr
7C
3 exhibits the strongest metallic character. The elastic constants, elastic anisotropies and theoretical hardness of the carbides are also presented, which are important parameters for the structural materials and surface coatings.
The effects of orientation and lamellar spacing on the interface microstructure and corrosion behavior of a directionally solidified (DS) Fe-B alloy in a hot-dip galvanization bath were investigated. ...The results indicated that the microstructure of the DS Fe-B alloy consisted of oriented α-Fe and Fe2B grains. The oriented Fe2B with 002 preferred growth orientation displayed low-angle grain boundaries on the Fe2B (001) basal plane. The DS Fe-B alloy with Fe2B vertical to the corrosion interface possessed the best corrosion resistance to liquid zinc owing to the formation of an interface-pinning multilayer induced by the Fe2B orientation. The epitaxially grown columnar ζ-FeZn13 products were controlled by the geometric constraint of Fe2B grain orientation and size, and a mechanism model that explains the interfacial orientation-pinning behavior is discussed in detail. Transmission electron microscopy (TEM) results revealed that the possible orientation relationships of the oriented Fe2B and columnar ζ-FeZn13 products are (001)Fe2B//(−402)ζ-FeZn13 and 002Fe2B//110ζ-FeZn13. The corrosion damage of the DS Fe-B alloy with Fe2B 002 orientation vertical to the corrosion interface in liquid zinc was governed by the competitive mechanisms of Fe2B/FeB transformation and microcrack-spallation resistance, which is proposed as being the result of a multiphase synergistic effect in the micro-structures.
Display omitted
Display omitted
•A novel Sc/Zr/Ti/Er modified Al-Cu alloy was successfully fabricated by wire-arc directed energy deposition technology.•The fine equiaxed grain structure was obtained due to the ...heterogeneous nucleation caused by the primary Al3X dispersoids.•Homogenization prefabricates more Al3X dispersoids, which promote the transition from θ″ to θ′ precipitate.•Homogenization ripens the existing pores, resulting in a better tensile isotropy.
In this study, a novel Sc/Zr/Ti/Er-modified 2219 aluminum alloy was designed and wall-shaped components were then fabricated by wire-arc directed energy deposition (DED). Due to the heterogeneous nucleation promoted by primary Al3X dispersoids, the as-deposited alloy exhibits a highly equiaxed fine grain structure. Two heat treatment schedules were conducted to tailor the strength and thermal stability of the alloy. Compared with the classical T6 heat treatment, the additional homogenization step before the T6 heat treatment (HT6) prefabricated more Al3X dispersoids, which indirectly promoted the transition from θ″ precipitates to θ′ precipitates, resulting in a slight reduction in strength. However, this homogenization step helps to avoid the aggregation of pores by promoting pores ripening and significantly alleviates the anisotropy of ductility. In addition, the better thermal stability of the HT6 heat treated alloy is due to more Al3X dispersoids and finer θ′ precipitates with high coarsening resistance caused by the segregation of Sc and Mn at the θ′/Al interface during thermal exposure. In general, the wire-arc DEDed micro-alloyed 2219 Al alloy exhibits a promising combination of strength and ductility after heat treatment and its mechanical properties are superior to those of the wrought micro-alloyed Al-Cu alloys.
▶ With the increasing chromium additions, the boride changes from Fe
2B to (Fe,Cr)
2B-type boride. ▶ The matrix of Fe–3.5B alloy transforms to supersaturated α-(Fe,Cr) solid solution when high ...chromium concentration is added. ▶ The fracture toughness of boride increases with the increase of chromium addition. ▶ Secondary phase precipitates during the heat treatment of Fe–3.5B alloy with various chromium concentrations.
The cast low carbon Fe–3.5B alloys containing various chromium concentrations were prepared in a 10
kg medium frequency induction furnace and the effects of chromium concentration on microstructure and properties of Fe–3.5B alloys have been examined by means of optical microscope (OM), scanning electron microscope (SEM), back-scattered electron microscope (BSE), electron probe microanalyzer (EPMA), energy dispersive spectrum (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and Vickers hardness. As a result, the as-cast structures of Fe–3.5B–
XCr (
X
=
0, 2, 5, 8, 12, 18, mass fraction) alloys are mainly composed of dendrite ferrite, martensite, pearlite and boride. The boride in the alloy without chromium addition comprises the eutectic Fe
2B, which is continuous netlike or fish-bone structure distributed over the metallic matrix. With the increase of chromium concentration in Fe–3.5B alloy, matrix structure turns into the supersaturated
α-Fe solid solution while the morphology of boride becomes dispersed due to the transformation of boride from simple Fe
2B to (Fe,Cr)
2B when the chromium concentration in Fe–3.5B alloy exceeds 8
wt.%. Meanwhile, some primary M
2B-type borides may precipitate under this condition. The bulk hardness of the as-cast alloy ranges from 41.8 to 46.8 HRC. However, the bulk hardness of the heat treated alloy rises first and falls later mainly because of the morphology variation of structure. Fracture toughness of boride is improved gradually owing to the entrance of chromium into Fe
2B, which may be attributed to the change of spatial structure of boride.
PDF
