Elasticity and thermodynamic properties of α-Ta4AlC3 under pressure

First-principles calculations of the crystal structure and the elastic properties of α-Ta₄AlC₃ have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The pressure dependence of the elastic constants c_ij, the aggregate elastic moduli (B, G, E), the Poisson's ratio, and the elastic anisotropy has been investigated. Using the quasi-harmonic Debye model considering the phonon effects, the temperature and pressure dependencies of isothermal bulk modulus, and the thermal expansions, and Grüneisen parameters, as well as Debye temperatures are investigated systematically in the ranges of 0–60 GPa and 0–1500 K as well as compared to available data.     

Mechanical properties and oxidation resistance of CrAlN/BN nanocomposite coatings prepared by reactive dc and rf cosputtering

CrAlN/BN nanocomposite coatings were deposited through reactive cosputtering, i.e., pulsed dc and rf sputtering, of CrAl and h-BN targets, respectively. X-ray diffraction (XRD) and selected area electron-diffraction (SAED) analysis indicated that the CrAlN/BN coating consists of very fine grains of B1 structured CrAlN phase. With an increasing BN volume fraction of over 8 vol.%, the nanocrystalline nature of the grains is revealed through a dispersion of fine grains in the CrAlN/BN coating. A cross-sectional observation using a transmission electron microscope (TEM) clarified that the coating demonstrating the highest level of hardness has a fiber-like structure consisting of grains that are ~ 20 nm in width and ~ 50 nm in length. X-ray photoelectron spectroscopy (XPS) analysis revealed that the coating consists mainly of CrAlN and h-BN phase. The indentation hardness (H_IT) and effective Young's modulus (E*) of the coatings increased with the BN phase ratio, reaching a maximum value of ~ 46 and ~ 440 GPa at ~ 7 vol.% of BN phase; it then decreased moderately to ~ 40 and ~ 350 GPa at 18 vol.% of BN, respectively. Furthermore, CrAlN/BN coatings showed superior oxidation resistance compared with CrAlN coatings. After annealing at 800 °C in air for 1 h, the indentation hardness of CrAlN coatings decreased to 50% of the as-deposited hardness; in contrast, the hardness of CrAlN/BN nanocomposite coatings either stayed the same or increased, attaining a value of about 46 GPa. After annealing at 900 °C for 1 h, the hardness of all the coatings decreased to about 40%.     

Enhanced mechanical properties due to structural changes induced by devitrification in Fe–Co–B–Si–Nb bulk metallic glass

Fe₃₆Co₃₆B_19.2Si_4.8Nb₄ bulk glassy rods were synthesized by copper mould casting. The effects of annealing treatments on the microstructure, elastic and mechanical properties of this alloy are investigated. Annealing below the glass transition temperature induces the formation of atomic clusters with pseudo-tenfold symmetry with a close relationship to the Fe₂₃B₆ phase. Annealing at sufficiently high temperatures promotes the formation of stable Fe₂B and FeB phases and Fe(Co) solid solution. The as-cast alloy exhibits ultra-high hardness (H > 14 GPa), high reduced Young’s modulus (E_r > 200 GPa) and good wear resistance. These properties are further enhanced after thermal treatments (H > 18 GPa and E_r > 260 GPa are achieved in the fully crystallized sample). The mechanical hardening is accompanied with an increase of the elastic recovery and a decrease of the Poisson’s ratio. The different microstructural mechanisms responsible for these annealing-induced changes in mechanical and elastic properties are discussed.     

Effect of alloying elements on the elastic properties of Mg from first-principles calculations

The influence of different alloying elements on the lattice parameters and elastic properties of Mg solid solution has been studied using first-principles calculations within the generalized gradient approximation. The solute atoms employed herein are Al, Ba, Ca, Cu, Ge, K, Li, Ni, Pb, Si, Y and Zn. A supercell consisting of 35 atoms of Mg and one solute atom is used in the current calculations. A good agreement between calculated and available experimental data is obtained. Lattice parameters of Mg–X alloys are found to be dependent on the atomic radii of the solute atoms. A correlation between the bulk modulus of Mg–X alloys and the nearest-neighbor distance between Mg and X is shown. Addition of solute atoms belonging to the s-block and p-block of the periodic table results in a lower bulk moduli than d-block elements. A strong dependence of the elastic modulus of Mg–X alloys on the elastic properties of the solute atoms is also observed. Using the bulk modulus/shear modulus ratio (B/G), the change in the ductility of Mg due to the addition of the solute atom is briefly described. Linear regression coefficients for the elastic constants of each of the alloys are obtained as a tool for predicting the trend in the elastic properties of Mg as a function of concentration of the solute atoms.     

In situ synthesis and sintering of ZrB2 porous ceramics by the spark plasma sintering–reactive synthesis (SPS–RS) method

Zirconium diborides (ZrB₂) porous ceramics were synthesized by the Spark Plasma Sintering-Reactive Synthesis (SPS–RS) technique using ZrO₂ and B₄C as precursors which undergo solid state reaction that lead to pore formation. Phase analysis of the products indicated that the reaction started between 1200 °C and 1300 °C and was carried out at 1600 °C within 10 min under SPS conditions, which was consistent with the thermodynamic calculations. The as-prepared ZrB₂ porous ceramics had a relatively smaller crystallite size (~ 1 μm), a lower oxygen content (~ 1.04 wt.%) and a relative density of 29.9%. The oxygen impurities decreased with the sintering temperature and holding time. In addition, the measured results showed that the reaction was carried out within 10 min holding time at the temperature of 1600 °C and the synthesized ZrB₂ products had high purity in comparison to commercial ZrB₂ powder product.     

Influence of thermal treatment on structure development and mechanical properties of amorphous Fe73.5Cu1Nb3Si15.5B7 ribbon

Ribbon-shaped amorphous samples with the stoichiometric composition Fe_73.5Cu₁Nb₃Si_15.5B₇ prepared by the melt spinning process were annealed at temperatures ranging from 693 K to 1123 K for 1 h under vacuum. In the early annealing stage, the alloy undergoes a specific nucleation process where Cu clusters precipitate from an amorphous matrix. Further heating initiates the partial crystallization of alloy forming the α-Fe–Si nanocrystallites. Subsequent Vickers hardness tests showed high values depending on the annealing temperature. It was found that the hardening process includes two stages. This behavior correlates well with results of density dislocation calculations. A crystallite size of 10 nm for the α-Fe–Si particles correlated very well with a maximum hardness of the material.     

Anisotropic elastic constants and thermal expansivities in monocrystal CrB2, TiB2, and ZrB2

The elastic constants and thermal expansivities in monocrystals of three transition-metal diborides with the AlB₂ structure, CrB₂, TiB₂, and ZrB₂, have been investigated in the temperature ranges from 300 to 1373 K and from 300 to 1073 K. The anisotropic parameters deduced from the measured elastic constants and thermal expansivities indicate that of the three diborides, the anisotropy is the most and least significant in CrB₂ and ZrB₂, respectively. The factors determining the significance in anisotropy in atomic bonding in AlB₂-type diborides are analyzed by an approach similar to the valence-force-field method and are discussed in terms of the deformation of the electronic charge around the metal atoms occurring to fit themselves in the (0 0 0 1) basal plane.     

Effect of Pr and Dy substitution on the impact resistance of sintered Nd–Fe–B magnets

The impact resistance of sintered Nd–Fe–B magnets with nominal compositions (Nd_1−xDy_x)₁₆Fe₇₈B₆ (x = 0, 0.05, 0.10, 0.15) and (Nd_1−yPr_y)₁₆Fe₇₈B₆ (y = 0, 0.33, 0.67, 1) has been investigated by the falling-weight impact test. The bending strength and the Vickers hardness were measured. It shows that the Dy substituted magnets have better impact resistance than the Pr substituted magnets. The impact resistance of the investigated magnets improves monotonously with increasing Dy content, and reduces with increasing Pr content, which seems to relate closely to the change of the density or of the hardness.     

Mechanical behavior of Fe65.5Cr4Mo4Ga4P12C5B5.5 bulk metallic glass

Fe_65.5Cr₄Mo₄Ga₄P₁₂C₅B_5.5 bulk amorphous rectangular bars with a cross-section of 2×2 mm² and a length of 30 mm were produced by copper mold casting. The as-cast bars as well as annealed samples were investigated by compression and Vickers hardness tests. The fracture strength for the as-cast samples σ_f is 2.8 GPa and the fracture strain ε_f is 1.9%. Upon annealing at 715 K for 10 min, i.e. at a temperature below the calorimetric glass transition, the fracture strain drops to 1.6% and no plastic deformation is observed. The Vickers hardness HV for the as-cast samples is about 885, and increases to 902 upon annealing. The fracture behavior of this Fe-based bulk glassy alloy is significantly different in comparison with the well-studied Zr-, Cu- or Ti-based good glass-formers. The fracture is not propagating along a well-defined direction and the fractured surface looks irregular. Instead of veins, the glassy alloy develops a high number of microcracks.     

Correlation between microhardness and electronic charge density of hafnium hydrides

Quantitative Vickers microhardnesses for α + δ′-, δ′-, δ-, δ + ?- and ?-phase hafnium hydrides (HfH_x; 1.46 ≤ x ≤ 2.02) and deuterides (HfD_x; 1.55 ≤ x ≤ 1.94) at room temperature have been measured using a Vickers hardness tester. The Vickers microhardnesses of the HfH_x and HfD_x gradually decreased with an increase of the hydrogen concentration, and those for 1.46 ≤ x ≤ 1.70 were higher than that of α-phase metallic Hf. It was revealed by a first-principles calculation as well as an X-ray photoelectron spectroscopy (XPS) measurement that the hydrogen concentration dependence of the microhardnesses for HfH_x and HfD_x was ascribed mainly in terms of its influence on charge transfer from Hf 5d to H 1s and reduction of cohesive energy.     

Theoretical elastic stiffness,structural stability and thermal conductivity of La2T2O7 (T = Ge,Ti, Sn,Zr, Hf) pyrochlore

In order to achieve better understanding of the structural/property relationships of La₂T₂O₇ (T = Ge, Ti, Sn, Zr, Hf) pyrochlore, first-principles calculations were conducted to investigate the bonding characteristics, elastic stiffness, structural stability and minimum thermal conductivity. The results show that the relatively weak La–O bonds play a predominant role in determining the structural stability, mechanical and thermal properties of these compounds. In addition, the elastic and thermal properties are influenced when the T atom changes from Ge to Hf. When the bonding strength is enhanced by applying hydrostatic pressure, apart from c₁₁, c₁₂, and B, which normally increase at high pressures, it is found that the shear elastic moduli, c₄₄ and G, which relate to the shear deformation resistance, abnormally remain almost constant. The underlying mechanism may help to explain the damage tolerance of pyrochlore compounds. After comprehensive consideration of the elastic anisotropy, a modified David Clarke-type equation is used to calculate the minimum thermal conductivity of the studied pyrochlore materials, which display an extraordinary low thermal conductivity.     

Investigation on the preparation and machinability of the B4C/BN nanocomposites by hot-pressing process

The machinable B₄C/BN nanocomposites were fabricated by the hot-pressing process using the B₄C/BN nanocomposite powders at 1850 °C for 1 h under the pressure of 30 MPa. The nanocomposite powders with the microstructure of micro-sized B₄C particles coated with amorphous nano-sized BN particles were prepared by the chemical reaction of H₃BO₃ and CO(NH₂)₂ on the surface of B₄C particles at high temperature. Then the amorphous BN transformed into the hexagonal-BN (h-BN) after the hot-pressing process at 1850 °C. The microstructure investigations of the B₄C/BN nanocomposites sintered samples showed that the nano-sized h-BN particles were homogenously distributed within the matrix grains as well as at the matrix grains boundaries. With the increasing content of h-BN, the relative density of the B₄C/BN nanocomposites decreased gradually. The fracture strength and fracture toughness of the B₄C/BN nanocomposites decreased gradually, the fracture strength and fracture toughness of the B₄C/BN nanocomposites with the h-BN content of 10 wt.% and 20 wt.% achieved high values. The Vickers hardness of the B₄C/BN nanocomposites decreased remarkably with the increasing content of h-BN, while the drilling rates and machinability of the B₄C/BN nanocomposites increased significantly. The B₄C/BN nanocomposites with the h-BN content more than 20 wt.% exhibited excellent machinability.     

Titanium diboride composite with improved sintering characteristics

Titanium diboride (TiB₂) and its ceramic composites were prepared by hot pressing process. The sintering process, phase evolution, microstructure and mechanical properties of TiB₂ ceramics prepared by using different milling media materials: tungsten carbide (WC/Co) or SiAlON was studied. It was found that the inclusion of WC/Co significantly improved the sinterability of the TiB₂ ceramics. A core/rim structure with pure TiB₂ as the core and W-rich TiB₂, i.e. (Ti,W)B₂ as the rim was identified. Microstructure analysis revealed that this core/rim structure was formed through a dissolution and re-precipitation process. In addition, silicon carbide (SiC) was also introduced to form TiB₂–SiC composites. The addition of SiC as the secondary phase not only improved the sinterability but also led to greatly enhanced fracture toughness. The optimum mechanical properties with Vickers hardness ~ 22 GPa, and fracture toughness ~ 6 MPa m^1/2 were obtained on TiB₂–SiC composites milled with WC/Co.     

Elastic constants of binary Mg compounds from first-principles calculations

Elastic constants (C_ij's) of 25 compounds in the Mg–X (X = As, Ba, Ca, Cd, Cu, Ga, Ge, La, Ni, P, Si, Sn, and Y) systems have been predicted by first-principles calculations with the generalized gradient approximation and compared with the available experimental data. Ductility and the type of bonding in these compounds are further analyzed based on their bulk modulus/shear modulus ratios (B/G), Cauchy pressures (C₁₂–C₄₄), and electronic structure calculations. It is found that MgNi₂ and MgCu₂ have very high elastic moduli. Mg compounds containing Si, Ge, Pb, Sn, and Y, based on their B/G ratios, are inferred as being brittle. A metallic bonding in MgCu₂ and a mixture of covalent/ionic bond character in Mg₂Si, as inferred from their electronic structures, further explain the corresponding mechanical properties of these compounds.     

Deposition and characterization of superhard biphasic ruthenium boride films

Recently, the superhardness of rhenium diboride films was reported. In this study the first successful preparation and characterization of ruthenium boride films is presented. The morphology, topography, microstructure and hardness of films, prepared by pulsed laser deposition, were investigated. The films, which are 0.7 μm thick, have a dense grain texture, and are composed of two phases Ru₂B₃ (main phase, 65% volume fraction) and RuB₂ (35%). The RuB₂ phase does not show any preferred orientation, while Ru₂B₃ is textured preferentially along the (1 1 4) and (1 0 5) directions, with crystallite growth parallel within 1.9° of average mismatch. The composite Vickers microhardness of the film–substrate systems was measured, and the intrinsic hardness of the films was separated using an area law-of-mixtures approach. The obtained films were found to be superhard, the intrinsic film hardness value (49 GPa) being much higher than that for the RuB₂ bulk used as the target for film deposition and than that for the Ru₂B₃ bulk.     

H2V3O8 single-crystal nanobelts: Hydrothermal preparation and formation mechanism

The formation mechanism of highly pure H₂V₃O₈ single-crystal nanobelts is clarified in a hydrothermal synthesis process with a specially designed precursor solution containing V⁵⁺ and V⁴⁺ in a fixed ratio of 2/1. This specially designed precursor solution provides an additional merit for the rapid fabrication of highly pure H₂V₃O₈ nanobelts through a simple hydrothermal route. During the hydrothermal synthesis process, V⁵⁺ species initially reacts with some V⁴⁺ to form a metastable, whisker-like V₁₀O₂₄ · nH₂O (n < 12). The V⁵⁺ species dissolved from the whisker-like V₁₀O₂₄ · nH₂O reacts continuously with residual V⁴⁺ ions in the precursor solution to form seeds of H₂V₃O₈ single-crystals. The anisotropic growth of H₂V₃O₈ single-crystal nanobelts with length > 10 μm and width between 50 and 150 nm occurs with prolonging the hydrothermal time. Finally, highly pure H₂V₃O₈ single-crystal nanobelts are obtained when the hydrothermal time reaches 4 h. The textures of vanadium oxides prepared at different hydrothermal times are systematically compared through X-ray diffraction, transmission electron microscopic and X-ray photoelectron spectroscopic analyses to clarify the synthesis mechanism of H₂V₃O₈ single-crystal nanobelts.     

Improving hardness and tribological characteristics of nanocrystalline Cr–C films obtained from Cr(III) plating bath using pulsed electrodeposition

Effect of pulsed electrodepostion on the nanocrystal size, composition, hardness, coefficient of friction and wear resistance was investigated for the Cr–C electrodeposits obtained from a trivalent chromium bath. The electrodeposits were shown to contain about 9% of carbon. Pulsed electrodeposition does not virtually affect the carbon content. At the same time, an increase in the off time duration leads to a decrease in the nanocrystals size. The hardness and wear parameters of the electrodeposits may be sufficiently improved when using pulsed current. For instance, at t_on = t_off = 1 s, the hardness reaches the values of ~ 1200 ÷ 1300 HV (meanwhile, it is close to 850 ÷ 950 HV at a steady-state electrolysis).     

Synthesis,crystal structure and thermal behavior of Na4[B10O16(OH)2]·4H2O

New hydrated sodium borate Na₄[B₁₀O₁₆(OH)₂]·4H₂O has been synthesized under mild hydrothermal conditions at 170 °C. The structure was determined by single-crystal X-ray diffraction and further characterized by FT-IR, Raman spectra and DTA-TG. It crystallizes in the monoclinic space group Pc with a unit cell of dimension a = 11.323(2) Å, b = 6.5621(14) Å, c = 12.244(3) Å, α = 90°, β = 91.050(3)°, γ = 90°, V = 909.7(3) Å³, Z = 2. The crystal structure of Na₄[B₁₀O₁₆(OH)₂]·4H₂O consists of Na–O polyhedra and [B₁₀O₁₆(OH)₂]⁴⁻ polyborate anions. Dehydration of this compound occurs in three steps and leads to an amorphous phase which undergoes crystallization.     

Elastic properties of silver–phospho–vanadate glasses

The elastic properties of xAg₂O–(50 ? x) P₂O₅–50V₂O₅ glasses are investigated using ultrasonic pulse-echo measurements and their elastic properties have been characterized at room temperature. Results from the studies show that both longitudinal and transverse velocities decrease with increase of Ag₂O concentration. The elastic constants C₁₁, C₄₄ and Young's modulus show decreasing trend while constant C₁₂, bulk modulus and Poisson's ratio show an increasing trend as the fraction of Ag₂O increases. Another notable observation is that the glass with 15 and 40 mol% of Ag₂O concentrations exhibits the low velocities and low elastic moduli. This behavior of the elastic properties is related to the change in the structure of glasses as well as the interatomic bonding.     

Effect of graphene platelets on tribological properties of boron carbide ceramic composites

The friction and wear behaviour of hot pressed boron carbide/graphene platelets (GPLs) composites have been investigated using the ball-on-flat technique with SiC ball under dry sliding conditions at room temperature. The hardness and fracture toughness of the investigated materials varied from 18.21 GPa to 30.35 GPa and from 3.81 MPa·m^1/2 to 4.60 MPa·m^1/2, respectively. The coefficient of friction for composites were similar, however the wear rate significantly decreased ~ 77% in the case of B₄C + 6 wt.% GPLs when compared to reference material at a load of 5 N, and ~ 60% at a load of 50 N. Wear resistance increased with increasing GPLs content in regards to the present graphene platelets, which during the wear test pulled-out from the matrix, exfoliated and created a wear protecting graphene-silicon based tribofilm.