Changes in the potential and vibrational energies upon the formation of vacancies in the oxygen-alloyed cores of crystallite-conjugation regions of polycrystalline Cr, Cu, and Ir

The changes in the potential and vibrational energies upon the formation of vacancies in the oxygen-alloyed (OA) cores of crystallite-conjugation regions (CCRs) in polycrystals of d transition metals Cr, Cu, and Ir have been determined. The potential energy increases upon the formation of vacancies (upon the formation of complexes of vacancies with oxygen atoms) in the OA cores of CCRs in the polycrystalline Cr, Cu, and Ir. However, the vibrational energy decreases upon the formation of vacancies in the OA cores of CCRs in these polycrystals, as in the OA cores of CCRs in polycrystals of the 4d and 5d transition bcc metals Mo, Ta, and W. The volume of the OA cores of CCRs in polycrystals of Cr, Cu, and Ir decreases upon the formation of vacancies. The changes in the interatomic interactions and dynamic properties in the regions of vacancy localization in the OA cores of CCRs coincide with analogous changes introduced into the lattices of metals by split interstitials.     

Changes in the vibrational energies and interatomic spacings upon the formation of vacancies in the volumes and in the cores of crystallite-conjugation regions of polycrystalline transition metals with cubic lattices

The changes in the vibrational energies and the signs of changes in the interatomic spacings upon the formation of vacancies in the bulk of metal and in the cores of the crystallite-conjugation regions (CCR) in polycrystalline transition metals with bcc and fcc lattices have been determined. The vibrational energy increases upon the formation of a vacancy in the bulk of metal because of a positive “relaxation” contribution to the change in the force constant of the atoms surrounding a vacancy. Positive “relaxation” contributions to the changes in the force constants and, correspondingly, an increase in the vibrational energy of the atoms surrounding a vacancy arise also upon the formation of “split” vacancies (S vacancies) in the cores of CCRs of polycrystalline transition metals with a face-centered cubic lattice. The positive “relaxation” contributions to the changes of the force constant of atoms in the region of localization of S vacancies are caused by a decrease in the interatomic spacings upon their formation, just as upon the formation of conventional vacancies in the bulk of metals. The vibrational energy of the nearest environment of the vacancies that are formed in the CCR cores in the polycrystalline d transition metals with a bcc lattice decreases because of a negative “relaxation” contribution to the change in the force constants. The cores of the high-angle CCRs in polycrystalline d transition metals with a bcc lattice are characterized by a negative internal pressure. Therefore, vacancies with positive relaxation volumes ν_BCC > 0 are formed in them, causing an increase in the interatomic distances in the nearest environment of such vacancies.     

Crystallite-conjugation regions and adjacent lattice regions in polycrystalline iridium: III. Enthalpies of formation of vacancies and the energies of interaction of partners in vacancy complexes with oxygen formed in the cores of crystallite-conjugation regions in polycrystalline iridium

The interatomic spacings in the cores of crystallite-conjugation regions (CCRs) and adjacent lattice regions (ALRs) of polycrystals either decrease or increase upon alloying of nominally pure metals with oxygen and vacancy complexes with oxygen (mVacO, where m is the number of vacancies in the complex) that are formed during annealing. These changes in the interatomic spacings lead to an increase or decrease in the isomer shifts δ of the components of the Mössbauer spectra of atomic probes ⁵⁷Fe that are localized in the CCR cores and ALRs of polycrystals [1–6]. The enthalpies Q _mcmpl1, of formation of vacancy-oxygen complexes mVacO in the CCR cores have been measured for Ir and Cr polycrystals, and the enthalpies Q _Vac,1 of formation of vacancies in CCR cores have been determined for Ta, W, Ir, and Cr polycrystals. The enthalpies E _mcmpl of the interaction between the partners of the complexes increase with increasing number m of vacancies in the complexes.     

Changes in the potential energy upon the formation of vacancies in the volume and in the cores of crystallite-conjugation regions in cubic polycrystalline metals

Changes in the potential energy of atoms that constitute the nearest neighborhood of vacancies formed in the bulk of d transition and precious cubic metals have been determined. These changes agree with the available first-principles calculations of changes in the potential energy of atoms of the nearest neighborhood of vacancies. In the cores of crystallite-conjugation regions (CCRs) of bcc polycrystalline d transition metals, the formation of vacancies is accompanied by positive changes in the potential energy of atoms of their nearest neighborhood. The absolute magnitudes of these changes are several times less than the changes in the potential energy of atoms of the nearest neighborhood of vacancies in the bulk of these metals, in accordance with the relationship between the enthalpies of formation of vacancies in these regions of polycrystals. The changes in the potential energy of atoms of the nearest neighborhood of vacancies formed in the cores of CCRs of fcc polycrystalline metals are negative because of the split structure of vacancies in the CCR cores of such metals.     

Solubility and ordering of Ti,Ta, Mo and W on the Al sublattice in L12-Co3Al

Co–Al–W-based alloys are promising new materials for high-temperature applications. They owe their high-temperature strength to hardening by ternary L1₂-Co₃(Al_1−xW_x) precipitates, which may form even though binary Co₃Al is not stable. In the current work, density functional theory calculations are performed to study the solubility and ordering of the transition metals W, Mo, Ti, and Ta at the Al sublattice in L1₂-Co₃Al. The sublattice disorder is modelled with a newly parametrised cluster expansion and compared to results using special quasi-random structures. Our results for W and Mo show that the mixing energy exhibits a minimum at approximately x = 0.7. However, the computed small values of the mixing energies indicate that W and Mo atoms are fully disordered with the Al atoms already at low temperatures. For Ti and Ta we find no sizeable driving force for ordering with the Al atoms. The computed solubilities on the Al sublattice obtained are in the range of 40–80 meV/atom for W and Mo and less than 25 meV/atom for Ti and Ta.     

Tensile and compressive behavior of tungsten,molybdenum, tantalum and niobium at the nanoscale

In situ nanomechanical tests are carried out to investigate the tensile and compressive behavior of 〈0 0 1〉-oriented body-centered cubic (bcc) metals W, Mo, Ta and Nb with nanometer dimensions. We find that the strength of these metals exhibits strong size dependence. The compressive size effect in Nb, as evaluated by the log–log slope of strength vs. nanopillar diameter, is ?0.93, a factor of 2.1 greater than that for the other three metals W, Mo and Ta (?0.44). In tension, however, Ta and Nb show higher size effect slopes (?0.80 and ?0.77) as compared with W and Mo (?0.58 and ?0.43). We also report that while the yield strength of these metals is a strong function of size, the strain-hardening behavior does not present any size-dependent trends. We further discuss the effects of strain-rate on deformation behavior and provide transmission electron microscopy analysis of microstructural evolution in the same Mo nanopillar before and after compression.     

Partitioning and site occupancy of Ta and Mo in Co-base γ/γ′ alloys studied by atom probe tomography

The detailed understanding of solute partitioning and site occupancies of these solutes within the ordered γ′ (L1₂) precipitates holds a key role in the development of cobalt-base γ/γ′ alloys with optimized properties. The present atom probe tomography study utilizes both structural and compositional information to determine the partitioning behavior of transition elements like Ta and Mo between γ matrix and γ′ precipitates and their site occupancy within the γ′ phase. The addition of Ta, which enhances the formation of γ′, to a ternary Co–Al–W alloy with stoichiometric Co₃(Al,W) precipitates, results in the substitution of only the W in the γ′ precipitates to form Co₃(Al, W, Ta) precipitates. Interestingly, Mo, typically considered a γ solid solution strengthener in nickel-base alloys, also partitions strongly to γ′ precipitates when added to the Co–Al–W alloy and displaces only the W atoms. The experimentally observed equal atomic substitution of W by both Ta and Mo, without any change in the Al content within the γ′ precipitates, gives insights into the energetics of relative site substitutions in this ordered compound.     

Crystallite-conjugation regions and adjacent lattice regions in polycrystalline iridium: II. Composition and properties of cores of crystallite-conjugation regions in polycrystalline iridium during annealing in an ultrahigh vacuum

Composition and properties of cores of crystallite-conjugation regions (CCRs) formed during annealing of polycrystalline iridium (poly-Ir) in an ultrahigh vacuum (UHV) have been studied using the method of intercrystalline diffusion in combination with Mössbauer spectroscopy (ID+MS) that has been developed in our previous works. Upon annealing in a UHV, poly-Ir is doped with oxygen from the atmosphere of the vacuum chamber. Complexes containing two vacancies per oxygen atom are formed in the CCR cores of poly-Ir because of a rearrangement of the atomic structure of the CCR cores upon their doping with oxygen. Using the ID+MS method, we for the first time revealed a “compensated” state of CCR cores in poly-Ir samples annealed in a UHV and of CCR cores in poly-Cr annealed in technical vacuum. The cause of the appearance of “compensated” states in CCR cores is the mutual compensation of relaxation volumes with opposite signs characteristic of different point defects. The relaxation volume of an oxygen atom in the CCR core of poly-Ir is by an order of magnitude greater than that in poly-Cr.     

Corrosion resistance of tantalum base alloys. Elimination of hydrogen embrittlement in tantalum by substitutional alloying

The corrosion behaviour of substitutional Ta–Mo, Ta–W, Ta–Nb, Ta–Hf, Ta–Zr, Ta–Re, Ta–Ni, Ta–V, Ta–W–Mo, Ta–W–Nb, Ta–W–Hf and Ta–W–Re alloys has been investigated in various corrosive media, i.e. (1) concentrated sulfuric acid at 250°C and 200°C, (2) boiling hydrochloric acid of azeotropic composition, (3) concentrated hydrochloric acid at 150°C under pressure, (4) HF-containing solutions and (5)0.5% H₂SO₄ at room temperature (anodisation). In highly corrosive media such as concentrated H₂SO₄ at 250°C and concentrated HCl at 150°C tantalum is hydrogen embrittled, probably by stress induced precipitation of β-hydride. Both corrosion rate and hydrogen embrittlement in concentrated H₂SO₄ at 250°C are strongly influenced by alloying elements. Small alloying additions of either Mo or Re decrease the corrosion rate and the hydrogen embrittlement, while Hf has the opposite effect. Hydrogen embrittlement in concentrated H₂SO₄ at 250°C is completely eliminated by alloying Ta with 1 to 3 at % Mo (0.5 to 1.5 wt % Mo). These results can be explained in terms of the oxygen deficiency of the Ta₂O₅ film and the electronic structure of these alloys.     

Effects of tantalum on the partitioning of tungsten between the γ- and γ′-phases in nickel-based superalloys: Linking experimental and computational approaches

Atom probe tomography (APT) and first-principles calculations are implemented to study the partitioning of W to the γ (face-centered cubic)- and γ′ (L1₂)-phases in Ni-based alloys. APT observations indicate that whereas W partitions preferentially to the γ′-phase in a ternary Ni–Al–W alloy, its partitioning behavior is reversed in favor of the γ-phase in multi-component alloys. Furthermore, the degree of W-partitioning to the γ′-phase decreases with the addition of Ta to a Ni–Al–Cr–W alloy, a trend which is consistent with Thermo-Calc simulations. First-principles calculations of the substitutional formation energies of W and Ta at 0 K predict that both elements prefer energetically sharing the Al-sublattice sites of the γ′-phase, whereas Ta has a larger tendency to partition to the γ′-phase than does W. This implies that Ta displaces W from the γ′-phase into the γ-phase in multi-component Ni-based alloys.     

Site preference of early transition metal elements in C15 NbCr2

The site preference of early 3d (Ti, V), 4d (Zr, Mo) and 5d (Hf, Ta, W) transition metal elements in C15 NbCr₂ Laves phase was studied using first-principles calculations. According to the present calculations, at T = 0 K, Zr, Hf and Ta consistently have a preference for the Nb sites in Nb-rich, Cr-rich and stoichiometric NbCr₂, while the site preference of Ti, V, Mo and W varies strongly with alloy composition. Using a statistical–mechanical Wagner–Schottky model based on the canonical ensemble, the finite temperature site occupancy behavior of those transition metal elements in NbCr₂ was further predicted. It was found that the site preference of Ti, V, Mo and W also depends strongly on temperature. The calculated results compare favorably with the experimental measurements using ALCHEMI and synchrotron X-ray diffraction techniques.     

Equilibrium vacancy concentrations in FeAl and FeSi investigated with an absolute technique

Absolute vacancy concentrations in FeAl and FeSi alloys are being determined by measuring simultaneously the relative changes in macroscopic length ΔL/L and X-ray lattice parameter Δa/a as a function of temperature in the same specimen. With the high-resolution differential dilatometer vacancy concentrations of about 10⁻⁵ to a few percent can be measured, so that thermal vacancies can be investigated within a wide temperature range of about 700–800 K. The temperature dependence of vacancy formation is correlated with the phase fields according to the phase diagrams referenced by Kubaschewski (Kubaschewski O. Iron-binary phase diagrams. Berlin: Springer, 1982). In the B2′ and B2(l) phase of FeAl and the D0₃(l) phase of FeSi, vacancy formation obeys an Arrhenius law with effective formation enthalpies about 1 eV and effective formation entropies about 4–5 k_B. For higher temperatures, in the B2(h) phase field of FeAl and the D0₃(h) phase field of FeSi, vacancies are no longer formed as easily as before. This results in a flatter line, effective formation enthalpies about 0.3–0.5 eV may be fitted. In the disordered A2 phase of FeAl the vacancy concentration increases again, effective formation enthalpies about 1.5 eV may be fitted.     

Partition behavior of alloying elements and phase transformation temperatures in Co–Al–W-base quaternary systems

The phase equilibria among γ (A1), γ′ (L1₂), χ (D0₁₉), β (B2) and μ (D8₅) phases and the γ′ solvus and γ solidus temperatures were investigated in the Co–Al–W-based quaternary systems with alloying elements of Ti, V, Nb, Ta, Cr, Mo, Mn, Fe, Ni, Si, Zr, Hf, Ru and Ir by electron probe microanalysis (EPMA) using multiphase alloys and by differential scanning calorimetry (DSC). It was found that Ta, Nb, Ti, V, Mo and W are partitioned to the γ′ or χ phase rather than to the γ phase, while Cr, Mn and Fe tend to be distributed to the γ phase. The correlation between the partition coefficient of alloying elements between γ/γ′, γ/χ and γ/β phases and ab initio formation energy of Co₃X (L1₂), Co₃X (D0₁₉) and CoX (B2) was respectively obtained. It was also found that the γ′ solvus temperature increases by the addition of the γ′ former elements such as Ta, Nb and Ti, which decreases the γ solidus temperature.     

Microstructure and mechanical properties of (Ti,W,Ta)C-xMo-Ni cermets

(Ti,W,Ta)C-xMo-20Ni (x = 0–20 wt.%) system cermets were prepared by using (Ti,W,Ta)C solid solution as hard phase, without adding secondary carbides in this work. The microstructures exhibited two kinds of carbide grains observable in the BSE mode: one had a dark core–gray rim structure; the other had a dark core–white inner rim–gray outer rim structure. Several (Ti,W)C white particles with high W content were also observed in the microstructures. Results showed that molybdenum has completely dissolved in the Ni binder and mainly was found in the rims of the carbide particles. As the Mo content increased, the wettability between (Ti,W,Ta)C particles and metallic phase was improved, and the shrinkage of cermets increased, the porosity decreased. Results also revealed that finer microstructures were obtained with the increase of Mo addition. But excess Mo addition caused the aggregation of particles. Mechanical properties such as hardness and TRS improved with the increase of Mo content, but the TRS of cermets reached a peak value at the 15 wt.% Mo addition, then declined with the further increase of Mo addition.     

钨中氘热脱附特性的定量研究

金属钨(W)是未来聚变堆反应环境中面向等离子体的主要候选材料之一,开展W中氢同位素输运和滞留行为的定量研究,对评估钨的服役性能及聚变堆燃料的物料平衡至关重要。本工作采用热脱附谱方法定量研究了多晶W经能量为100 eV/D、注量为3.8×10²⁴ D/m²的D⁺辐照作用后,在不同升温速率下氘的热脱附特性。研究发现,氘的热脱附量在不同升温速率下均为10²² D₂/m²量级,随着升温速率的增大,氘的热脱附峰峰位向高温方向移动。多晶W中氘的热脱附行为符合一级反应特征,钨中空位是氘在钨中的主要俘获态,D原子的热脱附能为1.04 eV。     

Hardness of hexagonal AlB2-like diborides of s,p and d metals from semi-empirical estimations

The high-level ab initio calculations of elastic moduli of materials now become a routine procedure, and the renewed attention is paid to the semi-empirical macroscopic models, providing the correlations between the Vickers hardness H_V and some elastic moduli. Such correlations, building up a bridge between Vickers hardness and elastic parameters, seem a helpful way to semi-quantitative estimations of hardness and to theoretical search of novel hard materials, based on the calculations of their elastic properties. Here, using the results of our calculations of elastic parameters of 14 hexagonal AlB₂-like diborides of s, p and d metals MB₂ (where M = Mg, Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W), we have probed six recently proposed correlations between H_V and bulk (B), shear (G), Young's moduli (E), as well as Poisson's ratio (ν), and the parameter k (the G/B ratio), and we compare the obtained results with the available experimental data. Our analysis reveals that for this family of related materials the correlations H_V ~ G, and H_V ~ E will be the most suitable approximations which enable us to roughly evaluate the Vickers hardness of diborides numerically.     

Effects of X (V, W, Mo, Hf, Ta, Zr) additions on the ideal cleavage fracture of Cr2Nb: First-principles determination

The effects of the additive elements X (V, W, Mo, Hf, Ta and Zr) on the ideal cleavage fracture of the C15 Cr₂Nb were investigated using the first-principles method. The brittle cleavage energy G_c and critical stress σ_c were calculated. The results show that V on Cr sites and X (W and Zr) on Nb sites can increase the cleavage strength of Cr₂Nb, while X (W and Mo) on Cr sites and X (Mo, Ta and Hf) on Nb sites can reduce the cleavage strength of Cr₂Nb. Our results are consistent with available experimental ones. We also find that the effect of the element W on the cleavage strength of Cr₂Nb strongly depends on its site preference in Cr₂Nb. The charge densities induced by the additive elements X were also calculated in order to reveal the origin of the effects of X on the ideal cleavage fracture of Cr₂Nb.     

Mechanical alloying of Ni3Al with molybdenum and arrangement of Mo atoms in sublattices of the intermetallic

Mechanochemical synthesis (MS) of Ni₇₀Al₂₅Mo₅ (composition 1) and Ni₇₅Al₂₀Mo₅ (composition 2) mixtures, in which 5 at % Mo substitutes for the equal amount of Ni or Al, leads to the formation of Ni-based nanocrystalline (coherent domains are ～7–12 nm in size) solid solutions; in this case, some amount of molybdenum remains free. A comparison of the lattice parameters of solid solutions, which were determined experimentally, with the magnitudes determined theoretically using Vegard law and Bozollo-Ferrante simulation, which takes into account volume modules of elasticity of elements, showed an increase in interactions between atoms composed the solid solution and the formation of regions characterized by short-range order. The heating of mechanically synthesized three-component Ni(Al, Mo) solid solutions to 720°C in a calorimeter chamber forms the ordered γ′ phase (L1₂) at T ～ 450°C. An analysis of the ratio of relative intensities of superlattice and fundamental reflections showed that, whatever the composition of initial mixture, Mo atoms always occupy positions in the Al sublattice. This arrangement of Mo atoms was confirmed by calculations of coefficients of concentrational variations of the lattice parameters. When molybdenum is added to Ni₃ Al, Mo atoms, rather than Ni atoms, complete the Al sublattice. In this case, vacancies compensate for the lack of atoms in the Ni sublattice.     

Density of thermal vacancies in γ-Ti–Al–M,M=Si,Cr, Nb,Mo, Ta or W

Modifications to alloy chemistry are often used to tailor the intrinsic flow behavior of structural materials. Models of creep in intermetallic alloys must account for the influence of chemistry on the available intrinsic creep mechanisms. As in simple metals the presence of vacancies strongly influences bulk diffusion processes in these materials. Limiting the density of constitutional and thermal vacancies by alloying may produce materials with enhanced creep properties. The energy of intrinsic and substitutional point defects in L1₀ TiAl is calculated within a first principles, local density functional theory framework. Relaxed structures and energies for vacancies, antisites and solid solutions are calculated using a plane-wave-pseudopotential method. Calculated defect energies are used within a canonical ensemble formalism to estimate the point defect densities as a function of temperature and composition. The density of vacancies is found to be sensitive to the underlying stoichiometry of TiAl. The dependence of the vacancy concentration for solid solutions of Si, Cr, Nb, Mo, Ta and W is also predicted.     

The effects of Cr,Al, Ti,Mo, W,Ta, and Cb on the cyclic oxidation behavior of cast Ni-base superalloys at 1100 and 1150°C

A series of cast nickel-base y/y′ superalloys as systematically varied at two nonzero levels of Cr, Al, Ti, Mo, Cb, W, and Ta with nominally fixed levels of 10 at. % Co, 0.06 at. % C, 0.05 at. % Zr, and 0.05% B. A 1/4-replicate 2⁷-factorial statistical design supplemented by 18 additional alloys was used to estimate main effects and all two-factor interactions. The cyclic oxidation resistance was determined from specific weight change data as a function time for 1-hr cycles in static air at 1100 and 1150°C. A derived oxidation attack parameter, K_a, was fitted by multiple linear regression over the alloy sample space at each temperature. At the levels tested, Al was the most important alloy addition. The lowest K_a was achieved at the highest (13 at. %) test level of Al and the lowest (10 and 2 at. %, respectively) test levels of Cr and Ti. The results for the refractory metal additions were somewhat ambiguous, but the most oxidation-resistant composition was estimated to be 3 at. % Mo-1 at. % Cb-3 at. % W-3 at. % Ta for the compositions tested. X-ray diffraction analysis of the alloy scales indicated that at the lowest K_a value, most oxidation resistant alloys were associated with alumina-aluminate spinel formation at the high Al levels. Higher boron contents, which were unintentionally varied, were clearly detrimental even for the most resistant compositions. High boron contents apparently trigger nonprotective NiO formation. In addition, tri-rutile type oxides were found on all test alloys, but these were not considered to be harmful to oxidation resistance.