Changes in the potential and vibrational energies upon the formation of vacancies in oxygen-doped cores of crystallite-conjugation regions of polycrystalline 4d and 5d BCC transition metals

The changes in the potential (u _cmpl) and vibrational (?_cmpl) energies and the signs of changes in the interatomic spacings (Δa _cmpl) upon the formation of vacancies in oxygen-alloyed (OA) cores of crystallite-conjugation regions (CCR) in polycrystalline 4d and 5d transition metals Mo, Ta, and W have been determined. The potential energy upon the formation of vacancies (upon the formation of vacancy complexes with oxygen atoms—vacO complexes) in the OA cores of CCRs in polycrystalline Mo, Ta, and W increases (u _cmpl are positive), as upon the formation of vacancies in pure CCR cores of transition bcc d metals. The vibrational energy upon the formation of vacancies in OA CCR cores in polycrystalline Mo, Ta, and W decreases (?_vacO are negative). The negative sign of changes in the vibrational energy upon the formation of vacancies in the OA CCR cores in Mo, Ta, and W (?_vacO < 0) agrees with the independent determinations of the sign of changes in the vibrational energy in the OA CCR cores in polycrystalline tungsten (?_vacO)_W using Mössbauer measurements of the Debye temperature. The signs of changes in the interatomic spacings upon the formation of vacancies in the OA CCR cores in polycrystals of Mo, Ta and W are negative (Δa _VacO < 0), in contrast to positive (0 < Δa _BCC) changes in the interatomic distances in the nearest neighborhood of vacancies formed in pure CCR cores in Mo, Ta, and W.     

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.     

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.     

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.     

储氚合金ZrCo0.8M0.2 (M = Co, Cu, Cr, Mn, Al)的性能研究

采用电弧熔炼的方法在氩气气氛中熔炼了ZrCo_0.8M_0.2 (M = Co、Cu、Cr、Mn、Al)合金。合金的主相均为ZrCo相,但Cr、Mn和Al部分替代Co后形成了第二相。Cr替代形成了Zr₂Co和ZrCr₂相,Mn替代形成了Zr₂Co和ZrMn₂相,Al替代形成Zr₃Co和Zr₆CoAl₂相。Cr和Mn的替代使合金的晶胞体积减小,而Cu和Al的替代使晶胞体积增大。Cu、Cr、Mn和Al替代后,ZrCo_0.8M_0.2合金的平台压变化不明显,但吸氢量出现了不同程度的降低。Cr和Mn元素的替代明显改善了ZrCo合金在室温下的活化性能。Cr和Mn元素的替代降低了高温下ZrCo合金发生歧化反应的速率,这主要是由于Cr和Mn元素的掺杂减少了氢原子占据不稳定位置8f₂ 和8e的数量,从而降低了歧化反应发生的驱动力。     

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.     

The influence of doping by transition metal elements on the vacancy formation energy in Fe-Al <Emphasis Type="Italic">B</Emphasis>2 phase

Based on ab initio calculations for Fe-Al B2 phase with and without the vacancies, the authors studied the influence of alloying by Ni, Cr, and V on the energy of vacancy formation. The authors predict that the presence of these alloying elements in the vicinity of the vacancy on the Fe site increases the energy of the vacancy formation. The present article discusses the changes of the electron charge redistribution in the vicinity of the vacancy in Fe-Al alloyed with these transition metals and link the preference of their site occupation with the peculiarities of the electron density redistribution. In addition, the authors report on the results of calculations of different types of antisite defects in Fe-Al and show that the formation of the antisite on the Al site is the energetically preferable configuration for this defect. The obtained results are in agreement with the experimental data.     

Effect of the magnetism of impurities on their diffusion in metals: Bulk diffusion of iron,cobalt, and rhodium in iridium single crystals

The coefficients and parameters of the temperature dependences of the coefficients of bulk diffusion of Fe, Co, Rh, and Au atomic probes (APs) in iridium single crystals (mono-Ir) have been determined from the diffusion profiles obtained using secondary-ion mass spectrometry of the diffusion zones. The enthalpies of activation of diffusion of Fe, Co, and Rh APs are considerably lower than the enthalpy of activation of selfdiffusion in mono-Ir. This is caused by the negative contributions of the intraatomic exchange energy and energy of relaxation of the environment of the d transition APs to the enthalpy of interaction of magnetically active APs with the vacancies in the iridium lattice. The interaction energy of partners in such complexes and the relationships between the magnetic moments of d transition APs in complexes with vacancies have been estimated. The Rh APs in complexes with vacancies in iridium possess stable magnetic moments.     

First-principles study on the lattice stability of elemental Co,Rh, and Ir in the VIIIB group

Lattice constants, total energies, and densities of state of transition metals Co, Rh, and Ir in the VIIIB group with different crystalline structures were calculated via generalized gradient approximation (GGA) of thetotal energy plane wave pseudopotential method in first-principles. The lattice stabilities of Rh and Ir are ΔG ^bcc-hcp > ΔG ^fcc-hcp > 0, agreeing well with those of the projector augmented wave method in first-principles and the CALPHAD method in spite of elemental Co. Analyses of the electronic structures to lattice stability show that crystalline Rh and Ir with fcc structures have the obvious characteristic of a stable phase, agreeing with the results of total energy calculations. Analyses of atomic populations show that the transition rate of electrons from the s state to the p or d state for hcp, fcc, and bcc crystals of Co and Rh increases with the elemental period number to form a stronger cohesion, a higher cohesive energy, or a more stable lattice between atoms in heavier metals.     

The formation of interstitial solid solutions based on solvents showing the fcc structure: elastic versus chemical interaction

Unit cell parameters and densities were measured for the terminal solid solutions of the late transition metals showing the fcc structure (α-Co, Rh, Ir, Ni, Pd, Pt) with boron and partly with carbon. Especially the solid solutions Pd(B) and Pd(C) were investigated because of the large homogeneity range up to the atomic fractions x^f_B=0.186 and x^f_C≃0.05, respectively. The unit cell parameter increases and the density decreases with increasing boron and carbon content. Comparison of the calculated and measured number of atoms in the Pd(B) and Pd(C) unit cells confirms the interstitial nature of both solid solutions. The difference between the measured dependence of the Pd–B shortest interatomic distances on the solute content and those calculated according to the hard sphere model indicates a distinct negative deviation from additivity of the pure form volumes. The chemical (electronic) interaction plays a more important role than the elastic interaction (characterized by a size factor) in the formation of solid solutions of metals with the interstitially dissolved metalloids H, B, C, N and O.     

5d过渡金属在Ni3Al中掺杂效应的第一性原理研究

5d过渡金属Hf、Ta、W、Re、Ir被广泛地应用于第4、5代镍基单晶高温合金中,但对于其机理却没有系统的理论研究。采用基于密度泛函理论(density functional theory)的第一性原理平面波超软赝势方法研究5d过渡金属Hf、Ta、W、Re、Ir掺杂镍基单晶高温合金γ′-Ni3Al相前后系统的晶格常数、形成热、结合能、态密度、差分电荷密度及电荷布局。计算结果表明:5d过渡金属Hf、Ta、W、Re、Ir掺杂Ni3Al系统后有优先占据Al原子位置的倾向,且与周围的Ni 3d电子和Al 3p电子发生强烈的轨道杂化,使电子被束缚,离域性变小,峰变窄;掺杂前系统中主要是Ni原子与最近的Al原子之间的共价键作用,掺杂后系统中主要是Ni原子与最近的X原子(Hf、Ta、W、Re、Ir)之间的共价键作用,且随原子序数的增大共价键逐渐增强。     

Control of crystallinity in sputtered Cr–Ti–C films

The influence of Ti content on crystallinity and bonding of Cr–Ti–C thin films deposited by magnetron sputtering have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy. Our results show that binary Cr–C films without Ti exhibit an amorphous structure with two non-crystalline components; amorphous CrC_x and amorphous C (a-C). The addition of 10–20 at.% Ti leads to the crystallization of the amorphous CrC_x and the formation of a metastable cubic (Cr_1?xTi_x)Cy phase. The observation was explained based on the tendency of the 3d transition metals to form crystalline carbide films. The mechanical properties of the films determined by nanoindentation and microindentation were found to be strongly dependent on the film composition in terms of hardness, elasticity modulus, hardness/elasticity ratio and crack development.     

Improving metal dusting resistance of. transition‐metals and Ni‐Cu alloys

The present study focuses on a new technique for the prevention of metal dusting in carbonaceous gas environments at intermediate temperature. Preliminary laboratory metal dusting test was conducted for transition‐metals and Ni‐x%Cu binary alloys in a simulated 60%CO‐26%H₂‐11.5%CO₂‐2.5%H₂O (in vol.%) gas mixture at 650°C for 100 h. The metal dusting caused no coke deposition on transition‐metals of Cu, Ag, and Pt, while those of Fe, Co, and Ni have a large amount of coke and lost mass. Whether or not coking behavior of Ni‐Cu binary alloys formed any oxide scales in the simulated gas environment depended on the Cu content. Specimens containing low Cu were entirely covered with coke and showed rough metal surfaces due to the degradation of metal. Alloys of 20% and more Cu, on the contrary, had no coke deposition and smooth metal surfaces, suggesting alloys with an adequate Cu do not react with CO in the gas mixture without an oxide scale barrier. Based on these results, we conclude that Cu does not protect by formation of the oxide scale but has a “Surfactant‐Mediated Suppression” against metal dusting. This effect can be explained in terms of atomistic interaction of CO with transition‐metal surfaces by electronic structure analyses. The concept can be also useful for the practical material design of Ni‐Cr base alloy with excellent metal dusting resistance.     

空位对Cu/Sn无铅焊点界面元素扩散的影响

界面柯肯达尔空洞形成的过程伴随着空位的形成与扩散,对空位行为的研究有利于深入理解界面扩散和空洞形成过程. 运用分子动力学方法模拟Cu/Cu₃Sn界面上空位对扩散的影响,计算空位形成能、扩散势垒及空位扩散激活能. 结果表明,相同条件下含空位的模型发生扩散的几率要高于不含空位的模型. 另外,计算表明铜晶体的空位形成能大于Cu₃Sn晶体中铜空位的形成能;Cu₃Sn晶格中不同晶位的Cu空位(Cu1空位和Cu2空位)的形成能比较接近,但均小于锡的空位形成能. 此外,对Cu/Cu₃Sn界面的空位扩散势垒及空位扩散激活能的计算结果表明,Sn原子的空位扩散激活能高于Cu原子.     

Local melting in silicon driven by retrograde solubility

High point-defect enthalpy of formation leads to retrograde solubility for a number of solutes in silicon, especially d-shell transition metals. We present direct experimental evidence for a thermodynamic pathway leading to local retrograde melting in silicon driven by the retrograde solubility of low-concentration metallic solutes at temperatures above the bounding liquid-silicide–silicon invariant reaction. We experimentally demonstrate this local melting pathway using in situ synchrotron-based X-ray microprobe measurements of silicon supersaturated with nickel, wherein solute precipitation into liquid droplets is observed. No significant energy barrier is observed for the nucleation of the liquid droplets at the surface, suggesting that in the Ni–Si system, retrograde melting will occur upon supersaturation if favorable heterogeneous nucleation sites are available.     

Electronic and crystal structure and bonding in Ti-based ternary solid solution nitrides and Ti-Cu-N nanocomposite films

Complex transition metal nitrides (TMN) have lately gained special attention in an effort to improve the properties of their binary counterparts. In this work we review a very wide range of binary and ternary transition metal nitrides of the form: Ti_xTM_1 − xN (TM = Zr,Hf,Nb,Ta,Mo,W) over the whole composition x range (0 < x < 1) grown by Pulsed Laser Deposition (PLD), Dual Ion Beam Sputtering (DIBS) and Magnetron Sputtering (MS). We identify the bonding mechanism and, despite the possible different valence electron configuration of the constituent elements, we show that TMNs are completely soluble to each other due to the hybridization of the d and sp electrons of the metals and nitrogen, respectively. Optical absorption bands are manifested due to the N-p → Me-d interband transition and the t_2g → e_g transition due to splitting of the metals’ d band, proving the partial ionic character of the bonds in TMNs. In addition, we consider the growth of Ti-Cu-N nanocomposites by PLD and MS. We investigate the difference in the atomic structure and bonding in both cases of ternary and nanocomposite nitrides, for sequential deposition of Ti and TM or Cu (achieved in PLD) or simultaneous deposition (achieved in DIBS, MS) and get insights for the kinetic effects.     

Determining the minimum grain size in severe plastic deformation process via first-principles calculations

Although the stacking fault energy (SFE) is a fundamental variable determining the minimum grain size (d_min) obtainable in severe plastic deformation (SPD) processes, its accurate measurement is difficult. Here we establish the SFEs of binary Pd–Ag, Pd–Cu, Pt–Cu and Ni–Cu solid solutions using the axial interaction model and the supercell model in combination with first-principles theory. The two models yield consistent formation energies. For Pd–Ag, Pd–Cu and Ni–Cu, the theoretical SFEs agree well with those from the experimental measurements. For Pt–Cu no experimental results are available, and thus our calculated SFEs represent the first reasonable predictions. We discuss the correlation of the SFE and d_min in SPD experiments and show that the d_min values can be evaluated from first-principles calculations.     

Effect of magnetism on the solubility of 3d elements in BCC iron: Results of first-principle investigations

The methods of quantum-mechanical simulation have been used to study alloys of bcc iron with 3d transition metals in the ferromagnetic and paramagnetic states. It has been shown that the main factor that determines the solubility of the 3d elements is their electronic structure. The energy of the solution, mixing, and effective interatomic interactions vary regularly depending on the position of the element in the Periodic Table and on the magnetic state of the matrix. In some cases, depending on the magnetic state, changes in these quantities lead to the violation of the Hume-Rothery rules that determine the solubility of substitutional elements in alloys. The results obtained help us to understand the microscopic mechanisms that determine the solubility of alloying elements and their effect on the phase stability and structural state of steels.     

On the ω phase formation in Cr–Al and Ti–Al–Cr alloys

The interaction between Al and the transition metals Ti and Cr on the stability of the ω phase in metastable β-based structures was studied. Alloys were quenched from the melt to retain at room temperature a metastable β phase (B2 structure), which is stable at high temperatures. The structural study of the ω phase was carried out by correlating the deviation of ω structure from the ideal ω phase to the compositions of the parent β phase. Deviation of ω structures from the ideal one was related to the electron concentration of the parent β phase. A diffuse ω structure is reported in the Cr₂Al phase (C11_b structure) for the first time. The results are consistent with our previous suggestions that Al stabilises the ω phase in transition metals by lowering the spatial conduction electron concentration in the parent β phase and by enhancing p–d hybridisation of valence electrons. In the ternary Ti–Al–Cr alloys, prolonged annealing of the Ti–30Al–10Cr and Ti–20Al–10Cr alloys at 450°C led to the formation of two types of ordered crystalline ω structure.     

Some aspects in designing passive alloys with an enhanced corrosion resistance

The technical and biomedical use of alloys containing reactive metals in aggressive environments is possible due to the spontaneous formation of thin oxide layers which slow down corrosion reactions by several orders of magnitude. Oxide films of important technical metals like Cr and Cu are cation semiconductors, and therefore they are electrochemically unstable and susceptible to anodic decomposition in the contact with electrolyte solutions. Alloying with elements that easily undergo oxidation and formation of highly charged cations, which interact electrostatically with the mobile cation vacancies, results in an enhanced electronic conductivity of their oxide films.This is an effective way how to design an alloy with significantly greater corrosion resistance in aggressive environments in comparison with its constitutive elements. The present paper describes several selected systems investigated under conditions of their real application.