Binary Alloy Surfaces Research Articles

Al2O3 Coatings Deposited By ALD on Al Alloys

The key question that was addressed in this work is the following: would a uniform surface layer of aluminium oxide on aluminium alloys be protective against corrosion despite the presence of intermetallic Al-Cu particles? To answer this question, well defined aluminium oxide coatings were deposited by atomic layer deposition (ALD) on the surface of a model binary aluminium-copper alloy (Al-Cu5%wt.) in which copper-rich particles were produced by ageing. The corrosion resistance of uncoated, mechanically polished aluminium alloy samples covered by a native oxide layer was compared with the one of aluminium alloy samples protected by a 20 nm thick Al2O3 layer deposited by Atomic Layer Deposition (ALD). Detailed surface analysis was performed by X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Scanning Electron Microscopy (SEM) on the samples before and after immersion in 0.01M NaCl + 0.3 % vol. H2O2 (pH=6.2). The polished Al-Cu alloy with a thin native oxide layer (around 5 nm, as measured by XPS), exhibits a copper enrichment at the oxide/Al alloy interface, clearly evidenced by ToF-SIMS depth profiling. The ToF-SIMS chemical images and EDX analysis confirmed the presence of copper-rich intermetallics (Al2Cu) in the interfacial oxide/substrate region. A short immersion (~7 min) of this alloy in a near neutral NaCl solution lead to severe surface modifications: formation of mixed aluminium-copper oxide layer rich in CuO and Cu(OH)2 (as shown by XPS), and formation of an inhomogeneous, thick aluminium oxide deposit (as shown by SEM and ToF-SIMS imaging). Localized corrosion in the vicinity of the intermetallic particles (well known as trenching around particles) is clearlyobserved by SEM and ToF-SIMS imaging. A significant improvement on corrosion resistance was observed after deposition of the 20 nm-thick Al2O3 ALD coating. Despite the presence of intermetallics at the oxide/substrate interface (observed in the SEM) no significant corrosion occurred in the chloride solution, the Al2Cu particles remained intact, and no trenching around them was observed. In conclusion, the formation of a good quality alumina layer on the Al-5%Cu alloy is beneficial for its corrosion resistance. The coating can be deposited by ALD without prior removal of the intermetallic particles.

Germanium Silicon Alloy Anode Material Capable of Tunable Overpotential by Nanoscale Si Segregation.

We developed the novel electrode that enables fine control of overpotential by exploiting surface segregation that is the enrichment of one component at the surface of binary alloy. To realize this approach, we controlled the proportion of Si with low Li diffusivity at the surface by annealing the SiGe nanowire in H2 environment at various temperatures. The resulting SiGe nanowires annealed at 850 °C exhibited high reversible capacity (>1031 mA·h·g(-1)), and long cycle life (400 cycles) with high capacity retention (89.0%) at 0.2 C. This superior battery performance is attributed to the remaining unlithiated part acting as support frame to prevent pulverization of anode material, which results from the fine-tuning of overpotential by controlling the degree of Si segregation.

Thermal oxidation of the surface of binary aluminum alloys with rare-earth metals

The kinetics of oxidation of the surface of Al alloys with 1–2.5 at % rare-earth metals (REMs) at 400–500°C in air was studied by ellipsometry and X-ray photoelectron spectroscopy (XPS). The addition (1–2.5 at % REM) of all rare-earth metals to aluminum was shown to increase the thickness of the oxide layer. The addition of surfactant and chemically active REMs (Yb, Sm, La, and Ce) increased the rate of oxidation of solid aluminum most effectively. The oxidation can be accelerated by the polymorphic transformations of the individual REM oxides in the film. The surface activity of Sm with respect to solid Al was confirmed by XRS.

Ab initio atomistic thermodynamics study on the oxidation mechanism of binary and ternary alloy surfaces.

Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.

(Corrosion Division H.H. Uhlig Award) Perspectives on Passivity and Its Breakdown

Aluminum, stainless steels, and many other metals and alloys are protected by a passive film that protects the surface from corrosion. However, in environments that contain aggressive anions such as chloride, Cl‾, the passive film becomes unstable and degrades locally causing oxide film breakdown and pitting corrosion. This presentation will discuss two aspects related to passivity and its breakdown that have been a focus of our research; the surface charge character of the oxide film (1-2) and chloride adsorption and incorporation into the oxide film (3-6). Most of the discussion will be related to passivity and its breakdown of aluminum, Al, in Cl‾ solutions.The first step in the initiation of metastable or stable pitting corrosion of Al is the adsorption of Cl‾ on the surface of the passive oxide. Chloride adsorption by the oxide film has been primarily discussed in terms of surface attractive forces. The isoelectric point (IEP) or pH of zero charge (pHpzc) of an oxide defines the surface charge character as a function of pH. The pHpzc of Al is ~ 9 in aqueous solutions. Thus, at pH values less than 9, which is the case for most natural water systems, the surface charge of Al is positive and this leads to the attraction of negative ions such as Cl‾ to the surface. The relationship between the pitting potential value of oxide-covered Al and the pHpzc, of its oxide was studied by ion implanting aluminum (99.999% purity) with elements whose oxides were of known pHpzc (1-2). The results showed that all the implanted ions chosen because of the low pHpzc of their oxides, i.e. lower than that of Al, (Si, Cr, Zr, Nb, Ta, and Mo) produced binary surface alloys that had higher (more positive) pitting potential values than Al. Conversely, the implanted ions chosen because of the high pHpzc of their oxides relative to Al (Li and Zn) produced binary surface alloys that had pitting potential values that were lower than that of Al. The implantation of Al into Al had no effect on the pitting potential, indicating that the increased and decreased pitting potential values observed for the other implant ions were chemical in nature and not a result of the ion implantation process.The second aspect of this presentation will discuss Cl‾ adsorption and incorporation into the oxide film. X-ray absorption spectroscopy (XAS), specifically X-ray absorption near edge spectra (XANES), and X-ray photoelectron spectroscopy (XPS) were used to investigate Cl‾ adsorption and incorporation into the oxide film on aluminum (3-6). Samples were polarized at selected potentials below the pitting potential in deaerated 0.1M NaCl solutions. XANES and XPS spectra both showed the presence of two distinct Cl‾ species associated with the oxide film on Al for samples that were polarized below the pitting potential. Chloride was found to be present as an adsorbed species at the surface, as well as an incorporated species within the passive oxide. Acknowledgements The author gratefully acknowledges the Naval Research Laboratory and the Office of Naval Research for financial support of this work. The author also gratefully acknowledges his colleagues especially the co-authors of the cited papers without whose contributions this work would not have been possible.

Theoretical Methodology for Studying Oxygen Reduction Reaction (ORR) on Disordered Binary Alloy Surfaces

ABSTRACTThe catalytic activity of disordered binary alloy metal surfaces is investigated for the oxygen reduction reaction (ORR) by generating free energy diagrams and performing calculations on d-band centers of alloys. The disorder was simulated using virtual crystal approximation; then, based on periodic, self-consistent density functional theory (DFT) methods, we calculated adsorption energies of reaction intermediates. Alternative pathway for ORR mechanism, involving proton/electron transfer to adsorbed oxygen and hydroxyl, is considered. The methodology was applied to (111) surface of PdxCu1-x disordered binary alloys, with different values of x concentration. This study found that at the ORR equilibrium potential of 1.23 V, the reactivity of all surfaces is shown to be limited by the rate of OH removal from the surface. Among the surfaces studied, the surface of Pd0.80Cu0.20 shows the highest reactivity and is more active than other non-Pt alloys. These results are in excellent agreement with earlier experimental and theoretical work.

Development and Validation of a ReaxFF Reactive Force Field for Fe/Al/Ni Alloys: Molecular Dynamics Study of Elastic Constants, Diffusion, and Segregation

We have developed a ReaxFF force field for Fe/Al/Ni binary alloys based on quantum mechanical (QM) calculations. In addition to the various bulk phases of the binary alloys, the (100), (110) and (111) surface energies and adatom binding energies were included in the training set for the force field parametrization of the Fe/Al/Ni binary alloys. To validate these optimized force fields, we studied (i) elastic constants of the binary alloys at finite temperatures, (ii) diffusivity of alloy components in Al/Ni alloy, and (iii) segregation on the binary alloy surfaces. First, we calculated linear elastic constants of FeAl, FeNi(3), and Ni(3)Al in the temperature range 300 to 1100 K. The temperature dependences of the elastic constants of these three alloys, showing a decrease in C(11), C(12), and C(44) as temperature increases, were in good agreement with the experimental results. We also performed ReaxFF molecular dynamics (MD) simulations for Al or Ni diffusion in the system modeled as Al/Ni mixed layers with the linear composition gradients. At 1000 K, Al diffusivity at the pure Al end was 2 orders of magnitude larger than that in the Al trace layers, probably explaining the nature of different diffusion behavior between molten metals and alloys. However, the diffusivity of Ni at the pure Ni end was only slightly larger than that in the Ni trace layers at the system temperature much lower than the melting temperature of Ni. Third, we investigated the surface segregation in L1(2)-Fe(3)Al, Fe(3)Ni, and Ni(3)Al clusters at high temperature (2500 K). From the analysis of composition distribution of the alloy components from the bulk to the surface layer, it was found that the degree of segregation depended on the chemical composition of the alloy. Al surface segregation occurred most strongly in Fe(3)Al, whereas it occurred most weakly in Ni(3)Al. These results may support the segregation mechanism that surface segregation results from the interplay between the energetic stability of the ordered bulk phase and the surface reconstruction. In addition, the surface segregation induced the depletion layers of segregating metal species (Al in Fe(3)Al and Ni(3)Al, and Ni in Fe(3)Ni) next to the segregation layers. These simulation results qualitatively agreed with early experimental observations of segregation in Fe/Al/Ni binary alloys.

Ab initio based multiscale modeling of alloy surface segregation

A fully integrated ab initio based multiscale model for analysis of segregation at alloy surfaces is presented. Major components of the model include a structure-energy analysis from the first-principles density functional theory (DFT), a Monte Carlo/molecular dynamics (MC/MD) hybrid simulation scheme for atomic transport, and a reactive force field formalism that binds the two. The multiscale model accurately describes the atomic transport processes in a multi-component alloy system at finite temperature, and is capable of providing quantitative predictions for surface compositions. The validity of the model was demonstrated by investigating the temperature-dependent segregation behavior of B2 FeAl binary alloy surfaces with a detailed description of the segregation mechanism. Based on the model’s prediction capabilities, potential extension of the model to the analysis of systems undergoing rapid chemical reactions is discussed.

Atomic structure of two-dimensional binary surface alloys: Si(111)-√21 × √21 superstructure

The atomic structures of Au and Ag co-adsorption-induced √21×√21 superstructure on a Si(111) surface, i.e., (Si(111)-√21×√21-(Au, Ag)), where the Si(111)-5×2-Au surface is used as a substrate, have been investigated using reflection high-energy positron diffraction (RHEPD) and photoemission spectroscopy. From core-level spectra, we determined the chemical environments of Ag and Au atoms present in the Si(111)-√21×√21-(Au, Ag) surface. From the rocking curve and pattern analyses of RHEPD, we found that the atomic coordinates of the Au and Ag atoms were approximately the same as those of the Au and Ag atoms in other Si(111)-√21×√21 surfaces with different stoichiometries. On the basis of the core-level and RHEPD results, we revealed the atomic structure of the Si(111)-√21×√21-(Au, Ag) surface.

Formation of Irregular Al Islands by Room-Temperature Deposition on NiAl(110)

ABSTRACTSTM studies reveal that irregular non-equilibrium two-dimensional Al islands form during deposition of Al on NiAl(110) at 300 K. These structures reflect the multiple adsorption sites and diffusion paths available for Al adatoms on the binary alloy surface, as well as the details of inhibited edge diffusion and detachment-attachment kinetics of Al adatoms for numerous distinct step edge configurations. We attempt to capture these features by multi-site lattice-gas modeling incorporating DFT energetics for adatoms both at adsorption sites and transition states. This formulation enables description and elucidation of the observed island growth shapes.

Self-assembly of metal nanostructures on binary alloy surfaces

Deposition of metals on binary alloy surfaces offers new possibilities for guiding the formation of functional metal nanostructures. This idea is explored with scanning tunneling microscopy studies and atomistic-level analysis and modeling of nonequilibrium island formation. For Au/NiAl(110), complex monolayer structures are found and compared with the simple fcc(110) bilayer structure recently observed for Ag/NiAl(110). We also consider a more complex codeposition system, (Ni + Al)/NiAl(110), which offers the opportunity for fundamental studies of self-growth of alloys including deviations for equilibrium ordering. A general multisite lattice-gas model framework enables analysis of structure selection and morphological evolution in these systems.

A multiscale modelling study of Ni–Cr crack tip initial stage oxidation at different stress intensities

In this study, different scale analyses have been performed to understand the very beginning stage of the Ni–Cr(1 1 1) binary alloy surface oxidation mechanism. The finite elements method (FEM) has been applied to find the stress intensity effect on a compact tension (CT) specimen crack tip, and a 2-μm rectangular region has been prepared for the quasi-continuum (QC) model. The displacement load calculated by FEM was applied at the upper and lower boundaries of the QC model. The obtained atomic positions of the deformed crystal structure were considered for quantum chemical molecular dynamics (QCMD) analysis by placing water molecules on the surface. Water molecules dissociate on the top site followed by the quick diffusion of hydrogen atoms on the surface through the hollow sites. Hydrogen is easily transported into the lattice due to its small atomic size. Increases in stress intensity factor ( K) act to deform the structure, which augments oxygen penetration and reduces hydrogen diffusion time; as a consequence, changes in K result in an increase of metallic surface oxidation. Oxygen preferentially bonds with chromium to develop a protective passive film on the surface, and the quickly diffused hydrogen is trapped on the metal surface. Hydrogen bonds with the metal by receiving electrons and, therefore, works as an oxidant on the surface and weakens the metal atomic bond at the primary stage. Initially, the oxidized surface forms a strong bond with oxygen, resulting in the breakage of metal–metal bonds. Therefore, this hydrogen action enhances the very early stage surface oxidation.

The initial oxidation of Al–Mg alloys: Depth-resolved quantitative analysis by angle-resolved x-ray photoelectron spectroscopy and real-time in situ ellipsometry

Real-time in situ spectroscopic ellipsometry and in situ, angle-resolved x-ray photoelectron spectroscopy (AR-XPS) have been applied to establish the relationships between the growth kinetics and the developing microstructure of ultrathin (<3 nm) oxide films grown on clean Al–1.12 at. % Mg alloy surfaces by thermal oxidation in the temperature range T=300–485 K. To this end, procedures for the depth-resolved quantitative AR-XPS analysis of ultrathin, multiple-element, and/or multiphase oxide overgrowths on binary alloy surfaces were developed. As a result, the relationships between the relative depth distributions of Al and Mg in the developing oxide films, the oxide-film growth kinetics, and the oxidation-induced compositional changes in the AlMg alloy substrate could be established as a function of the oxidation conditions.

Mixing and magnetic properties of surface alloys: The role of the substrate

In this paper, we have performed ab initio density functional theory calculations to compare the miscibility and magnetic properties of two-dimensional binary surface alloys of the form M x N 1− x (M = Fe or Co; N = Pt, Au, Ag, Cd or Pb) on two different substrates – Rh(1 1 1) and Ru(0 0 0 1). The trends in miscibility for the two substrates are found to be strikingly similar. The magnetic moments show qualitatively similar behavior, but their magnitudes differ: surface alloys on Rh(1 1 1) have larger magnetic moments than on Ru(0 0 0 1). We infer that strain plays the determining role in stabilizing these two-dimensional alloys, whereas the differences in magnetic moments can be ultimately attributed to the different number of d-electrons in Rh and Ru.

Study of the segregation phenomena on the surface of binary alloys and steels in oxygen environment

The elemental composition of the surfaces of Pd25V75, Pd44V56, and Cr20Ni80 binary alloys and 40Kh13 instrumental steel exposed for a long time in air has been investigated by X-ray photoelectron spectroscopy (SPECS system, monochromatic radiation AlKα line). The composition of the samples was also investigated as a function of the heating temperature. The features of the dependences of the concentrations of the main components of binary metal systems on the sample processing temperature are explained within the segregation theories of ternary systems of highly diluted solutions.

Kinetics of phase transformations in a binary alloy surface layer at the selective dissolution. II. Ag-Au|Ag+* system

Based on the theoretical investigation [1], the conditions of phase transformation of gold on Ag-Au alloy surfaces, which are selectively dissolved in a nitrate environment at the overcritical anodic polarization, are experimentally found. Kinetic regularities of the formation of Au° individual gold phase on the alloy surface depending on the alloy composition and overpotential, as well as on the presence of surface active organic substances in the solutions and the passivating alloying additives in the alloy are determined.

Combinatorial Density Functional Theory-Based Screening of Surface Alloys for the Oxygen Reduction Reaction

A density functional theory (DFT) -based, combinatorial search for improved oxygen reduction reaction (ORR) catalysts is presented. A descriptor-based approach to estimate the ORR activity of binary surface alloys, wherein alloying occurs only in the surface layer, is described, and rigorous, potential-dependent computational tests of the stability of these alloys in aqueous, acidic environments are presented. These activity and stability criteria are applied to a database of DFT calculations on nearly 750 binary transition metal surface alloys; of these, many are predicted to be active for the ORR but, with few exceptions, they are found to be thermodynamically unstable in the acidic environments typical of low-temperature fuel cells. The results suggest that, absent other thermodynamic or kinetic mechanisms to stabilize the alloys, surface alloys are unlikely to serve as useful ORR catalysts over extended periods of operation.

Kinetics of phase transformations in a binary alloy surface layer at the selective dissolution. I. Theoretical analysis

A procedure of estimating the formation rate of the individual phase of an electrochemically positive component B0 at the anodic selective dissolution or corrosion of an A-B alloy in a potential range above the critical potential of the development of a morphologically unstable surface is proposed and theoretically founded.

Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces

The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichment of Pt atoms in the surface layer and of Pd atoms in the first subsurface layer has been found in Pt-rich PtPd alloy models, despite the lower surface energy of pure Pd compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr occupy preferentially surface sites due to their lower electronegativity and thus stronger oxygen affinity compared to Pt and Co, respectively. In either cases, the calculated oxygen adsorption energies on the alloy surfaces are larger than on the pure components when the more noble components are present in the subsurface layers.

Metastable magnetic configurations of (V, Cr, Mn and Fe) on Ir(0 0 1) surfaces

We present a theoretical study of the electronic and magnetic structure of the 3d-transition metals (M = V, Cr, Mn and Fe) in several overlayer systems. The electronic as well as magnetic structures are investigated for pseudomorphic overlayers (M/Ir(0 0 1)), ordered alloyed overlayers of the type M 0.5Ir 0.5/Ir(0 0 1) and ordered binary surface alloys of V, Cr, Mn and Fe transition metals on Ir(0 0 1) substrates. The calculations are performed with a self-consistent tight-binding method using the unrestricted Hartree-Fock approximation within the Hubbard model. We obtained metastable c(2 × 2) configurations for V, Cr and Mn and a p(1 × 1) configuration for Fe pseudomorphic overlayers. However, ferrimagnetic configuration has been obtained for the ordered surface alloys M 0.5Ir 0.5 and the binary alloyed overlayers on Ir(0 0 1) surfaces.