Near-surface Region Research Articles

Redistribution of carbon and residual stress in low-temperature gaseous carburized austenitic stainless steel during thermal and mechanical loading

The effect of thermal annealing and mechanical loading on the redistribution of carbon- and residual stress-depth profiles in low-temperature carburized 316 L austenitic stainless steel was investigated. Gaseously carburized 316 L austenitic stainless steel was strained with a custom-built four-point bending device and isothermally annealed at temperatures in the range 300–500 °C for up to 200 h. Experimental results show that the carburized case is stable at 300 °C and that carbides precipitate only at the highest temperature investigated. At temperatures above 300 °C, the carbon atoms in the carburized case continue to diffuse inwardly, causing an increase of the case depth. Consequently, the surface concentration of carbon, the surface hardness and compressive residual stress change with time. Four-point bending during isothermal annealing accelerates the diffusion-controlled redistribution of carbon over the carburized case for both tensile and compressive side of the strained specimen. Since the carbon-induced volume expansion of austenite during carburizing is accommodated elasto-plastically, a substantial reduction of the carbon content in the surface-near region leads to a change from compressive to tensile residual stresses in the surface region.

Characterization of a coarse synthetic silver ore as a model ore material for heap leaching research

Leaching and micro-focus X-Ray computed tomography (µXCT) characterization studies were performed on a synthetic silver ore prepared from cement mixtures with embedded silver grains. This was done with the aim to understand the leaching behaviour of coarse ore particles in laboratory-scale heap leaching experiments. The synthetic ore was developed to possess specific ideal properties including uniform grain dispersion and size, homogenous porosity, and even grade distribution, and the “homogenized ore” was evaluated for these characteristics. The study validated that the ore had achieved its homogenized properties; however, the inner-particle pores were not found to be continuous at the µXCT detection limit of 4.8 µm. Poor extraction from the coarse particles in the leaching investigation suggested the pore network was not well established and may exist only at the sub-micron scale. Coarse particle leaching extraction data was analysed with the diagnostic shrinking core and the extended mixed topology models, which demonstrated that a diffusion–reaction mechanism governed the dissolution extraction from the larger particles. The results showed that leaching was restricted to the near-surface regions, but did not extend to the inner core, resulting in low extraction. Coarse synthetic particle leaching under controlled conditions could be utilized as a potential deterministic technique for understanding the reaction performance in coarse particle heap leaching.

Dynamic deuterium retention properties of tungsten measured using laser-induced breakdown spectroscopy

Dynamic deuterium (D) retention properties of tungsten (W) are investigated under steady-state D plasma exposure in the PISCES-A linear plasma device. In contrast to static retention, dynamic retention is quickly released from the material at the termination of the incident plasma flux, and thus in-operando laser-induced breakdown spectroscopy (LIBS) measurements have been conducted during steady-state D plasma exposure. A procedure is, first, established to extract the dynamic retention component from the in-operando LIBS D I 656.1 nm line intensity, which can contain contributions from dynamic and static retention as well as background D/D2 gas, excited by both steady-state and laser-induced plasmas. Using the developed procedure, the dynamic D retention in W is systematically examined while scanning the following plasma exposure parameters: incident ion energy, E i, sample temperature, T s, incident ion flux, Γi. No clear E i dependence is seen in the range of E i ∼ 45–175 eV, as expected from a small variation in the average implantation depth (∼3–6 nm) of D in W. The dynamic retention is found to monotonously decrease with increasing T s from 348 to 573 K, while the total static retention is reported to peak at T s ∼ 500–600 K. It is revealed that the dynamic retention linearly increases with increasing Γi, and then saturates at Γi ⩾ 0.75 × 1021 m−2 s−1. Possible physical mechanisms for the observed dependence of the dynamic retention on T s and Γi are discussed. Furthermore, sequential pure He, followed by pure D, plasma exposures show that the dynamic D retention is not strongly affected by He bubbles in the near-surface region.

Activating Inert Surface Pt Single Atoms via Subsurface Doping for Oxygen Reduction Reaction.

The performance of single-atom catalysts strongly depends on their particular coordination environments in the near-surface region. Herein, we discover that engineering extra Pt single atoms in the subsurface (Ptsubsurf) can significantly enhance the catalytic efficiency of surface Pt single atoms toward the oxygen reduction reaction (ORR). We experimentally and theoretically investigated the effects of the Ptsubsurf single atoms implanted in different positions of the subsurface of Co particles. The local environments and catalytic properties of surface Pt1 are highly tunable via Ptsubsurf doping. Specifically, the obtained Pt1@Co/NC catalyst displays a remarkable performance for ORR, achieving mass activity of 4.2 mA μgPt-1 (28 times higher than that of commercial Pt/C) at 0.9 V versus reversible hydrogen electrode (RHE) in 0.1 M HClO4 solution with high stability over 30000 cycles.

EFFECT of surface treatment on corrosion control of P91 hydrothermal aqueous solutions

ABSTRACT The proven efficacy of surface treatment on ferritic-martensitic (F/M) steels in subcritical and supercritical water environments has spawned interest in its application for the construction of biomass hydrothermal liquefaction (HTL) conversion reactors. The objective of this paper is to determine the relative extent to which well-established surface treatments, including mechanical abrasion (baseline), sandblasting, shot peening, and pickling, affect corrosion of the steels in a simulated HTL biomass conversion environment. These treatments were administered to P91 steel for the purpose of improving corrosion resistance towards that exhibited by Type 304 austenitic stainless steel. Samples were immersed in a simulated HTL conversion environment at 310 °C and 10 MPa for 10 days using a static autoclave testing system. None of the surface treatments administered improved corrosion control of P91. The structure and chemistry of the oxide scale formed during immersion were not altered by the surface treatments administered, i/e., none of which promoted of the formation of a more protective Cr-enriched scale. Shot peening was particularly problematic as it promoted the formation of Cr-rich carbides in the near-surface region, which, when incorporated into the oxide scale that formed during high temperature immersion, served to reduce the amount Cr available for oxidization.

Hot corrosion and internal spallation of laser-cladded inconel 625 superalloy coatings in molten sulfate salts

Hot corrosions of laser-cladded inconel 625 in molten salt of Na2SO4-MgSO4 mixture have been studied at 900 °C to address its anticorrosion behaviors. Elemental and microstructural characterizations provide direct evidence for the formation of MgO/Cr2O3 external layers and the outward-diffusion of Cr atoms. The Cr-depletion promotes precipitation of δ-Ni3(Nb/Mo), nucleation and accumulation of dislocations, pores, and internal spallation of the corroded alloy. They also provide evidence for the diffusion of sulfur into the near-surface region, leading to the formation of internal sulfide scales (i.e., Nb4−xCrxS6, CrS, and MnCr2S4). Grain boundaries play important roles in the atomic diffusions and sulfide formations.

Sputtering of Oxides from LaNi5 Surface

The changes in chemical composition of the intermetallic alloy LaNi5 surface monolayers were studied using secondary ion mass spectrometry (SIMS) in the process of the alloy interaction with oxygen. The investigated samples were pellets made by pressing the fine-grained LaNi5 alloy. Ar+ ions having energies of 10-18 keV were used as primary ions. The primary beam current density was 9-17 μA·cm-2, which corresponds to the dynamic SIMS mode. The emission intensities of secondary ions were measured within the dynamic range of at least 6 orders of magnitude. Before the measurements, the samples were annealed in residual vacuum at a temperature of ~ 1000 K. After the annealing, the sample surface was cleaned using the primary ion beam until the mass-spectrum composition and secondary ion emission intensity stabilized completely. The gas phase composition was monitored using a gas mass spectrometer. The conducted studies showed that a complex chemical structure including oxygen, lanthanum, and nickel is formed on the surface and in the near-surface region of LaNi5 as a result of its exposure to oxygen. Oxygen forms strong chemical bonds in such a structure with both components of the alloy. This is evidenced by the presence of a large set of oxygen containing emissions of positive and negative secondary ions with lanthanum, with nickel, and oxygen containing lanthanum-nickel cluster secondary ions in mass spectra. The resulting oxide compounds have a bulk structure and occupy dozens of monolayers. In such a bulk oxide structure, the outer monolayers are characterized by the highest ratio of oxygen atom number to the number of matrix atoms. This ratio decreases along the transition from the surface to the underlying monolayers. This process occurs uniformly, without any phase transformation. The observed secondary ions are not a product of association between sputtered surface fragments and oxygen in the gas phase at the fly-off stage after sputter-ejection, but they are products of the oxide compounds being sputtered, hence they characterize the composition of surface and near-surface region.

Tensile and Impact Toughness Properties of a Zr-Based Bulk Metallic Glass Fabricated via Laser Powder-Bed Fusion.

In the past few years, laser powder-bed fusion (LPBF) of bulk metallic glasses (BMGs) has gained significant interest because of the high heating and cooling rates inherent to the process, providing the means to bypass the crystallization threshold. In this study, (for the first time) the tensile and Charpy impact toughness properties of a Zr-based BMG fabricated via LPBF were investigated. The presence of defects and lack of fusion (LoF) in the near-surface region of the samples resulted in low properties. Increasing the laser power at the borders mitigated LoF formation in the near-surface region, leading to an almost 27% increase in tensile yield strength and impact toughness. Comparatively, increasing the core laser power did not have a significant influence. It was therefore confirmed that, for BMGs like for crystalline alloys, near-surface LoFs are more detrimental than core LoFs. Although increasing the border and core laser power resulted in a higher crystallized fraction, detrimental to the mechanical properties, reducing the formation of LoF defects (confirmed using micro-computed tomography, Micro-CT) was comparatively more important.

Carbon and Neon Ion Bombardment Induced Smoothing and Surface Relaxation of Titania Nanotubes.

Titania nanotube arrays with their enormous surface area are the subject of much attention in diverse fields of research. In the present work, we show that not only 60 keV and 150 keV ion bombardment of amorphous titania nanotube arrays yields defect creation within the tube walls, but it also changes the surface morphology: the surface relaxes and smoothens in accordance with a curvature-driven surface material’s transport mechanism, which is mediated by radiation-induced viscous flow or radiation-enhanced surface diffusion, while the nanotubes act as additional sinks for the particle surface currents. These effects occur independently of the ion species: both carbon and neon ion bombardments result in comparable surface relaxation responses initiated by an ion energy of 60 keV at a fluence of 1 × 10 ions/cm. Using atomic force microscopy and contact angle measurements, we thoroughly study the relaxation effects on the surface topography and surface free energy, respectively. Moreover, surface relaxation is accompanied by further amorphization in surface-near regions and a reduction in the mass density, as demonstrated by Raman spectroscopy and X-ray reflectivity. Since ion bombardment can be performed on global and local scales, it constitutes a versatile tool to achieve well-defined and tunable topographies and distinct surface characteristics. Hence, different types of nanotube arrays can be modified for various applications.

Quantitative hydrogen state analysis in the leached layer of soda-lime-silica glass by combination of nuclear reaction analysis and C60-X-ray photoelectron spectroscopy

The effects of acid treatment on the surface layer of soda-lime-silica glasses were investigated by nuclear reaction analysis (NRA) and C60-X-ray photoelectron spectroscopy (XPS) in a depth-resolved manner. As the time of treatment with HCl increased, the amount of H increased and was distributed in a region at a depth of almost 150 nm. In the near-surface region, however, the damage due to 15N2+ beam irradiation for conducting the NRA was found to be substantial as observed by the fast decay of signal intensity with increasing ion dose. The depletion of sodium, magnesium, and calcium in the surface layer was examined by C60-XPS. Sodium was depleted at a depth of 150 nm by the acid treatment, but the depletion of magnesium and calcium occurred at a depth within approximately 30 nm. Assuming the ion exchange reaction between the alkaline and alkaline earth elements with H3O+, the Si-OH concentration profile was evaluated from the depletion profiles of sodium, magnesium, and calcium. From the difference between the Si-OH profile and H distribution obtained by NRA, the H2O/Si-OH ratio in the leached layer was estimated at 1.3.

High activity and durability of carbon-supported core-shell PtPx@Pt/C catalyst for oxygen reduction reaction

Alloying Pt with transition metals can significantly improve the catalytic properties for the oxygen reduction reaction (ORR). However, the application of Pt-transition metal alloys in fuel cells is largely limited by poor long-term durability because transition metals can easily leach. In this study, we developed a nonmetallic doping approach and prepared a P-doped Pt catalyst with excellent durability for the ORR. Carbon-supported core-shell nanoparticles with a P-doped Pt core and Pt shell (denoted as PtPx@Pt/C) were synthesized via heat-treatment phosphorization of commercial Pt/C, followed by acid etching. Compositional analysis using electron energy loss spectroscopy and X-ray photoelectron spectroscopy clearly demonstrated that Pt was enriched in the near-surface region (approximately 1 nm) of the carbon-supported core-shell nanoparticles. Owning to P doping, the ORR specific activity and mass activity of the PtP1.4@Pt/C catalyst were as high as 0.62 mA cm−2 and 0.31 mA μgPt−1, respectively, at 0.90 V, and they were enhanced by 2.8 and 2.1 times, respectively, in comparison with the Pt/C catalyst. More importantly, PtP1.4@Pt/C exhibited superior stability with negligible mass activity loss (6% after 30000 potential cycles and 25% after 90000 potential cycles), while Pt/C lost 46% mass activity after 30000 potential cycles. The high ORR activity and durability were mainly attributed to the core-shell nanostructure, the electronic structure effect, and the resistance of Pt nanoparticles against aggregation, which originated from the enhanced ability of the PtP1.4@Pt to anchor to the carbon support. This study provides a new approach for constructing nonmetal-doped Pt-based catalysts with excellent activity and durability for the ORR.

Oxidation of graphene-modified HfB2-SiC ceramics by supersonic dissociated air flow

The oxidation resistance of ultra-high-temperature ceramic material (HfB2-30 vol%SiC)-2 vol%rGO (rGO: reduced graphene oxide) under long-term exposure (2000s) to a supersonic air flow has been studied. The ceramics were obtained by reactive hot pressing of HfB2-(SiO2-C)-rGO composite powder at a temperature of 1800°C (pressure 30 MPa, holding time 15 min, Ar). The surface temperature of graphene-modified ceramics under the influence of heating by high-enthalpy air flow (heat flow q reached 779 W·cm–2) did not exceed 1700°C, which is 650–700°C less than for the HfB2-30 vol%SiC baseline ceramics. This may be related to an increase in the efficiency of heat transfer from the sample to the water-cooled module, due to the higher thermal conductivity of the rGO-containing material. Thereby, a decrease in the material degradation degree has been noted, i.e. decrease in the recession rate and decrease in the total thickness of the oxidised ceramic layer by tenth. The peculiarities of the oxidised surface and near-surface region microstructure upon aerodynamic heating of the graphene-modified ceramic material, have been shown.

Deep reinforcement learning for predicting kinetic pathways to surface reconstruction in a ternary alloy

The majority of computational catalyst design focuses on the screening of material components and alloy composition to optimize selectivity and activity for a given reaction. However, predicting the metastability of the alloy catalyst surface at realistic operating conditions requires an extensive sampling of possible surface reconstructions and their associated kinetic pathways. We present CatGym, a deep reinforcement learning (DRL) environment for predicting the thermal surface reconstruction pathways and their associated kinetic barriers in crystalline solids under reaction conditions. The DRL agent iteratively changes the positions of atoms in the near-surface region to generate kinetic pathways to accessible local minima involving changes in the surface compositions. We showcase our agent by predicting the surface reconstruction pathways of a ternary Ni3Pd3Au2(111) alloy catalyst. Our results show that the DRL agent can not only explore more diverse surface compositions than the conventional minima hopping method, but also generate the kinetic surface reconstruction pathways. We further demonstrate that the kinetic pathway to a global minimum energy surface composition and its associated transition state predicted by our agent is in good agreement with the minimum energy path predicted by nudged elastic band calculations.

Structure, Surface Morphology, Chemical Composition, and Sensing Properties of SnO2 Thin Films in an Oxidizing Atmosphere.

We conducted experiments on SnO2 thin layers to determine the dependencies between the stoichiometry, electrochemical properties, and structure. This study focused on features such as the film structure, working temperature, layer chemistry, and atmosphere composition, which play a crucial role in the oxygen sensor operation. We tested two kinds of resistive SnO2 layers, which had different grain dimensions, thicknesses, and morphologies. Gas-sensing layers fabricated by two methods, a rheotaxial growth and thermal oxidation (RGTO) process and DC reactive magnetron sputtering, were examined in this work. The crystalline structure of SnO2 films synthesized by both methods was characterized using XRD, and the crystallite size was determined from XRD and AFM measurements. Chemical characterization was carried out using X-ray photoelectron (XPS) and Auger electron (AES) spectroscopy for the surface and the near-surface film region (in-depth profiles). We investigated the layer resistance for different oxygen concentrations within a range of 1–4%, in a nitrogen atmosphere. Additionally, resistance measurements within a temperature range of 423–623 K were analyzed. We assumed a flat grain geometry in theoretical modeling for comparing the results of measurements with the calculated results.

Evolution of grain boundary character distribution in near-surface regions of a cold-rolled nickel-based superalloy during induction heating process

The influence of induction heating on the grain boundary character distribution (GBCD) in near-surface regions of a cold-rolled Nickel-based superalloy was researched. After induction heating, most of low-Σ coincidence site lattice (CSL) boundaries were Σ3 boundaries, which were mainly formed via the growth accident model. Moreover, the grain structures evolution during induction heating had a great influence on the evolution of GBCD. At the low strain of 0.1, both the fraction of Σ3 boundaries and grain size increased with the increasing temperatures, while the former was closely related to the better development of grain-clusters at the higher temperature. In addition, the coherent Σ3 boundaries were easier to be formed at the higher temperature during induction heating, owing to their low interface energy and mobility. At the large strain of 0.5, the fraction of Σ3 boundaries also increased with the increasing temperatures, but the grain size exhibited the opposite trend, which was closely related to the well development of static recrystallization (SRX) behaviors. Meanwhile, there was a “symbiotic relationship” between the SRX grains and Σ3 boundaries during induction heating. Through the electrochemical corrosion tests, it was proved that induction heating can contribute to the improvement of corrosion properties of superalloys via increasing the fraction of Σ3 boundaries, while the best corrosion resistance appeared in the samples treated at 800 °C with the strain of 0.5. Moreover, the evolution of both Σ9 and Σ27 boundaries was also closely related to strains and induction heating temperatures, but their fractions were less than 4%.

Diffusion Kinetics of Adsorbed Species on Pyrite Surfaces

Surface diffusion can bring redox pairs closer together, which is necessary for electron transfer to happen, thereby strongly influencing the overall kinetics. However, little is known about the diffusion kinetics of redox-active species on pyrite and other sulfides, which aid in catalyzing many redox reactions. Here, we calculate the diffusion of oxidant (UO22+) and reductant (Fe2+, HS–) species on pyrite {100} surfaces using quantum-mechanical calculations. Energy curves along different diffusion paths are derived for both inner- and outer-sphere complexes by moving the species in small increments (0.05–0.25 Å). The diffusion path along the molecular ridges formed by disulfide groups on the uppermost pyrite surface has the lowest energy barrier for the diffusion of all species tested. Single-particle diffusion coefficients along their optimal diffusion pathways are derived from diffusion energy barriers and attempt frequencies. Calculations are performed on flat defect-free pyrite surfaces, while on actual surfaces, diffusion is affected by defects, steps, and impurities. Calculated mobilities of the outer-sphere complexed uranyl and ferrous iron are about 4–5 times faster than their inner-sphere ones. Although the results here focus on single-particle diffusion, UO22+-HS– was used as an example for interdependent multiple-particle diffusion on the pyrite surface. Interactions between diffusing species (uranyl vs HS–), and to a limited degree jump correlations, were derived quantum-mechanically. Interactions are a combination of electronic interactions underneath the mineral surface and through the aqueous near-surface region; their interdependent diffusion can be approximated by apparent Coulomb interactions (with a dielectric constant of ∼7.7) for processing in subsequent Monte Carlo simulations.

Nitrogen ion induced CdS clusterization: Structural, morphological, and optoelectrical properties

Semiconducting nanostructures are unique system to study the nanoscale phenomenon for diverse applications in new generation electronic devices. Ion implantation is a promising approach for producing nanoscale crystals or clusters in the near-surface region for doping of semiconductors which repair the respective optical and electrical properties. In the present study, cadmium sulfide (CdS) thin films are grown on a glass substrate via an easy and economical chemical bath deposition method. Thereafter, CdS thin films are implanted with 80 keV nitrogen (N+) ions with different fluences. Implanted thin films are characterized with the support of various characterization techniques. X-ray diffraction (XRD) analysis indicates that the hexagonal phase of CdS nanoflakes remains consistent while scanning electron microscopy (SEM) imaging reveals the formation of defect clusterization after implantation. The defect clusterization in CdS thin films is explained on the basis of local heating after the transmission of implantation energy. Optical properties such as absorption and emission spectra of implanted CdS thin films also get affected on the account of the generation of donor and acceptor level owing to Cd and S vacancies. Furthermore, a considerable increase in the magnitude of the electric current is observed as compared to the pristine sample with the implantation dose.

Computational Fluid Dynamics Simulation of Fouling of Plate Heat Exchanger by Phosphate Calcium

A computational fluid dynamics (CFD) model was developed for fouling of calcium phosphate in heat exchangers which can consider the tendency of fluid components to precipitate and the surface properties of the materials. In the CFD model the precipitation of inversely soluble mineral due to the temperature increase in the near-surface region of the heated wall was modeled based on the saturation ratio. The precipitates form small, spherical particles that can adhere on the wall surface. The wall function model developed for adherence was modified to consider the turbulent viscosity modeled by CFD. The model was tested against experiments made with simulated milk ultrafiltrate, which is an aqueous solution simulating the mineral composition of milk. This preliminary adhesion CFD model paves the route to build a more comprehensive model for predicting the likely profile of milk deposition in heat exchangers.

Fluorescent PIV using Atomized Liquid Particles

It is shown that aerosolized fluorescent particles generated using a Venturi-type atomizer, from a solution of fluorescent Kiton Red 620 dye in a water/glycol fluid, provide effective flow seeding for fluorescent PIV. The atomized liquid particles were found to be of acceptable size for PIV purposes, with 92% of detected particles by number concentration measuring < 1 μm in diameter. A PIV application was conducted in a wind tunnel (freestream velocity U∞ = 27 m/s), using the particles for measurement of the boundary layer flow approaching a forward-facing step (approach boundary layer momentum thickness Reynolds number of Reθ = 5930), to identify potential benefits in near-wall regions normally affected by unwanted laser reflections from tunnel surfaces. Particles were generated from solutions with dye molar concentrations of 2.5 × 10−3 and 1.0 × 10−2 mol/L, and PIV images were obtained for both elastic Mie scattering and filtered, Stokes-shifted fluorescent light. Raw images indicate that the fluorescence yield of the 1.0 × 10−2 mol/L solution provides PIV images with high contrast, even in the near-surface regions where Mie scattering images are highly affected by surface reflections. Boundary layer profiles are processed in the adverse pressure gradient region leading up to the forward-facing step, where the fluorescent PIV performed comparably to the most optimized Mie scattering PIV; both obtained data as near to the wall as 30 μm, or 2 viscous wall units in our flow of interest. These results indicate that the new seeding method holds excellent promise for near-surface measurement applications with more complicated three-dimensional geometries, where it is impossible to arrange PIV cameras to reject surface-scattered light.

Hydrogen-Driven Surface Segregation in Pd Alloys from Atomic-Scale Simulations

The safe widespread application of hydrogen-based fuels requires sensors that are long-term stable, inexpensive, hydrogen-specific, and have a short response time. In this regard, optical sensing based on nanostructured Pd alloys has shown great potential, but challenges remain, including understanding and controlling the surface composition under operation conditions. While the latter is crucial for long-term functionality and stability, it is experimentally very challenging to obtain accurate atomic-scale information. Here, we therefore scrutinize the behavior of two particularly relevant surface alloys, {111} AuPd and {111} CuPd, in H environments ranging from vacuum to fully covered surfaces. To this end, we employ a combination of alloy cluster expansions trained using density functional theory calculations, Monte Carlo simulations, and thermodynamic analysis to obtain the H coverage as well as the layer-by-layer composition of the near-surface region as a function of H partial pressure, temperature, and bulk composition. To overcome the symmetry reduction implicit to surface systems, we exploit local symmetries, which enables us to achieve accurate and reliable models at a low computational cost. In the case of AuPd, Au segregates to the surface in vacuum while Pd segregates to the surface at 100% H coverage, and the transition between these regimes occurs over a narrow H pressure interval. In the case of CuPd, on the other hand, the H coverage varies much more gradually with H pressure and is coupled to a nonmonotonic variation of the Cu concentration in the topmost surface layer. While there is a pronounced Cu depletion both at 0 and 100% H coverage, Cu enrichment is observed at 50% coverage at Cu bulk concentrations up to at least 10%, providing a nontrivial explanation for an apparent discrepancy between experiment and calculations that was observed previously. At the same time, layer 2 continuously shifts from Cu enrichment to Cu depletion with increasing H coverage. The results demonstrate the rich behavior that can result from the coupling of metal–metal and metal–hydrogen interactions at surfaces, even in apparently simple but concentrated systems. Moreover, they underline the advantages of simulations that account for temperature and pressure effects as well as models that can accurately capture the interactions over a wide composition range.