Subsurface Alloy Research Articles

Crack initiation during environment-induced cracking of metals: current status

Abstract Environment-induced cracking (EIC) research spanning the last 80 years for ferrous and non-ferrous metals in aqueous environments at ambient and elevated temperatures has concentrated on crack propagation. Studies clearly reveal EIC involves two differentiable processes, one controlling initiation and the other propagation. Utilization of advanced high-resolution electron microscopy over the last 20 years has enabled more focused studies of crack initiation for stainless steel and nickel-based alloys at elevated temperatures exposed to environments associated with the nuclear industry. More recently, when coupled with advanced in-situ experimental techniques such as time-lapse X-ray computed 3D-tomography, progress has also been made for aluminum alloys suffering EIC at ambient temperatures. Conventional wisdom states that chemical processes are typically rate-controlling during EIC initiation. Additionally, experimental evidence based on primary creep exhaustion ahead of the introduction of an aggressive environment indicates that time-dependent mechanically-driven local microstructural strain accommodation processes (resembling creep-like behavior) often play an important role for many metals, even for temperatures as low as 40 % of their melting points (0.4 Tm). EIC studies reveal initial surface conditions and their associated immediate sub-surface alloy microstructures generated during creation (i.e. disturbed layers) can dictate whether or not EIC initiation occurs under mechanical loading conditions otherwise sufficient to enable initiation and growth. The plethora of quantitative experimental techniques now available to researchers should enable significant advances towards understanding EIC initiation.

Revealing the elusive role of water vapor in the oxidation behavior of a Mn-Si containing NiCr alloy at 950 °C

Detailed nanoscale characterizations revealed the formation of a considerably higher fraction of SiO2 particles in the external Cr2O3 layer on a NiCr alloy. In addition, atom probe tomography revealed substantially lower segregation of Mn at the Cr2O3 grain boundaries in wet air compared to dry air exposures, where the intriguing formation of Mn-rich clusters was observed in the Cr2O3 grains for the very first time. These findings provided additional evidence regarding the impact of distinct transport processes through the Cr2O3 layer on the resulting oxidation rates and subsurface alloy compositional changes.

Submonolayer Cu/Pt film as bifunctional catalyst for CO oxidation: An STM study

Considering the high activity in CO oxidation as well as the related reactions, the surface structure of Cu/Pt bimetallic system and its interaction with CO and O2 molecules have been revisited with low-temperature scanning tunneling microscopy (STM) in combination with density-functional theory calculations (DFT). The extensively concerned Cu/Pt subsurface alloy (Cu-SSA) is clearly demonstrated with relative deactivation to CO and drastically weakened binding to O. In contrast, the submonolayer Cu film on Pt(111) manifests a significant enhancement in activating the O2 molecules while reserving the strong binding to CO at the uncovered Pt area. Detailed control experiments corroborated by DFT calculations revealed that the Cu/Pt interface serves as the critical sites for both O2 activation and the CO oxidation reaction. Therefore, the submonolayer Cu film may register a more promising catalyst structure than the Cu-SSA configuration for the Cu/Pt bimetallic system.

Identification of Cu/Sc and Cu/Ti subsurface alloys for highly efficient CO electroreduction to C2 products

The electroreduction of CO to C2 products (primarily C2H4 and EtOH) provides a promising route to both carbon neutrality and energy storage and conversion under mild conditions. However, the commonly used copper catalyst suffers from high overpotentials, which has been shown to be closely related to CO dimerization reaction. Using density functional theory calculations, we study the effect of composition and structure of CuM(100) surface (M = Ru, Au, Zn and Ga) and Cu/M(100) subsurface (M = 3d Sc-Zn) alloys on the relative stability of 2CO and C2O2. We show that the higher stability of C2O2 relative to 2CO and enhanced CO electroreduction activity are achieved through increasing the interlayer separation of Cu (100) by the presence of subsurface Sc and Ti, which decrease the overlap between the dz2 orbitals of surface Cu and subsurface atoms and destabilizes the σ bond of Cu–CO more than that of Cu–C2O2. This modification also largely facilitates the C2H4 formation instead of EtOH formation through a substantial stabilization of the adsorbed intermediates. This work opens up an avenue for the design and development of catalyst in CO/CO2 electroreduction and other important reactions of technological interest.

Synchrotron XPS and Electrochemical Study of Aging Effect on Passive Film of Ni Alloys

To investigate aging effect on the passive film of Ni23Cr15Mo and Ni22Cr9Mo3Nb, synchrotron-based X-ray photoelectron spectroscopy (XPS) was used to analyze the structure and composition of the air-formed passive film on the alloys. The corrosion resistance of the two Ni alloys in 1 M NaCl solution was evaluated with electrochemical cyclic polarization measurement. The synchrotron XPS measurement provided detailed information about chemical states of alloying elements in the passive film, showing that the passive film consists of an inner oxide layer and an outer hydroxide layer. The XPS data allowed precise determination of the chemical composition and the thickness of the outer hydroxide layer, the inner oxide layer, and the underlying subsurface alloy layer. The Cr-oxide in the inner layer grows thicker with aging time, leading to Cr-depletion in the subsurface region. Mo and Nb in the alloy form mixed oxides and hydroxides, and aging in air leads to transformation of the lower valence oxides into higher valence oxides. The freshly formed oxide film exhibits similar barrier properties as the aged oxide film. The stability of the passive film formed on Ni22Cr9Mo3Nb seems to be better than that on Ni23Cr15Mo.

Thermal Annealing of Graphene Implanted with Mn at Ultralow Energies: From Disordered and Contaminated to Nearly Pristine Graphene

Ultralow-energy (ULE) ion implantation is increasingly being explored as a method to substitutionally dope graphene. However, complex implantation-related effects such as defect creation and surface contamination, and how they can be minimized by thermal annealing, remain poorly understood. Here, we address these open questions taking as the model case epitaxial graphene grown on Cu(111), which was subsequently ULE implanted with Mn at 40 eV and then studied as a function of annealing temperature under ultrahigh vacuum. While significant surface cleaning occurs at annealing temperatures as low as 200 °C, recovery from the implantation-induced disorder requires at least 525 °C. Upon high-temperature annealing, in the 600–700 °C range, the Mn atoms that were incorporated upon implantation as intercalated species (between graphene and the Cu surface) experience diffusion into the Cu layer, creating a subsurface alloy. Annealing at 700 °C restored implanted graphene to a nearly pristine state, with a well-ordered graphene lattice with substitutional Mn atoms and a well-defined Dirac cone. In addition to the insight into the complex physicochemical effects induced by thermal annealing, our results provide useful guidelines for future experimental studies on graphene that is modified (e.g., substitutionally doped) by using ULE ion implantation.

Designing Pt-based subsurface alloy catalysts for the dehydrogenation of perhydro-dibenzyltoluene: A first-principles study

Designing effective dehydrogenation catalysts for liquid organic hydrogen carriers is essential to the release and transport of hydrogen. The hydrogen release of perhydro-dibenzyltoluene using Pt-based subsurface alloys (Pt/M/Pt(111), where M = Pd, Cu, or Ni) and the effect of the applied biaxial strain on dehydrogenation performance of Pt/M/Pt(111) were systematically investigated. The doping of M atoms onto Pt/M/Pt(111) and strained Pt/M/Pt(111) could effectively tune the electronic properties of the Pt atoms, thus eventually affecting the hydrogen adsorption strength. The rate-determining step (RDS) of the dehydrogenation process on surfaces of Pt(111), Pt/M/Pt(111), strained Pt(111), and strained Pt/M/Pt(111) was identical: the first step of dehydrogenation in the middle ring of perhydro-dibenzyltoluene. The doping of the M atoms and application of tensile strain promoted dehydrogenation. Furthermore, it was revealed that d-band centers and reaction energies of the RDS correlated with the hydrogen adsorption energy, suggesting that hydrogen adsorption strength is a practical descriptor of dehydrogenation activity.

Behavior at 1100°C of nickel–based alloys strengthened by MC carbides with M = Tantalum and Hafnium

The metallurgical principle constituted by a chromium–rich metallic matrix associated with eutectic script monocarbides represents a promising way in terms of high temperature performance. In this work, focus was done on polycrystalline nickel–based alloys reinforced by monocarbides involving both tantalum and hafnium, with as objectives exploring the microstructures that can be obtained and the metallurgical, chemical and mechanical behaviors of what they can be able. Two alloys were synthesized by foundry under inert atmosphere. Their as–cast microstructures were visualized by electron microscopy and their behavior at high temperature were explored, for the following temperature and time parameters: 1100°C and 50 hours for all tests. The obtained results show that the carbide population obtained after solidification is mainly composed of MC carbides in both cases but it is composed of MC carbides only for the alloy with more hafnium than tantalum. The morphological resistance of the MC carbides is good for the test conditions for both alloys. Concerning oxidation, a chromia–forming behavior was observed in the two cases, with furthermore an obvious good resistance against scale spallation at cooling. The oxidation–induced subsurface alloy degradations and the heat–induced bulk microstructure deteriorations were very limited. The bending tests under constant load allow the sample demonstrating good creep resistance. To summarize both alloys appeared as good bases for developing quite usable superalloys for this temperature where the high performance ’ nickel–based single crystals start to encounter serious problems as consequences of the loss of their reinforcing particles.

Thickness and composition of native oxides and near-surface regions of Ni superalloys

The surface chemistry and thickness of the native oxide, hydroxide, and modified sub-surface layer of three Ni superalloys (alloy 59, 625, and 718) were determined by synchrotron X-ray Photoelectron Spectroscopy (XPS) and X-ray Reflectivity (XRR). Taking advantage of the synchrotron radiation techniques, a procedure for normalizing the photoelectron intensity was employed, which allowed for accurate quantitative analysis revealing a total oxide thickness for all samples of 12–13 Å, a hydroxide layer of 2–3 Å, and a thickness of the sub-surface alloy layer of 20–35 Å. The thickness results were compared to structural atomic models suggesting that the oxide thickness corresponds to four planes of metal cations in the oxide matrix. The XPS data revealed that the native oxides were enriched in Cr3+, Mo(4,5,6)+, and Nb5+, while no Ni oxide was detected. The hydroxide layer mainly contained Ni2+ and Cr3+ hydroxide. The sub-surface layer was enriched in Ni and depleted in Cr, Fe, Mo, and Nb. The obtained oxide composition can be explained using thermodynamics, and it was found that the oxide composition correlates with the enthalpy of oxide formation for the metal elements in the alloys. Finally, the advantages of synchrotron radiation for composition and thickness determination are discussed.

Multicomponent nanoporous Al–Ni–Cu–Pt–Pd–Co as highly stable anode catalysts in a flexible room-temperature pure ethanol–powered solid-state fuel cell

Developing highly active and durable anodic catalysts with low noble-metal content for ethanol electrooxidation is important for boosting the performance of direct room-temperature ethanol fuel cells. Herein, we use a general and scalable dealloying method to prepare a series of nanoporous multicomponent alloys with different alloy compositions. It is found that the subsurface alloy composition play an important role on the catalytic activity of the surface noble metals due to the surface strain and electronic effect. By screening, we find that the AlNiCuPtPdCo senary combination exhibits the highest electrocatalytic activity for ethanol oxidation in alkaline media. On the cathode electrode, a highly ORR active, durable, and ethanol-tolerant Co@N–C catalyst derived from MOF is used. Impressively, the designed solid-state room-temperature ethanol fuel cell using such anodic and cathodic catalysts can work immediately with the addition of pure ethanol, delivering a high power density of ~9.20 mW cm−2 with 1 mL ethanol. The flexible fuel cell can also work repeatedly when the added ethanol is consumed.

Isolating the contributions of surface Sn atoms in the bifunctional behaviour of PtSn CO oxidation electrocatalysts

The bifunctional mechanism is well-acknowledged for the promoted CO oxidation on Pt-based bimetallic electrocatalysts. However, the direct identification of the active oxygenated species and the nature and electrochemistry of the second component are still a matter of debate. Herein, Snad-Pt/C catalysts, where Sn ad-atoms are exclusively on the surface of Pt nanoparticles at low coverages ranging from 0.0033 to 0.2 monolayers to avoid sub-surface Sn and alloy formation, were prepared as a model system to resolve these issues using a surface organometallic chemistry approach. Effects of the Sn ad-atoms on CO oxidation were studied by CO stripping voltammograms as a function of Sn coverage. Using in situ XAS measurements, the Sn average oxidation state is estimated to increase from +0.2 to +3.1 as the potential increases from 0 to 0.8 VRHE, with the number of the oxygen neighbours increasing stepwise. Pt4.5-Sn-(OH)1.5 is revealed as the active species responsible for the bifunctional mechanism at low overpotentials and is generated via a redox couple corresponding to Pt4.5-Sn*/Pt4.5-Sn-(OH)1.5.

Atomic Relaxation and Vibration Properties of the Cu(111)–(√3 × √3)R30°–Cr Surface

The structural and vibration properties of the (√3 × √3)R30°–Cr adsorption structure on the Cu(111) surface and the Cu(111)–(√3 × √3)R30°–Cr two-dimensional alloy incorporated into the S – 1 substrate layer are studied using the tight binding approximation. Surface relaxation is discussed along with the dispersion of surface phonons and the local density of vibration states of adatoms and substrate atoms. It has been demonstrated that the presence of chromium atoms modifies the structural and vibration characteristics of the substrate and leads to the appearance of new phonon states in it. The analysis of the entire set of data has shown that the Cu(111)–(√3 × √3)R30°–Cr subsurface two-dimensional alloy is a more stable configuration.

Theory-Guided Machine Learning Finds Geometric Structure-Property Relationships for Chemisorption on Subsurface Alloys

Summary Developing physically transparent and quantitatively accurate models that relate the chemical interaction (chemisorption strength) between an adsorbate and a solid surface to the adsorption site's geometry is critical for our understanding of catalysis, corrosion, and electrochemistry. We develop a theory-guided machine-learning (ML) approach, which uses an interpretable class of ML models called generalized additive models (iGAM models), to discover predictive structure-property models that can quantify and explain the link between the geometric structure of an adsorption site and the chemisorption strength. We demonstrate the approach through several case studies, where we analyze the chemisorption strength of chemically distinct adsorbates (O, OH, S, and Cl) on subsurface metal alloy surfaces subjected to various strain- and ligand-induced changes in the local geometric structure. By comparing the ML-derived chemisorption models to previously established electronic-structure models, we clarify the critical physical parameters that control the chemisorption process on metal surfaces.

Reviving Inert Oxides for Electrochemical Water Splitting by Subsurface Engineering

Recently, it was theoretically predicted and experimentally validated that subsurface alloying of SrRuO3 (SRO) beneath the SrTiO3 (STO) capping layer can significantly promote the otherwise inert S...

Weakening the strength of CO binding on subsurface alloyed Pt(111)

The interaction of CO with Pt/3d/Pt(111) subsurface alloys (3d: Ni, Co, Fe) was investigated by combination of vibrational spectroscopy and temperature programmed desorption. The binding strength of CO is significantly reduced on Pt/Fe/Pt(111) and Pt/Co/Pt(111) and is sorted in the following order: Pt>Ni>Co≈Fe. Our analysis shows that the 3d admetal is stable in the subsurface region and CO is linearly bonded to the atop sites of the Pt(111) surface. At high CO coverage, compressed structures are obtained. At low CO coverage, the desorption activation energy of CO (∼143 kJ/mol for Pt(111)) drops to 124 kJ/mol for Pt/Ni/Pt(111), and goes further down to 111 and 110 kJ/mol for Pt/Co/Pt(111) and Pt/Fe/Pt(111), respectively. The enhancement in the surface activity is attributed to the modified backdonation to the antibonding states of adsorbed CO due to the downshifted 5d-band center of Pt upon embedding 3d metals in the subsurface region.

Modifying hydrogen binding strength of graphene

The effect of the substrate on the binding strength of hydrogen on single layer graphene grown on Pt(111) surfaces has been investigated via determining its desorption activation energy. We showed that subsurface alloys on Pt(111) can dramatical modify the CH bond strength in hydrogenated graphene. Various 3d metals, vanadium, iron, cobalt, and nickel were deposited in the subsurface layer to modify the chemical and electronical properties of the substrate. Analysis of the temperature programmed desorption data shows that subsurface alloys reduce the hydrogen desorption activation energy by weakening CH bond energy in graphene, down to ∼57 kJ/mol in the case of Pt/Co/Pt(111) as compared to ∼111 kJ/mol obtained from hydrogenated graphene grown on a bare Pt(111).

Surface alloy formation by adsorption of holmium on Ag/Mo(112) bimetallic surfaces

Work function change measurements, low energy electron diffraction (LEED) and density functional theory (DFT) are used to determine the structures formed on Ag/Mo(112) bimetallic surfaces upon deposition of 0.5 monolayer (ML) of holmium. As the bimetallic surfaces, we have chosen the Mo(112) substrate covered with 1 or 2 ML of Ag. Such surfaces have the same symmetry as the Mo(112) face but different electronic properties. LEED experiment indicates that the c(2 × 2) structure is formed on (1 ML Ag)/Mo(112) bimetallic surface upon deposition of 0.5 ML of Ho. DFT calculations show that a type of Ag–Ho surface alloy is formed, with Ho atoms 0.6 Å below the distorted layer of Ag. This is neither a substitutional nor a subsurface alloy. It is found that the adsorption structure formed on the (2 ML Ag)/Mo(112) bimetallic surface depends on the annealing temperature. After deposition of 0.5 ML of Ho at 300 K, the LEED pattern of p(2 × 2) symmetry is observed. Annealing of the overlayer at 640 K irreversibly changes the p(2 × 2) pattern into a pattern of c(2 × 2) type. The results of DFT computations show that the c(2 × 2) structure of the Ag–Ho surface alloy is energetically most favorable. In this structure, 0.5 ML of Ho is between the two monolayers of Ag, and the symmetry of the topmost layer is changed. The work function change calculated for the c(2 × 2) structure is in a good agreement with the measured value (0.22 eV). The results show that adsorption of Ho on the Ag/Mo(112) bimetallic surfaces is substantially different than on the clean Mo(112).

Corrosion of nickel-base alloys in primary medium of pressurized water reactors: New insights on the oxide growth mechanisms and kinetic modelling

The generalised corrosion of Ni-base alloys exposed to pressurized water reactors primary coolant leads both to the growth of a protective oxide scale and to the release of cations. Based on experimental results previously published, a mechanistic description of this phenomenon is proposed. Then, from a simplified view of theses mechanisms, a kinetic model is proposed, focusing on the link between the alloy subsurface defects density and the shape of oxidation kinetic curves. Comparing calculated curves and experimental data shows this first approach is promising. Nevertheless a better knowledge of some kinetic parameters is necessary to improve the model.

Use of thermogravimetry and thermodynamic calculations for specifying chromium diffusion occurring in alloys containing chromium carbides during high temperature oxidation

The chromium diffusion is of great importance for the high temperature oxidation behaviour of the chromium-rich carbides-strengthened superalloys. These ones contain high chromium quantities for allowing them well resisting hot corrosion by constituting and maintaining a continuous external scale of chromia. Knowing how chromium can diffuse in such alloys is thus very useful for predicting the sustainability of their chromia-forming behaviour. Since Cr diffusion occurs through the external part of the alloy already affected by the previous steps of oxidation (decarburized subsurface) it is more judicious to specify this diffusion during the oxidation process itself. This was successfully carried out in this work in the case of a model chromia-forming nickel-based alloy containing chromium carbides, Ni(bal.)–25Cr–0.5C (in wt.%). This was done by specifying, using real-time thermogravimetry, the mass gain kinetic due to oxidation, and by combining it with the post-mortem determination of the Cr concentration profiles in subsurface. The values of DCr thus obtained for 1000, 1050 and 1100 °C in the alloy subsurface are consistent with the values obtained in earlier works for similar alloy's chemical compositions.

SIMS and TEM investigation of hydrogen trapping on implantation defects in a nickel-based superalloy

This study aims at characterizing the interactions of hydrogen with point defects and their clusters in nickel-based superalloys. For this, nickel ion implantation was used to create defects in the sub-surface of alloy 690 coupons. The interaction of hydrogen with these defects was studied thanks to SIMS depth profile analysis performed after deuterium cathodic charging, and different annealing treatment. In parallel, TEM observations were carried out in order to qualify and quantify the defects responsible for the trapping, and to observe their evolution in the presence of hydrogen. All the results exhibit a strong interaction of hydrogen with implantation-defects, up to high temperatures (325 °C). Also, hydrogen is shown to assist defects reorganisation, in a way similar to the effect of temperature. The effect of hydrogen on the alloy atoms as explored by SIMS, combined with the TEM investigation of defects evolution in the presence of hydrogen, allows to suggest that the main mechanism for hydrogen-assisted motion of defects is by lowering the lattice friction, thus decreasing the activation volume for the unfaulting or motion of point defect clusters, up to dislocation loops.