Near-surface Region Research Articles

Melt thermal-rate treatment for uniform solute distribution and improved mechanical properties of an Al-Zn-Mg-Cu alloy prepared by direct-chill casting

Melt thermal-rate treatment (MTRT), a promising technique for improving microstructure uniformity, has been rarely applied in direct-chill (DC) casting. In this study, we investigated the effects of MTRT on the microstructure and mechanical properties of an Al-Zn-Mg-Cu alloy billet prepared using DC casting. Results indicate that MTRT improved the uniformity of solute distribution in the Al matrix, thereby enhancing mechanical properties such as hardness, tensile strength, and ductility. In particular, yield strength, ultimate tensile strength, and elongation of the near-surface region of the billet increased from 250 to 274 MPa, from 319 to 393 MPa, and from 2% to 8%, respectively. It has been reported that twinned dendrite grains (TDGs) form on the surface of billets because of directional solidification or DC casting at high cooling rates. However, this study reveals that the formation of TDGs was induced in the central region of the billet by MTRT. Furthermore, the effects of TDGs on the mechanical properties are evaluated and compared with those of regular grains of MTRT billets. Overall, these findings suggest that MTRT is an efficient technique used to improve the mechanical properties of Al alloys fabricated using DC casting.

A slow positron beam study of the oxidation behaviour of 310S stainless steel and alloy 800HT exposed to supercritical water

This paper reports a variable-energy slow positron beam characterisation of the surface oxide films in 310 S stainless steel and 800HT nickel-based alloy exposed to 550 °C supercritical water for 1000–4392 h. The evolution of the oxide layer was depth-profiled using positron beam energies from 0.5 to 20 keV, which enabled a probe of up to 2 mm of the near-surface region. SEM/EDS was used as a complementary characterisation tool to support and comprehend the positron annihilation spectroscopy data. A strong dependency on the positron trapping rate and the effective positron diffusion length on the microstructure of the studied materials enabled us to identify up to three non-bulk layers, which were preliminarily attributed to different oxide layers and Fe/Cr depletion regions. By calculating the effective positron diffusion length in the given multi-layer systems, we obtained the thicknesses of individual layers, which are in good agreement with the EDS analysis.

Effects of Carbon on Void Nucleation in Self-Ion–Irradiated Pure Iron

Impurities such as carbon atoms play a significant role in the void swelling of irradiated metals. The phenomenon is important to both materials designs in which impurities are intentionally introduced and accelerator-based ion irradiation testing in which impurities are introduced unintentionally as contaminants. Here, we report rate theory simulations of void nucleation in pure Fe, which are irradiated by 5-MeV Fe ions, as one typical irradiation condition used in nuclear material testing. Based on kinetics obtained previously from ab initio calculations, Multiphysics Object-Oriented Simulation Environment (MOOSE)–based numerical solvers were used to simulate defect distributions and void nucleation. Vacancy-carbon interactions increase the effective migration energies of carbon and decrease the diffusivity prefactors. The vacancy mobility reduction decreases both interstitial flux and vacancy flux. However, the vacancy flux reduction is more significant than that of interstitials, leading to reduced void nucleation in bulk. On the other hand, reduced vacancy flux toward the surface leads to local vacancy pileups, leading to locally enhanced void nucleation. These two combined effects make the void nucleation profile deviate from the displacements per atom (dpa) peak, and void swelling peaks shift to the near-surface region. The transition from deep swelling to near-surface swelling is plotted as a function of dpa rate, carbon concentration, and temperature. The study shows that the swelling peak shifting caused by the carbon effect can be avoided by either reducing dpa rates or increasing irradiation temperatures. The study is important to understand swelling behaviors and to optimize irradiation parameters for accelerator-based swelling testing.

Electron properties investigation of the near-surface region in crystalline semiconductors using the transverse acoustoelectric effect

Electron properties investigation of the near-surface region in crystalline semiconductors using the transverse acoustoelectric effect

Spectroscopic Evaluation of Surface Chemical Processes Occurring in MoS2 upon Aging.

Molybdenum disulfide (MoS2) coatings have attracted widespread industrial interest owing to their excellent lubricating properties under vacuum and inert conditions. Unfortunately, the increase in MoS2 interfacial shear strength following prolonged exposure to ambient conditions (a process referred to as "aging") has resulted in reliability issues when MoS2 is employed as solid lubricant. While aging of MoS2 is generally attributed to physical and chemical changes caused by adsorbed water and/or oxygen, a mechanistic understanding of the relative role of these two gaseous species in the evolution of the surface chemistry of MoS2 is still elusive. Additionally, remarkably little is known about the effect of thermally- and tribologically-induced microstructural variations in MoS2 on the aging processes occurring in the near-surface region of the coating. Here, we employed three analytical techniques, namely, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and grazing-incidence X-ray diffraction (GIXRD), to gain insights into the aging phenomena occurring in sputtered MoS2 coatings before and after tribological testing, while also evaluating the impact of thermally-induced variations in the coating structure on aging. The outcomes of XPS analyses provide evidence that a substantial surface oxidation of MoS2 only takes place under humid conditions. Furthermore, the correlation of XPS, ToF-SIMS, and GIXRD results allowed for the development of a qualitative model for the impact of shear-induced microstructural variations in MoS2 on the transport of water in the near-surface region of this material and on the extent of surface oxidation. These results add significantly to our understanding of the aging mechanisms of MoS2 coatings used in tribological applications and their dependence on environmental conditions.

An in situ ToF-LEIS characterization of the surface of Ti-based thin films under oxygen exposure and at elevated temperatures

Ti-based coatings are utilized in a wide variety of applications, from biomedical implants to mechanical tools. Insight into initial stages of processes triggered by gas exposure and temperature change in the near-surface region of such coatings is essential for the understanding of their macroscopic behavior. We present an in situ time-of-flight low-energy ion scattering (ToF-LEIS) approach for the non-destructive and depth-resolved study of composition and morphology of the immediate surface region with sub-nm resolution. Ti-based coatings, with increasing compositional complexity, starting from in situ grown Ti, followed by ex situ grown Ti, TiN, and (Ti,Al)N, are studied concerning effects of exposure to oxygen and elevated temperatures. On the clean in situ deposited Ti surface, a 1.9 nm thick oxide layer is observed after exposure to 4000 Langmuir oxygen at room temperature. In contrast, for ex situ grown samples, an oxidic surface layer not removable by surface ion sputtering is found to limit effects of further oxygen exposure. TiN does not show significant changes when exposed to oxygen at 370 °C. For (Ti,Al)N, a nm-thick Al-rich surface layer is observed at annealing temperatures above 600 °C, both in ultra-high vacuum and in 1.0 × 10-3 Pa of oxygen.

The universality of black hole thermodynamics

The thermodynamic properties of black holes — temperature, entropy and radiation rates — are usually associated with the presence of a horizon. We argue that any extremely compact object (ECO) must have the same thermodynamic properties. Quantum fields just outside the surface of an ECO have a large negative Casimir energy similar to the Boulware vacuum of black holes. If the thermal radiation emanating from the ECO does not fill the near-surface region at the local Unruh temperature, then we find that no solution of gravity equations is possible. In string theory, black holes microstates are horizonless quantum objects called fuzzballs that are expected to have a surface [Formula: see text] outside [Formula: see text]; thus the information puzzle is resolved while preserving the semiclassical thermodynamics of black holes.

Nucleation and growth of low resistivity copper thin films on polyimide substrates by low-temperature atomic layer deposition

The metallization of polyimide (PI) remains a formidable challenge when using an atomic layer deposition of copper. The present study employs Cu(hfac)2 and Et2Zn to deposit the copper thin films that are highly conformal and continuous, and possess low resistivity (5.6 μΩ·cm), through a low-temperature atomic layer deposition (120 °C) with multi-pulse of Cu(hfac)2 process on a treated polyimide surface. The process exhibited both self-limiting ALD and CVD-like reactions with a growth rate of 0.079 nm/cycle. The findings demonstrate that an increase in functional groups and adsorption area on the surface of treated PI substrates enhances the adsorption of Cu(hfac)2. A model has been developed to describe the nucleation and growth kinetics of atomic layer deposition of copper on polyimide substrates, based on the adsorption mechanism of Cu(hfac)2 and the interface structure of ALD-Cu/treated PI onto the surface and into the near-surface region of the PI substrates. The academic developing in this work provides theoretical and experimental basis for the deposition using other copper precursors onto polymeric substrates in the future.

Unveiling the potential of surface–beneath region doping by induced-diffusion in nickel-rich single crystal cathode for high-performance lithium-ion batteries

A critical challenge associated with Ni-rich single-crystal cathode materials is the significant degradation of their structural integrity caused by cation mixing during high-temperature sintering. To address this issue, we propose an effective strategy of uniformly doping Zn ions into the region below the surface of Ni0.90Co0.07Mn0.03OH2 using polyvinyl pyrrolidone. This approach enables the localized concentration of Zn ions in the near-surface region, which can effectively reduce the degree of Li/Ni mixing and improve the surface and thermal stability of the material under high-state-of-charge conditions. We observed that the near-surface region doped sample with 1 wt% Zn (SC-NCM90@Zn1) demonstrated superior rate performance (100.01 mA h g−1 at 10C) and excellent capacity retention (87.71% after 100 cycles) compared with those of Pristine SC-NCM90 (LiNi0.90Co0.07Mn0.03O2). Overall, this study provides a design approach for enhancing the performance of Ni-rich single-crystal cathode materials for Li-ion batteries.

Testing the complexity and chaotic nature of wave-dominated turbulent flows

This study employs chaos theory concepts to investigate the complexity and chaotic nature of surface-generated waves in comparison to steady-flow state. The flows are generated through laboratory-flume experiments. Two separate tests are carried out: steady-flow test; and test with the addition of waves to the steady-flow under identical flow conditions. For wave addition, two-different frequencies of wave (i.e., 1.1 Hz and 2.1 Hz) are considered. Two chaos theory-based approaches are employed to determine the complexity and chaotic nature of wave-current dynamics: False nearest neighbour (FNN) method; and Lyapunov exponent method. An effort is also made to confirm and understand the results from these chaos-theory methods with the results from the wavelet and Shannon entropy methods. The results from the chaos-theory methods suggest optimistic evidence of the presence of chaotic behaviour in the combined wave-current cases. The results show that greater complexity is found at the near-bed region with aperiodic nature of the eddy scales and the complexity becomes less when moving towards the near-surface region. These results are well supported by the tests with wavelet and Shannon entropy methods. The results further reveal that the complexity for the steady-flow case is high and the complexity decreases with the addition of waves.

Microstructural characterization of near-surface microstructures on rail wheels in service - an insight into "stratified surface layers".

Background: To decrease maintenance costs and improve safety in rail transportation, the understanding of rail and wheel defects is vital. Studies on "white etching layers" (WEL) on rails and wheels, prone to fatigue crack initiation, have been extensively studied. Recently, a relative named "brown etching layer" (BEL) and its combination, the so-called "stratified surface layer" (SSL), are observed in the field. This study presents an investigation on a rail wheel affected by mechanical and thermal loadings from service with focus on the different evolved layers in the near-surface region. Methods: Optical microscopy is performed on etched cross-sectional cuts to identify different evolved microstructures (WEL, BEL, SSL), further, specific regions are investigated in detail by scanning electron microscopy to evaluate the microstructural characteristics. To analyze the change in mechanical properties, low-load Vickers hardness investigations are executed in distinctive zones. Results: This study highlights the broad variety of evolved microstructures, however, a rough classification of WEL (fine mesh-like microstructure, 900 - 1200 HV0.01) and BEL (globular cementite particles, 400 - 600 HV0.01) is given. Further, results indicate that the BEL is commonly accompanied by a WEL, representing an SSL. Conclusions: The complex loading situation in a wheel-rail contact can lead to the formation of WEL, BEL and SSL. The observation of numerous initiated fatigue cracks within these regions demonstrates the relevance of in-depth studies on evolved microstructures in wheel-rail contacts.

Assessment of residual strain in laser shock peened additive manufactured Inconel 718 using synchrotron X-ray diffraction

The through-thickness residual strain profile due to laser shock peening (LSP) of an additively-manufactured IN718 sample is shown for the first time. The results provide tantalising insight into the potential of fatigue life extension of additive-manufactured samples using LSP. Residual strains were measured in as-manufactured and peened samples using state-of-the-art synchrotron radiation. Significant beneficial compressive in-plane residual strains, extending to 1.0 mm depth, were observed due to the peening process. No phase changes were observed due to LSP. A periodic strain variation was observed arising from the layer-by-layer manufacturing strategy of the samples. The peened sample showed no increase in diffraction peak width in the near-surface regions. This is contrary to reported studies using wrought laser shock peened alloys and suggests less grain disruption in additively-manufactured peened samples. The results are timely given a current drive to adopt additive-manufacturing for a step-change in efficiency of aero-engine component production.

Helium bubble size effects on the surface morphological response of plasma-facing tungsten

Abstract We report a simulation study on the effects of helium (He) bubble size on the morphological evolution and pattern formation on the surface of tungsten used as a plasma-facing component (PFC) in nuclear fusion devices. We have carried out a systematic investigation based on self-consistent dynamical simulations of surface morphological evolution according to an atomistically-informed, 3D continuum-scale model that captures well the relevant length and time scales of surface nanostructure formation in PFC tungsten. The model accounts for PFC surface diffusion, driven by the biaxial compressive stress originating from the over-pressurized He bubbles in the near-surface region of PFC tungsten as a result of He plasma exposure, combined with the formation of self-interstitial atoms in tungsten that diffuse toward the PFC surface and the flux of surface adatoms generated as a result of surface vacancy-adatom pair formation upon He implantation; this transport of surface adatoms contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. Our detailed analysis reveals that varying the average He bubble size plays an important role in the PFC surface growth kinetics as well as the resulting surface topography. Specifically, we find that an increase in the He bubble size leads to a deceleration in the growth rate of the tungsten nanotendrils that emanate from the PFC surface. We also find that the separation distance between the resulting surface features increases with increasing He bubble size, as well as over time. This coarsening effect is a thermally activated process resulting in an accurate description of the temperature dependence of the average surface feature separation by an Arrhenius relation.

FORMATION OF CRYSTALLINE SiC IN NEAR-SURFACE SILICON LAYERS BY METHOD OF COORDINATED SUBSTITUTION OF ATOMS

In this work, monocrystalline films of silicon carbide were synthesized on the surface of a Si(100) silicon wafer using the method of coordinated substitution of atoms. The films were synthesized at temperatures of 1200 °C and 1300 °C for 20 minutes in a CO gas flow at a pressure of 0.8 Pa. The effect of 1200–1300 °C temperatures on the formation of single- and polycrystalline layers, as well as nanostructured SiC phases in the near-surface region of silicon by the method of atom substitution, is analyzed. The formation of a high-quality crystalline silicon carbide film and the influence of synthesis conditions on the total volume of SiC structural phases, microstructure and nanostructure of the surface are shown. It was found that an increase in temperature from 1200 °C to 1300 °C led to a more intensive formation of silicon carbide and an increase in the number of Si–C bonds by 1.9 times due to an increase in the thickness of the synthesized silicon carbide layer. There is an increase in the proportion of the crystalline phase due to a more intense transformation of the nuclei of nanocrystals into micro- and nanocrystals. Intense processes of penetration of carbon atoms deep into silicon at a temperature of 1300 °C with amorphization of its structure and the formation of Si-C, which can transform into crystalline phases at temperatures above 1300 °C, are assumed. The proportion of the SiC crystalline phase increases to 50.2% of the film volume due to the intensive transformation of nanocrystal nuclei into micro- and nanocrystals. It has been experimentally shown that the formation of various SiC structures on Si (100) occurs in full accordance with the main principles of the method of coordinated substitution of atoms.

Microstructure Evolution of the Near-Surface Deformed Layer and Corrosion Behavior of Hot Rolled AA7050 Aluminum Alloy.

The current study investigates the influence of hot rolling on the microstructure evolution of the near-surface region on AA7050 aluminum alloy and the corrosion performance of the alloy. It is revealed that hot rolling resulted in grain refinement in the near-surface region, caused by dynamic recrystallization, and equiaxed grains less than 500 nm can be clearly observed. Fibrous grains were evident in the hot rolled AA7050 aluminum alloy with relatively lower rolling temperature or larger rolling reduction, caused by the more severe elemental segregation at grain boundaries, which inhibited the progression of dynamic recrystallization. The density of the precipitates in the fibrous grain layer was higher, compared with those in the equiaxed grain layer, due to the increased dislocation density, combined with more severe elemental segregation, which significantly promoted the nucleation of precipitates. With the co-influence exerted by low density of precipitates and dislocations on the improvement of the corrosion performance of the alloy, the rolled AA7050 alloy with decreased density of precipitates and dislocations exhibited better corrosion resistance.

Surface microstructure and phase structure of zirconia ceramics under intense pulsed ion beam irradiation

The surface microstructure and phase structure of zirconia (ZrO2) ceramics under intense pulsed ion beam (IPIB) irradiation were investigated in this work. Experiments were carried out on TEMP-4M accelerator with peak accelerating voltage, current density, and pulse duration of 220 kV, 150 A/cm2, and 80 ns. Micro-cracks with a height of up to 150 nm above the surface were observed and were deduced to be formed by the tensile stress during the shrinking process. The cross-sectional observation indicates that columnar structure thin layer with thickness about 1.23 μm was produced after IPIB irradiation. The GIXRD pattern indicates that obvious preferred orientation in (110) and (200) of t-ZrO2 was occurred in near-surface region after IPIB irradiation. Opposite micro-stress in bulk and near-surface region was revealed by W-H plot after Rietveld refinement. The evolution of mass loss and surface roughness of samples with pulse number was also investigated.

Effect of the free surface on near-surface void swelling in self-ion irradiated single crystal pure iron considering the carbon effect

The influence of ion-incident free surfaces on void swelling in near-surface regions was systematically studied using self-ion irradiation of single crystal pure iron. The key interest was to evaluate the effect of free surfaces not only on the formation of void-denuded zones but also on the swelling in the region immediately adjacent to the void-denuded zone. The irradiation matrix included beam energies varying from 1.0 to 5.0 MeV, peak displacements-per-atom levels of 50 and 100 dpa, and irradiation temperatures at 425 °C (at 6 × 10−3 peak dpa/sec, 475 °C (at 1.2 × 10−3 peak dpa/sec) and 525 °C (at 6 × 10−3 peak dpa/sec). The observed denuded depth Δx obtained from transmission electron microscopy was modified to incorporate the sputtering effect. The derived activation energy governing the denuding process is Ex4=1.65±0.03eV, which is significantly higher than the vacancy migration energy EVm known for Fe to be 0.67 eV. This difference is speculated to be due to the strong effect of relatively small amounts of dissolved carbon at 103 appm, which reduces the effective vacancy mobility and thereby increases the effective migration energy. Based on the effective vacancy migration energy extrapolated from a previous first-principle calculation for Fe containing various carbon levels, rate theory calculation was used to model the surface influence and the results support the speculation of carbon influence. For the region immediately adjacent to the void-denuded zone, swelling is locally suppressed, rising to reach the bulk swelling at a depth increment essentially equal to the denuded width. This suppression effect causes shifting of swelling vs. local dpa curves obtained from different peak dpa irradiations. Such shifting is proposed in the present study as a method to define the boundary of the surface-induced suppressed swelling zone beyond which the swelling can be considered to be fully free of surface influence.

Surface treatment of Ti and Ti composites using concentrating solar power and laser

Titanium and its composites are widely used in implants of bones and teeth. Besides mechanical properties also surface characteristics are very important in these biomaterials. Very important are properties such as surface topography, roughness, chemistry, and surface energy, wettability, and Ti oxides or Ti nitride layers thickness. The concentrated solar power was used successfully to nitride Ti Grade 2 and powder metallurgical Ti prepared from hydrogenated dehydrogenated Ti powder. The nitriding experiments were performed under nitrogen atmosphere at different temperatures and time in SF40 (40kW horizontal solar furnace) at PSA, Spain. Concentrated solar energy has been shown to be an economical alternative to conventional gas nitriding techniques in electric furnaces, CVD, PVD, plasma nitriding, or laser treatments. It has been observed that the solar process represents a significant reduction of the heating time to several minutes (up to 5 minutes at temperature range 500-1000 °C), a clean and non-polluting high-temperature process. The formation of continuous and homogeneous surface layers of TiN, Ti2N and their mixture according to the nitriding temperature was investigated using X-ray diffraction and electron microscopy. Laser surface treatment is of great significance in modifying surface morphology and surface and near-surface region microstructures. Effects of lase treatment parameters on machined surface morphology, surface roughness and chemistry are analyzed in this study and discussed from the point of view of application in dental implantology. The current advances of our research group in application of laser-treated powder metallurgy prepared Ti-based materials are analyzed and discussed.

Near-surface termination of upward-propagating strike-slip ruptures on the Yangsan Fault, Korea

We present a new example of the termination of strike-slip paleoearthquake ruptures in near-surface regions on the Yangsan Fault, Korea, based on multi-scale structural observations. Paleoearthquake ruptures occur mostly along the boundary between the inherited fault core and damage zone (N10–20°E/> 75°SE). The ruptures propagated upward to the shallow subsurface along a < 3-cm-wide specific slip zone with dextral-slip sense, along which the deformation mechanism is characterized mainly by granular flow in near-surface region. The ruptures either reach the surface or are terminated in unconsolidated sediment below the surface. In the latter case, the rupture splays show westward bifurcation, and their geometry and kinematics show a change to NNW-strike with low-angle dip and dextral-reverse oblique-slip sense in the strata. We suggest that the upward termination of the contractional strike-slip ruptures is controlled by the inherited fault geometry that is unfavorable with respect to the stress field (ENE–WSW σHmax) at basement depths in terms of movement on the fault, and the lack of extension of the fault into shallow subsurface; a depth-dependent change in stress from σHmax > σv > σHmin to σHmax > σHmin > σv at depth of a ~ 200 m; and the physical properties of unconsolidated sediment, which have low inter-granular cohesion, resulting in distributed deformation.

Velocity-selected spatial map ion imaging spectrometer for direct imaging of near-surface catalytic activity

We propose and demonstrate an approach permitting direct imaging of the spatial distribution of gas-surface reaction products with &amp;lt;60 μm lateral spatial resolution using a velocity filtered ion imaging technique. We demonstrate direct imaging of the density of hydrogen deuteride (HD) molecules desorbed from a patterned platinum (Pt) thin film exposed to molecular beams of hydrogen (H2) and deuterium (D2). Resonance enhanced multiphoton absorption was performed with a 2 + 1 scheme through the E,F state using a nanosecond UV laser at ∼201 nm. The generated cations of HD, D2, and H2 were velocity filtered and accelerated with ion imaging optics toward a multichannel plate and phosphor screen. To reduce the significant image blur caused by the translational energy of the parent molecules, a grounded pinhole with 50 μm diameter is placed at the velocity-mapped imaging plane of the ion optics, which velocity-filters the ions that form the image of the near-surface origination plane, improving the ion imaging resolution by a factor of ∼10. The instrument demonstrates the capability to directly image catalytic output in the gas phase in the near-surface region with tens of micrometers of spatial resolution simultaneously with mass and molecular velocity resolution.