Detecting debonding flaws at the epoxy–concrete interfaces in near-surface mounted CFRP strengthening beams using the impact–echo method

This paper discusses the feasibility of using the impact–echo method (IE method) to assess the debonding flaws at the epoxy–concrete interfaces of near-surface mounted carbon fiber reinforced polymers (NSM CFRPs). The effectiveness of NSM CFRP strengthening is influenced by whether epoxy bonds CFRP bars and existing concrete structures effectively and thoroughly, thereby mutually transmitting the stress in the reinforced zones with CFRP bars. Because of poor construction quality, aging, and deterioration, debonding occurs between epoxy and concrete surfaces, severely inhibiting the reinforcement effectiveness of NSM CFRPs. The IE method is based on the principle of stress waves and was adopted in this study as the nondestructive debonding flaw assessment technique. To clarify the characteristics of the stress wave propagation caused by impact forces on NSM CFRP structures, numerical analysis and experimental test were performed on the bar-like epoxy specimens with a cross-sectional size of 13×13 mm. The types of specimens used in this study included the bar-like pure epoxy specimen, bar-like epoxy specimens containing CFRP bars, and the concrete beams containing bar-like epoxy specimens with and without CFRP bars. For the concrete beams embedded with bar-like epoxy specimens, various debonding statuses at the epoxy–concrete surfaces were considered. The numerical analysis revealed that the impact responses in the bar-like pure epoxy specimen and bar-like epoxy specimens containing CFRP bars were dominated by the first few cross-sectional modes of vibration, and the IE test verified this result. The numerical and experimental results indicated that for the concrete beams containing bar-like epoxy specimens with and without CFRP bars and without debonding flaws, the impact response spectra each featured one high-amplitude peak at the fundamental mode frequency, referred to as the dominant frequency. When debonding occurred at the epoxy–concrete interfaces of the concrete beams containing bar-like epoxy specimens with and without CFRP bars, the impact responses showed that the dominant frequencies decreased significantly. When the dominant frequencies instead increased to levels nearly equal to those of the fundamental frequencies of the bar-like epoxy specimens, the epoxy thoroughly debonded from the concrete interfaces. According to the characteristics of the aforementioned impact responses, the IE method is capable of detecting the debonding flaws at epoxy–concrete interfaces in NSM CFRP strengthening.     

Atomic-scale structure and chemistry of ceramic/metal interfaces—II. Solute segregation at MgO/Cu (Ag) and CdO/Ag (Au) interfaces

The first quantitative measurements of solute segregation at ceramic/metal (C/M) heterophase interfaces are presented for the MgO/Cu (Ag) and CdO/Ag (Au) systems. Interfaces are produced by internal oxidation of ternary alloys. Solute segregation at C/M interfaces is induced by intermediate-temperature annealing treatments. The Gibbsian interfacial excess of solute, Γ_solute, at these interfaces is determined in a direct, quantitative manner by atom-probe field-ion microscopy (APFIM). These measurements are complemented in the MgO/Cu (Ag) system by a composition analysis of this interface employing electron energy loss spectroscopy (EELS). Analyses of 15 {222} MgO/Cu (Ag) interfaces by APFIM show an average segregation level of (4.0±1.9)×10¹⁴ atoms/cm² or 0.22±0.10 effective monolayers at 500°C. Analyses of three {222} CdO/Ag (Au) interfaces show an average segregation level of (3.0±1.0)×10¹⁴ atoms/cm² or 0.22±0.07 effective monolayers at 400°C. Whereas {222} CdO/Ag (Au) interfaces in unannealed specimens show no evidence of gold segregation. These results are discussed in view of recent models of interfacial segregation.     

Phosphorus segregation in nanocrystalline Ni–3.6 at.% P alloy investigated with the tomographic atom probe (TAP)

The microstructures of electroless plated and thermally aged nanocrystalline nickel–3.6 at.% phosphorus layers were investigated on an atomic scale with a tomographic atom probe (TAP). After heat treatments at 250 and 400°C, a continuous P-segregation in the grain boundaries of the nanocrystalline structure was directly proved for the first time. This segregation effect explains the comparatively high thermal stability of the material. Assuming the existence of a metastable equilibrium, a simple mass balance calculation, which uses experimentally determined data exclusively, makes it possible to predict grain sizes of other NiP alloys within the thermal stability region.     

Silver segregation to θ′ (Al2Cu)–Al interfaces in Al–Cu–Ag alloys

θ′ (Al₂Cu) precipitates in Al–Cu–Ag alloys were examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The precipitates nucleated on dislocation loops on which assemblies of γ′ (AlAg₂) precipitates were present. These dislocation loops were enriched in silver prior to θ′ precipitation. Coherent, planar interfaces between the aluminium matrix and θ′ precipitates were decorated by a layer of silver two atomic layers in thickness. It is proposed that this layer lowers the chemical component of the Al–θ′ interfacial energy. The lateral growth of the θ′ precipitates was accompanied by the extension of this silver bilayer, resulting in the loss of silver from neighbouring γ′ precipitates and contributing to the deterioration of the γ′ precipitate assemblies.     

Application of the cluster variation method to ε-Fe2N1-z and ζ-Fe2N: ordering of interstitial atoms

The regular and the irregular trigonal prism approximations of the cluster variation method (CVM) are applied to investigate the ε-Fe₂N_1−z and the ζ-Fe₂N phases. The ε-Fe₂N_1-z/ζ-Fe₂N phase boundary, the types of interstitial ordering and the lattice parameters of the ε and ζ phases are calculated and compared with the available experimental data. In the composition range from 25 to 33 at.% N, long-range order occurs. The changes in the N distributions are associated with the strong repulsive interactions between neighbouring N atoms. The CVM results show that with increasing the N content towards 33 at.% N, the repulsive N–N interactions favour the distortion of the iron host lattice, thereby increasing the nearest-neighbouring distance between occupied sites, i.e. transformation to the ζ-Fe₂N phase. The calculated interstitial site occupations are in good agreement with those proposed on the basis of experimental data obtained by Mössbauer spectroscopy and neutron diffraction.     

Equilibrium Diagram in the Presence of a Gaseous Phase and Its Application(Overview Part Ⅱ)

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First-principles calculations of the structural and thermodynamic properties of bcc,fcc and hcp solid solutions in the Al–TM (TM = Ti,Zr and Hf) systems: A comparison of cluster expansion and supercell methods

The thermodynamic properties of solid solutions with body-centered cubic (bcc), face-centered cubic (fcc) and hexagonal close-packed (hcp) structures in the Al–TM (TM = Ti, Zr and Hf) systems are calculated from first-principles using cluster expansion (CE), Monte-Carlo simulation and supercell methods. The 32-atom special quasirandom structure (SQS) supercells are employed to compute properties at 25, 50 and 75 at.% TM compositions, and 64-atom supercells have been employed to compute properties of alloys in the dilute concentration limit (one solute and 63 solvent atoms). In general, the energy of mixing (Δ_mE) calculated by CE and dilute supercells agree very well. In the concentrated region, the Δ_mE values calculated by CE and SQS methods also agree well in many cases; however, noteworthy discrepancies are found in some cases, which we argue originate from inherent elastic and dynamic instabilities of the relevant parent lattice structures. The importance of short-range order on the calculated values of Δ_mE for hcp Al–Ti alloys is demonstrated. We also present calculated results for the composition dependence of the atomic volumes in random solid solutions with bcc, fcc and hcp structures. The properties of solid solutions reported here may be integrated within the CALPHAD formalism to develop reliable thermodynamic databases in order to facilitate: (i) calculations of stable and metastable phase diagrams of binary and multicomponent systems, (ii) alloy design, and (iii) processing of Al–TM-based alloys.     

Equilibrium Diagram in the Presence of a Gaseous Phase and Its Application(Overview Part Ⅰ)

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Phase Relationship in the Al-rich(Al 0.15at %V)-Mg-Zn System

Isothermal sections at room temperature and 450℃ for Al-rich(Al 0.15at%V)-Mg-Zn system have been determined bymeans of X-ray diffraction,microscopic observation and DTA methods.Comparing with Al-Mg-Zn system the α-phase regionof in isothermal sections room at temperature and 450℃ for the system with 0.15at%V is slightly narrowed and expandedrespectively.The solubilitiy limits of Zn and Mg in α-Al have been found to be 3.3at%(7.6wt%)and 5.5at%(5.0wt%)respectively at room temperature,and to be 6.0at%(13.4wt%)and 11.0at%(10.0wt%)at 450℃.The τ phase in this system is aternary intermetallic compound,which undergoes an allotropic transformation at about 427℃.     

Influence of hydrogen on the deformation morphology of vanadium (1 0 0) micropillars in the α-phase of the vanadium–hydrogen system

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Structural variations in the ternary system HfAl2−xCux (x = 0.2–1.0)

Synthesized ternary intermetallic phases HfAl_2?xCu_x in a series with (x = 0.49, 0.88, 1.04) are characterized as Laves phase structures. X-ray diffraction revealed homogeneity within ranges 0.2 ≤ x ≤ 0.5 and 0.7 ≤ x ≤ 0.9 for structure types MgCu₂ and MgNi₂, respectively. When Cu atoms gradually replaced the Al atoms, the structure type altered in the sequence MgCu₂ → MgNi₂ → MgZn₂, and the Kagomé nets were distorted with varied bond lengths. Measurements of physical properties revealed these phases to be metallic, with resistances 4.35 (x = 0.5), 5.85 (x = 0.7), and 6.50 (x = 0.9) mΩ cm, respectively, at temperature 300 K. The coloring schemes reveal that, upon increasing the proportion of Cu atoms, the stability of these phases correlated with the arrangements of the Al and Cu atoms. Calculated electronic structures indicate that the bonding character is consistent with the experimentally observed phase width.     

Hydrogen sorption in the LaNi5-xAlx-H system (0 ≤ x ≤ 1)

A thermodynamic assessment of hydrogen sorption in LaNi_5-xAl_x-H (0 ≤ x ≤ 1) alloys was performed combining experimental and theoretical investigations. The occurrence of sloped plateaux in Pressure Composition Isotherms (PCI) is discussed on the basis of compositional inhomogeneities and of possible re-distribution of metallic elements between the hydrogen rich and the hydrogen poor phases.In order to provide input for the thermodynamic assessment, literature data were reviewed and PCI experiments were performed at increasing temperatures on alloys with different Al contents. The presence of a compositional distribution was investigated using Rietveld refinement of X-Ray diffraction patterns, volumetric and calorimetric measurements, both on as received and annealed samples with LaNi_4.8Al_0.2 nominal composition. Moreover, ab initio energies of formation for LaNi₅, LaAl₅ and the respective hydrides were calculated. A full CALPHAD assessment of the Gibbs energy for LaNi_5-xAl_xH_y (0 ≤ x ≤ 1, 0 ≤ y ≤ 7) phase was obtained, showing a good agreement between experimental and calculated results.The presence of sloped and flat plateaux has been related to the occurrence of ortho- and para-equilibrium conditions.     

Thermal stability of Al1−xInxN (0 0 0 1) throughout the compositional range as investigated during in situ thermal annealing in a scanning transmission electron microscope

The thermal stability of Al_1?xIn_xN (0 ? x ? 1) layers was investigated by scanning transmission electron microscopy (STEM) imaging, electron diffraction, and monochromated valence electron energy loss spectroscopy during in situ annealing from 750 to 950 °C. The results show two distinct decomposition paths for the layers richest in In (Al_0.28In_0.72N and Al_0.41In_0.59N) that independently lead to transformation of the layers into an In-deficient, nanocrystalline and a porous structure. The In-richest layer (Al_0.28In_0.72N) decomposes at 750 °C, where the decomposition process is initiated by In forming at grain boundaries and is characterized by an activation energy of 0.62 eV. The loss of In from the Al_0.41In_0.59N layer was initiated at 800 °C through continuous desorption. No In clusters were observed during this decomposition process, which is characterized by an activation energy of 1.95 eV. Finally, layers richest in Al (Al_0.82In_0.18N and Al_0.71In_0.29N) were found to resist thermal annealing, although the initial stages of decomposition were observed for the Al_0.71In_0.29N layer.     

Phase Equilibria in the Zn-Rich Corner of the Zn-Cu-Ti-RE (Ce,La) Systems at 450 °C

The Zn-rich corner of the Zn-Cu-Ti-RE(Ce, La) quaternary systems at 450 °C have been experimentally investigated by scanning electron microscopy coupled with energy dispersive x-ray spectrometer and x-ray diffraction. The liquid containing phase regions have been well identified in these two systems. The ternary compound τ (Cu₂TiZn₂₀) can equilibrate with all phases in the Zn-rich corner of these two systems. Neither Ce nor La can dissolve into TiZn₁₆, ε(CuZn₄) and τ(Cu₂TiZn₂₀). While the solubility of Cu in CeZn₁₁ and LaZn₁₃ can reach 2.9 at.% and 5.0 at.%, respectively. The results can help to design and analyze the Zn-Cu-Ti-RE alloys.     

Atomistic structure of the Cu(1 1 1)/α-Al2O3(0 0 0 1) interface in terms of interatomic potentials fitted to ab initio results

We propose an atomistic model to describe the copper/sapphire interface by means of simple interatomic potentials involving only a few fitting parameters. Successful results are achieved when the copper atoms in the monatomic layer closest to the interface have properties different from the bulk. This layer is to accommodate the ionic/covalent bonding in the ceramics to the metallic bonding in copper. For an oxygen terminated interface, we fit the parameters of the potentials to the results of a rigid tensile test (explained in the text) simulated from first principles. The results of atomic relaxation near the interface are shown to be consistent with ab initio and experimental results available in the literature. Calculations reveal highly interesting relaxation dynamics near the interface. In the early stage of relaxation, a periodic network of partial misfit dislocations is formed, which later transforms into an irregular network due to the instability of the layer of copper atoms atop the oxygen atoms. This explains the interface incoherency observed in high-resolution electron microscopy. Calculations based on the FK model reproduce this effect.     

Optimum glass formation at off-eutectic composition and its relation to skewed eutectic coupled zone in the La based La–Al–(Cu,Ni) pseudo ternary system

Glass formation and its relation to the dendritic and eutectic growth has been investigated for the La_100−x[Al_0.412(Cu,Ni)_0.588]_x (x=30–56.3) and La_86−yAl₁₄(Cu,Ni)_y (y=16–29) alloy systems. The experimental results show that in the La-rich pseudo ternary La–Al–(Cu,Ni) system, optimum glass formation actually occurs at an off-eutectic composition. A nearly fully amorphous rod with 12 mm in diameter can be obtained at an off-eutectic composition near La₆₂Al_15.7(Cu,Ni)_22.3, while only a 1.5 mm diameter rod can be obtained fully amorphous for the eutectic alloy La₆₆Al₁₄(Cu,Ni)₂₀. A strong dependence of GFA on the composition is observed for these alloys. In addition, formation of a composite (i.e. αLa dendrite reinforced glass matrix) in 12 mm diameter cast rods is observed over a wide range of composition, including the eutectic composition. It has been found that the GFA does not correlate well with the extent of the undercooled liquid region (ΔT_x) and is not even closely related to the reduced glass transition temperature (T_rg). These unusual observations in glass formation and its relation to the skewed eutectic coupled zone have been explained in terms of the competition between the growth of crystalline phases (i.e. eutectic and dendritic phases) and the formation of the amorphous phase.