Philippe André Buffat

The main strengthening mechanism for Inconel 718 (IN718), a Ni-based superalloy, is precipitation hardening by γ′ and γ″ particles. It is thus essential, for good alloy performance, that precipitates with the desired chemical composition have adequate size and dispersion. The distribution of the γ′ and γ″ phases and their chemical composition were investigated in the nickel-based Inconel 718 superalloy by taking advantage of the new capabilities of scanning transmission electron microscopy and energy-dispersive X-ray spectrometry using a windowless multiple detector, a high-brightness Schottky electron gun, and a spherical aberration corrector in the illumination probe optics. A small routine was developed to deconvolute the respective compositions of γ′ and γ″ nanoprecipitates embedded in the γ matrix. Keeping the electron probe current low enough—a few hundred pA—prevented excessive irradiation damage during the acquisition of element maps and brought their spatial resolution down to the atomic column level to track their element compositions. The present results agree with and complement atomic probe tomography observations and Thermo-Calc predictions from the literature. The presence of an Al enrichment at the γ′/γ″ interface—which may control the γ″ phase coarsening—is observed in the last row of Al-Nb-Ti columns along this interface. In addition, a few columns with similar composition changes are found randomly distributed in the γ′ phase.

Analytical scanning and transmission electron microscopy were used to study the microstructure of Ce,Er-doped Na_0.5La_0.5MoO₄ laser crystals. Crystals were grown by the Czochralski method from the melts with a nominal composition of Na_0.5La_0.5−xCe_xEr_0.005MoO₄, where x = 0.125 and 0.15, then annealed at 700 and 1000 °C in the oxidizing atmosphere. We found the secondary phase precipitation of Ce₂O₃ oxide in as-grown crystals, while after high-temperature annealing the CeO₂ precipitated crystals are always observed. Impurity ions Ce³⁺ occupy the La sites, and approximately 20% of the nominal Ce content is involved in the formation of Ce oxide secondary phase precipitates. The length of CeO₂ precipitated crystals ranged between 100 nm and 550 nm (average length was 200 nm) and their width was 30–70 nm. The mechanism of CeO₂ formation is discussed. The orientation relationships of Na_0.5La_0.5−xCe_xEr_0.005MoO₄/CeO₂, the degree of coherence of the interface, and the preferential directions of their growth in the matrix were established. CeO₂ crystals precipitated in the matrix cause light scattering with a wavelength comparable to the size of the precipitates and lead to deterioration of optical transparency of the material.

The ternary Cr-Fe-Si system was investigated with X-ray diffraction, energy dispersive X-ray spectrometry, scanning and transmission electron microscopy, and electron diffraction. Samples melted at 1723 K were examined right after cooling or after annealing at 1073 K for 3 days to determine phases, grain sizes, and interphase interfaces. During annealing, a polymorphic transformation of the tetragonal α-FeSi₂ to the orthorhombic β-FeSi₂ phase occurs, while CrSi₂ retains its hexagonal structure at high-temperature treatment. Thin layers of ε-FeSi with a cubic structure were observed and identified within the CrSi₂ grains. Crystallographic orientation relationships are determined at the interphase interfaces. The contributions of lattice mismatch and thermal expansion coefficient misfit to deformation are discussed.

This work reports experimental results of the quantitative determination of oxygen and band gap measurement in the TiN_x electrodes in planar TiN_x top/La:HfO₂/TiN_x bottom MIM stacks obtained by plasma enhanced atomic layer deposition on SiO₂. Methodological aspects of extracting structural and chemical information from (scanning) transmission electron microscopy imaging (bright field and high angular annular dark field), energy dispersive X-ray spectrometry and electron energy loss spectroscopy are thoroughly considered. The study shows that the oxygen concentration is higher in the TiN_xO_y bottom electrode (about 14.2 ± 0.1 at. %) compared to the TiN_xO_y top electrode (about 11.4 ± 0.5 at. %). The following average stoichiometric formulas are TiN₀.₅₂O₀.₂₀ top and TiN₀.₅₄O₀.₂₆ bottom for top and bottom electrodes, respectively. The amount of oxygen incorporated into TiN_x during PEALD because of oxygen impurities in the plasma is minor compared to that because of diffusion from SiO₂ and HfO₂. This asymmetry, together with results on a sample grown on a Si substrate, shows that incorporating oxygen impurity from the plasma itself is a minor part compared to diffusion from the SiO₂ substrate and HfO₂ dielectric during the PEALD growth. We observe the presence of TiO₂ at the interface between the Hf oxide layer and the Ti nitride electrodes as well as at the SiO₂ interface. EELS analysis led to a band gap ranging from 2.2 to 2.5 eV for the bottom TiN_xO_y and 1.7–2.2 eV for the top TiN_xO_y, which is in fair agreement with results obtained on the top TiN_x electrode (1.6 ± 01 eV) using optical absorption spectra. Measurement of sheet resistance, resistivity and temperature coefficient of resistance by a four-point probe on the top TiN_xO_y electrode from 20 to 100 °C corresponds to the typical values for semiconductors.

High-resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy provide information on the structural evolution, dielectric function, and bandgap values of nanocrystalline 10 nm thick lanthanum doped hafnia (La:HfO₂) layers in TiN/La:HfO₂/TiN/SiO₂/Si irradiated with 24, 46, 72, and 160 MeV (0.2–1.2 MeV/u) Xe ions. Swift heavy Xe ions were expected to create significant atomic rearrangements when passed through a solid losing energy mainly through electronic excitation and ionization of the target atoms. Local heating and subsequent rapid cooling in the region around the ion track can lead to re-solidification with the formation of a new stable crystalline phase, and direct crystal-to-crystal transformations are possible. The structure evolution of hafnia nanocrystals from the orthorhombic Pbcm in the pristine layer to the tetragonal P4₂/nmc phase in the 160 MeV Xe ion irradiated layer was observed. The mixture of tetragonal and orthorhombic phases was found in samples irradiated with ions of intermediate energies. Textured hafnia layers were formed as a result of ion irradiation. The changes in plasmon line shape and the blueshift of the plasmon energy-loss peak from 14.9 eV in the pristine layer to 15.4 eV in 46 MeV and 15.9 eV in 160 MeV Xe ion irradiated hafnia reflect structural transformations, the increase in the Hf coordination number, and crystal orientations. Valence-electron energy-loss spectroscopy measurements showed a slight increase in the bandgap value from 6.1 eV in the pristine sample to 6.2 eV and 6.3 eV in irradiated samples with 46 and 160 MeV Xe ions, respectively, and dielectric functions changed insignificantly in irradiated hafnia layers.

The morphology and structure of damaged regions (tracks) produced by swift heavy 167, 77, and 46 MeV ¹³²Xe²³⁺ ions in YBCO-based second generation industrial high temperature superconductors wires (2G HTS) were studied using transmission electron microscopy. It was shown that ions produce tracks aligned along the ion trajectory but of morphology depending on their energy: continuous, nearly continuous, or discontinuous tracks like prolate ellipsoids of 10–15 nm in length and spheroids of 5 nm in diameter. The damaged regions of about 5 nm in diameter contain an amorphous material with a lower density compared to the pristine YBCO. The material density drops from 6300 kg m⁻³ for YBCO matrix to 3600 kg m⁻³ inside the tracks. Barium enrichment was found in the vicinity of the track at a distance of about 10 nm from the center of the defect. Ion damage tracks with different morphologies showed different effectivenesses as pinning centers. Tracks composed of discontinuous pinning centers—spheroids of about 5 nm in diameter—bring the best enhancement of the critical current density. It occurs for the lowest ion energy (46 MeV) in the range of energy loss of 8.9 keV/nm–4.7 keV/nm for Xe ions. The samples showed highest critical current densities of 56 MA/cm² (4.2 K) and 3 MA/cm² (77 K) in self-field, while in magnetic fields of 8 T, the values of 17 MA/cm² (4.2 K) and 1.6 MA/cm² (77 K) were achieved. The reduction of the superconducting volume corresponding to the amorphous radiation defects did not exceed 4% from the total sample volume.

The paper “Structural and Mössbauer studies of evaporated Fe100−x Pdx thin films” by A. Laggoun et al. contains interpretation errors of XRD patterns and therefore cast some heavy doubt on the actual phases present in the Fe-Pd films.