Transmission electron microscopy study of the β′ phase (Al-Mg-Si alloys) and QC phase (Al-Cu-Mg-Si alloys): ordering mechanism and crystallographic structure

The β′ and QC rod-shaped precipitates, observed in the Al-Mg-Si and Al-Cu-Mg-Si alloys, respectively, are studied by transmission electron microscopy (TEM). Superstructure images and microdiffraction patterns confirm the likeness between the β′ and QC phases. The microdiffraction patterns are compared with computed ones in the aim to refine a previous structural model based on the existence of a latent disordered phase QP constituted by sub-unit triangular clusters. The space group of the β′ and QC phase is found to be hexagonal P6̄2m. Besides its smaller hexagonal parameter (a = 6.7 nm for QC and a = 0.71 nm for β′), the QC structure differs from the β′ one due to the Cu atoms contained in addition to the Si atoms in the triangular sites of the clusters. The phase transition of QC (or β′) from the latent phase QP (or βP) is concluded to be displacive. The complete phase transition (QP → QC → Q) or (βP → β′ → B′) during the elaboration of the materials is discussed.

Characterization at the Atomistic Level of Defective Structures in Complex Materials

Piyush Agrawal

Defects are key to enhance or deploy particular materials properties. In this thesis I present analyses of the impact of defects on the electronic structure of materials using combined experimental and theoretical Electron energy loss spectroscopy (EELS) in (Scanning) Transmission Electron Microscopy. The energy loss near-edge structure (ELNES) in EELS reflects the element-specific electronic structure providing insights into bonding characteristics of individual atomic species. New electron optical devices have boosted the analytical capabilities by which materials can be investigated with atomic resolution and single atom sensitivity using (scanning) transmission electron spectroscopy (STEM/TEM).With the help of aberration correctors for forming small electron probes, high intensity electron beams can nowadays be focused to clearly less than 100 pm which has enhanced the resolution and sensitivity in analytical scanning transmission electron microscopy (STEM). Various kinds of defects in different complex oxides were studied: point defects like oxygen vacancies in BiVO4 and SrMnO3, edge-dislocations in BiFeO3, and planar defects in GaAs. By comparison with experimental data, structures for these systems were proposed based on all-electron density functional theory (DFT) code Wien2k. By comparing theoretical calculations and experimental data, a pronounced surface reduction in the oxidation state of vanadium in BiVO4 from +5 to +4 was unveiled, which is due to a high density of oxygen vacancies, and its importance in potential application of BiVO4 in photoelectrochemical energy conversion. A similar study was performed on a series of SrMnO3 thin films with different epitaxial strain where theoretical investigations revealed the impact of oxygen non-stoichiometry and strain on the O-K ELNES. In the next study, molecular dynamics simulations were combined with FEFF-based EELS calculations and its comparison with experiments was helpful for the correct prediction of the edge dislocation core structure in BiFeO3. This study also confirmed the presence of Fe atoms in the core of the edge dislocation which possibly makes these defects ferromagnetic whereas the bulk structure is known to be antiferromagnetic. This thesis has established methodologies for utilizing different codes, illustrating how links between experimental and theoretical ELNES can be used in revealing structural information around defects and how defects affect materials properties. This tandem methodology of theory and experiments is applicable to various future materials where the reliable interpretation of EELS data is pivotal in unfolding mysteries of such technologically important materials.

EPFL2017

Energy dependent structure of Xe ion tracks in YBCO and the effect on the superconductive properties in magnetic fields

Philippe Buffat, Elena Suvorova Buffat

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 kgm−3 for YBCO matrix to 3600 kgm−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.

2019

Optimization of composition and structure of cemented carbide cutting tools

Samy Adjam

WC-Co cemented carbides are composites, which combine a hard phase consisting of WC grains and a metallic ductile phase as a binder. Their excellent mechanical properties, combining high hardness, toughness and refractory properties, make them excellent materials for cutting processes such as turning, drilling and milling. For such applications, cemented carbides are coated with thin ceramic films that increase their resistance to thermal stress as well as to the mechanical and chemical wear inherent in extreme machining conditions. Three main characteristics determine the performance and wear resistance of cutting tools: the cohesion of the hard phase above 1200 K, the ductility of the cobalt binder, and the mechanical and thermal resistance of the coatings. This work focuses on the impact of the microstructural behaviour of the cobalt binder on the tool-life of the cutting tools. The aim is to develop new binders for cemented carbides with improved impact resistance. The techniques of transmission electron microscopy at room temperature and in-situ at high temperature, as well as mechanical spectroscopy in a forced torsion pendulum have proved to be essential for the identification of the microstructure of cobalt and for the study of the relaxation phenomena occurring therein. Mechanical spectroscopy has evidenced the presence of two relaxation peaks, P1 and P2 in the cobalt phase and a peak (P3) related to WC-WC grain boundary sliding. The peak P1 appears to be correlated with the cutting tool life measured in interrupted cutting tests. In this work, the role of the binder is highlighted in particular by the identification of a glass-type transition observed for cobalt. The thermodynamics of the transition is evidenced by the behavior of P1 whose relaxation time diverges according to a Vogel-Fulcher-Tammann model, which is characteristic of glass transitions. The state of the material corresponding to temperatures below this transition shows a strong dependence on its thermomechanical history. This dependence is confirmed by a divergence in plastic deformations over two identical temperature cycles with different thermomechanical histories. Such a divergence on a standard protocol, called (ZFC/FC), seems to indicate a break in the thermodynamic hypothesis of ergodicity, even though it could not be attributed solely to the behaviour of cobalt. Transmission electron microscopy at room temperature has determined that the low temperature phase of the cobalt binder has a face-centred cubic structure and is composed of twinned nanodomains containing nanotwins. This state is therefore characterised by a short-range order without the long-range order, which is characteristic of glassy materials. In-situ observations have highlighted a detwinning process corresponding to the temperatures of the cobalt relaxation peak identified by mechanical spectroscopy. This detwinning process continues until the appearance of a long-range ordered face-centred cubic phase above 1050K, which is favourable to deformation. Mechanically, this transition is similar to a brittle-to-ductile transition, greatly influencing the properties of the material and its toughness. The cutting tool life could benefit from lowering the brittle to ductile transition temperature.