Uncovering temperature-tempted coordination of inclusions within ultra-high-strength-steel via in-situ spectro-microscopy

Funding Information: Authors acknowledge Academy of Finland grant #311934 and Kvantum Institute, University of Oulu (Project CLEAN2STEEL) for the financial support. We also thank the crew of the MAX IV laboratory for their support during the beamtime operation. Authors would like to thank Mr. Tun Nyo for the assistance in sample preparation for SEM and X-PEEM, Mr. Jaakko Hannula for insightful discussions, and the Centre for Material Analysis, University of Oulu is also acknowledged for in-house characterizations. Publisher Copyright: © 2022 The Author(s)Despite the common challenge of investigating non-metallic inclusions within ultra-high-strength-steel (UHSS) at sub-micrometer scale via conventional methods, probing nitride inclusions at elevated temperatures is vital for guiding steel’ performance. Herein, an in-situ spectro-microscopic determination using advanced Synchrotron X-ray absorption spectroscopy (XAS) coupled with photoelectron emission microscopy (PEEM) is employed to explore the local structure and electronic properties of selective h-boron nitride (h-BN) containing inclusions (A1 and A2) embedded within steel matrix. While the variation in the relative intensity of π∗/σ∗ excitonic peaks at spatially different locations refers to the polarization and or thickness effects. Several minute features observed in the 192–195 eV energy range show oxygen (O) substituted nitrogen (N) defects (ON,2N,3N), which are more dominant in A2 inclusion. The observed dominance further explains the relatively high intense π∗ peak in A2 due to increased localization. Weak shoulder on the left side of π∗ peak in both room and high-temperature XAS spectra is ascribed to the interaction between h-BN and the local environment, such as Ca-based inclusion or steel matrix. Defects are commonly found in h-BN, and precise identification of the same is vital as they affect the overall physical, chemical, and mechanical properties. Moreover, significant changes in high-temperature B K-edge XAS spectra, such as relative intensity of π∗/σ∗ excitonic peaks at the same location and reduced intensity of defects, suggest the adjusting nature of BN inclusion, complicating their precise prediction and control towards clean steel production.Peer reviewe

In situ study of ultra-thin magnesium oxide growth on metallic and semiconductor substrates

Ce travail de thèse s’est inscrit dans un cadre fondamental d’étude de la formation contrôlée d’oxydes en couches ultra-minces. Un bâti ultra-vide dédié à la croissance contrôlée de ces oxydes et à leur analyse in-situ (STM-STS, AES et LEED) a été développé. Par une méthode originale de phases alternées de dépôt d’une monocouche atomique de Mg et oxydation à RT, les mécanismes impliqués dans la formation de deux systèmes à fort potentiel applicatif : MgO/Si(100) et MgO/Ag(111) ont été étudiés .Le système MgO/Si(100) a révélé la formation d’une couche ultra-mince de Mg2Si à l’interface entre le MgO et le substrat. En bon accord avec les calculs thermodynamiques réalisés, une cristallisation de cette couche interfaciale dans un processus de dissociation partielle du Mg2Si en MgO à température ambiante a été observée. Identifiée ex-situ par TEM, la relation d’épitaxie associée à cette cristallisation a permis de conclure à la formation d’une hétérostructure MgO / Mg2Si (11-1) / Si(001), témoignant d’une grande qualité d’interface avec le silicium et de la formation d’une couche ultra-mince et amorphe de MgO homogène et isolante (gap de 6 eV).Pour le système MgO/Ag(111), nos résultats expérimentaux couplés aux calculs ab initio de nos partenaires du LAAS ont révélé l’absence de formation d’un alliage de surface ainsi qu’une croissance « liquid-like » du magnésium à RT. Un double empilement O/Mg/O/Mg/Ag(111) suivi d’un recuit UHV à 430°C a ensuite permis la stabilisation d’une couche ultra-mince polaire de MgO(111) qui a été caractérisée par LEED et STM-STS. Les propriétés physico-chimiques et origines possibles de stabilité de cet oxyde polaire ont ensuite été discutées.This PhD work was dedicated to studying the fundamental mechanisms driving the controlled growth of ultra-thin oxide films. An experimental set-up was designed to finely control the growth parameters under UHV conditions while allowing the study of such oxide layers in situ with STM-STS, AES and LEED. Using an original method based on alternate cycles of Mg monolayer adsorption and RT oxidation, we focused on the formation of systems exhibiting a wide range of potential applications: MgO/Si(100) and MgO/Ag(111). The MgO/Si(100) system revealed the growth of an ultra-thin Mg2Si layer at the interface between the MgO and the silicon. In agreement with thermodynamic calculations, a crystallization of this interlayer driven by a partial decomposition of the Mg2Si to a MgO oxide was shown to occur at RT. From ex situ TEM experiments, the involved epitaxial relationship highlighted the formation of an MgO / Mg2Si (11-1) / Si(001) heterostructure. A sharp interface with the silicon was formed, as much as an ultra-thin and amorphous MgO layer exhibiting both a good homogeneity and a high insulating character (bandgap of 6 eV).In the MgO/Ag(111) system, no interfacial alloy formation and a « liquid-like » growth for the Mg were evidenced at RT, using our experimental results coupled with the ab initio calculations performed by our co-workers at LAAS laboratory. Later, a double-layering O/Mg/O/Mg/Ag(111) grown at RT followed by UHV annealing at 430°C resulted in the stabilization of a polar MgO(111) ultra-thin film, which was characterized using LEED and STM-STS. The physicochemical properties of this polar oxide and the potential origin of its stability were discussed

Study of the Very First Stages of Mg Growth onto Si(100)

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MgO monolayer epitaxy on Ni (100)

International audienceThe growth of two-dimensional oxide films with accurate control of their structural and electronic properties is considered challenging for engineering nanotechnological applications. We address here the particular case of MgO ultrathin films grown on Ni (100), a system for which neither crystallization nor extended surface ordering have been established previously in the monolayer range. Using Scanning Tunneling Microscopy (STM) and Auger Electron Spectroscopy (AES), we report on experiments showing MgO monolayer (ML) epitaxy on a ferromagnetic nickel surface, down to the limit of atomic thickness. Alternate steps of Mg ML deposition, O2 gas exposure, and ultrahigh vacuum (UHV) thermal treatment enable the production of a textured film of ordered MgO nano-domains. This study could open interesting prospects for controlled epitaxy of ultrathin oxide films with high magneto-resistance (MR) ratio on ferromagnetic substrates, enabling improvement in high-efficiency spintronics and magnetic tunnel junction devices

Ultra-thin MgO(111)-polar sheets grown onto Ag(111)

International audienceAn ultra-thin MgO film of two monolayers (ML) was successfully grown onto Ag(111) by repeating twice RT adsorption of Mg atomic monolayer, RT oxidation and 725 K thermal annealing under UHV conditions. Using Auger electron spectroscopy, low electron energy diffraction and scanning tunneling microscopy/spectroscopy, we report here formation of MgO(1x1) and MgO(1x1)-R30 epitaxial layers, consistent with the growth of a MgO(111) polar film onto the silver surface and composed of patches having extended slight corrugation oriented along defined directions with respect to the substrate

Growth, stability and decomposition of Mg 2 Si ultra-thin films on Si (100)

International audienceUsing Auger Electron Spectroscopy (AES), Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Low Energy Electron Diffraction (LEED), we report an in-situ study of amorphous magnesium silicide (Mg2Si) ultra-thin films grown by thermally enhanced solid-phase reaction of few Mg monolayers deposited at room temperature (RT) on a Si(100) surface. Silicidation of magnesium films can be achieved in the nanometric thickness range with high chemical purity and a high thermal stability after annealing at 150 °C, before reaching a regime of magnesium desorption for temperatures higher than 350 °C. The thermally enhanced reaction of one Mg monolayer (ML) results in the appearance of Mg2Si nanometric crystallites leaving the silicon surface partially uncovered. For thicker Mg deposition nevertheless, continuous 2D silicide films are formed with a volcano shape surface topography characteristic up to 4 Mg MLs. Due to high reactivity between magnesium and oxygen species, the thermal oxidation process in which a thin Mg2Si film is fully decomposed (0.75 eV band gap) into a magnesium oxide layer (6–8 eV band gap) is also reported

<i>In Situ</i> Investigation of Grain Migration by TGZM during Solidification in a Temperature Gradient

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MAXPEEM : a spectromicroscopy beamline at MAX IV laboratory

MAXPEEM, a dedicated photoemission electron microscopy beamline at MAX IV Laboratory, houses a state-of-the-art aberration-corrected spectroscopic photoemission and low-energy electron microscope (AC-SPELEEM). This powerful instrument offers a wide range of complementary techniques providing structural, chemical and magnetic sensitivities with a single-digit nanometre spatial resolution. The beamline can deliver a high photon flux of ≥1015 photons s−1 (0.1% bandwidth)−1 in the range 30–1200 eV with full control of the polarization from an elliptically polarized undulator. The microscope has several features which make it unique from similar instruments. The X-rays from the synchrotron pass through the first beam separator and impinge the surface at normal incidence. The microscope is equipped with an energy analyzer and an aberration corrector which improves both the resolution and the transmission compared with standard microscopes. A new fiber-coupled CMOS camera features an improved modulation transfer function, dynamic range and signal-to-noise ratio compared with the traditional MCP-CCD detection system

A perfect wetting of Mg monolayer on Ag(111) under atomic scale investigation: first principles calculations, scanning tunneling microscopy and Auger spectroscopy

International audienceFirst principles calculations, scanning tunneling microscopy, and Auger spectroscopy experiments of the adsorption of Mg on Ag(111) substrate are conducted. This detailed study reveals that an atomic scale controlled deposition of a metallic Mg monolayer perfectly wets the silver substrate without any alloy formation at the interface at room temperature. A liquid-like behavior of the Mg species on the Ag substrate is highlighted as no dot formation is observed when coverage increases. Finally a layer-by-layer growth mode of Mg on Ag(111) can be predicted, thanks to density functional theory calculations as observed experimentally

Electrostriction, Electroresistance and Electromigration in Epitaxial BaTiO3 - based Heterostructures: Role of Interfaces and Electric Poling

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