Characterisation and absolute quantification of nanosized V and Nb precipitates in high manganese steel using DualEELS

Bobynko, Joanna (2018) Characterisation and absolute quantification of nanosized V and Nb precipitates in high manganese steel using DualEELS. PhD thesis, University of Glasgow.

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The dispersion strengthening is a process that strengthens the material's matrix with use of the smallest precipitates. The most power comes from the character of the nanostructures, their chemical compositions, crystallographic structures, sizes, shapes and interfaces between matrix and precipitates. Through analytical methods it is possible to study and achieve the optimal mechanical properties of elements produced from high-manganese steels with dispersive nanoparticles such as carbides or carbo-nitrides. So far the most effective research methods are transmission electron microscopy and spectroscopy with use of multiple detectors such as Dual Electron Energy Loss Spectrometer (DualEELS), Energy Dispersion X-ray detector, (EDX) or Gatan Imaging Filter (GIF). The DualEELS, which is mostly developed during this thesis, is a technique which results in spectrum imaging of all elements presented in the analyzed area.
The material studied during this Ph.D. is a high-manganese steel with vanadium and niobium carbides or carbo-nitrides with various sizes from few to dozen of nanometers, but the average is less than 20nm.
The main goal of this Ph.D. is to advance the current available methods of nano-analysis using DualEELS of the smallest precipitates embedded in the matrix supported by improved and optimized sample preparation method with Focused ion Beam (FIB) lamella. The advancement is made in several fields including development of acquisition conditions, development of sample preparation technique using low voltages at FIB, and development and adaptation of the novel post-processing routines of the acquired data using Gatan DualEELS QuantumER post-column detector. The post-processing routine involved quantitative and qualitative analysis of precipitates, which required new values of partial cross-sections and mean free paths of inelastic scattering events estimated and calculated based on experimental (standards) approach. The results consist of fully quantified precipitates (V,Ti)(N,C) and ((Nb,Ti)C) supported by chemical profiles, comparison discussion between specimen, with a pseudo-3D reconstruction of spectrum imaging of these precipitates, which means that all elements have individually calculated thicknesses. The results gave a glimpse into the nucleation of the smallest precipitates, unexpectedly different than in most literature, showing that in this case there is no core-shell structure in mixed transition metal carbides. The Ti, which is not an intentional part of the alloy, acts as a nucleation sites, but is not built over core for the V or Nb mixed precipitates. These results are shown by four examples of V-steel and three of Nb-steel. They are presented as spectrum imaging signals, separated from matrix, with pseudo 3D representation and quantified in absolute manner with the accuracy down to a few atoms.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Nanocharacterisation, DualEELS, precipitates, vanadium, niobium, 3d.
Subjects: Q Science > QC Physics
Q Science > QD Chemistry
Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
Supervisor's Name: MacLaren, Dr. Ian and McGroughter, Dr. Damien
Date of Award: 2018
Depositing User: Mrs Joanna Bobynko
Unique ID: glathesis:2018-30670
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 11 Jul 2018 12:44
Last Modified: 05 Dec 2023 12:05
Thesis DOI: 10.5525/gla.thesis.30670
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