| 1. |
- Katnagallu, Shyam, et al.
(författare)
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Impact of local electrostatic field rearrangement on field ionization
- 2018
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Ingår i: Journal of Physics D. - : IOP PUBLISHING LTD. - 0022-3727 .- 1361-6463. ; 51:10
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Tidskriftsartikel (refereegranskat)abstract
- Field ion microscopy allows for direct imaging of surfaces with true atomic resolution. The high charge density distribution on the surface generates an intense electric field that can induce ionization of gas atoms. We investigate the dynamic nature of the charge and the consequent electrostatic field redistribution following the departure of atoms initially constituting the surface in the form of an ion, a process known as field evaporation. We report on a new algorithm for image processing and tracking of individual atoms on the specimen surface enabling quantitative assessment of shifts in the imaged atomic positions. By combining experimental investigations with molecular dynamics simulations, which include the full electric charge, we confirm that change is directly associated with the rearrangement of the electrostatic field that modifies the imaging gas ionization zone. We derive important considerations for future developments of data reconstruction in 3D field ion microscopy, in particular for precise quantification of lattice strains and characterization of crystalline defects at the atomic scale.
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| 2. |
- Klaes, Benjamin, et al.
(författare)
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A model to predict image formation in the three-dimensional field ion microscope?
- 2021
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Ingår i: Computer Physics Communications. - : ELSEVIER. - 0010-4655 .- 1879-2944. ; 260
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Tidskriftsartikel (refereegranskat)abstract
- Field ion microscopy (FIM) was the first technique to image individual atoms on the surface of a material. By a careful control of the field evaporation of surface atoms, the bulk of the material is exposed, and, through digital processing of a sequence of micrographs, an atomically-resolved threedimensional reconstruction can be achieved. 3DFIM is particularly suited to the direct observation of crystalline defects that underpin the physical properties of materials: vacancies and vacancy clusters, interstitials, dislocations, or grain boundaries. Yet, further developments of 3DFIM are necessary to turn it into a routines technique. Here, we introduce first a protocol for 3DFIM image processing and subsequent tomographic reconstruction. Second, we propose a numerical model enabling simulation of the FIM imaging process. The model combines the meshless algorithm for field evaporation proposed by Rolland et al. (Robin-Rolland Model, or RRM) with fundamental aspects of the field ionization process of the gas image involved in FIM. The proposed model enables the simulation of imaging artefacts that are induced by non-regular field evaporation and by the disturbed electric field distribution near atomic defects. Our model enables more precise interpretation of 3DFIM characterization of structural defects. (C) 2020 Elsevier B.V. All rights reserved.
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| 3. |
- Kontis, Paraskevas, et al.
(författare)
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The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys
- 2018
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Ingår i: Metallurgical and Materials Transactions. A. - : SPRINGER. - 1073-5623 .- 1543-1940. ; 49A:9, s. 4236-4245
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Tidskriftsartikel (refereegranskat)abstract
- Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850 degrees C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850 degrees C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850 degrees C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of gamma precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed gamma matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF.
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