| 1. |
- Stiller, Krystyna Marta, 1947, et al.
(författare)
-
Atom probe tomography of oxide scales
- 2013
-
Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 79:3-4, s. 227-238
-
Tidskriftsartikel (refereegranskat)abstract
- Atom probe tomography, APT, is the only microstructural method that can routinely analyse and position individual atoms in a material with a spatial resolution of 0.1-0.5 nm. Recent implementation of pulsed-laser to APT made investigation of less conducting materials, such as oxides, feasible. In this paper a short description of the principle of the techniques is presented, followed by examples of recent APT studies of thermally grown oxide scales produced on alumina formers (Pt-modified NiAl diffusion coating and FeCrAl alloy), at the crack tips in a Ni-based alloy and on a Zr-alloy. Additionally, results from preliminary studies of ZnO and MgO bulk materials are shown. The obtained information on the atomic scale about the chemistry variations in the scales and at the metal oxide interfaces provides valuable insights into oxidation processes. © 2012 Springer Science+Business Media New York.
|
|
| 2. |
- Thuvander, Mattias, 1968, et al.
(författare)
-
Quantitative atom probe analysis of carbides
- 2011
-
Ingår i: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 111:6, s. 604-608
-
Tidskriftsartikel (refereegranskat)abstract
- Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti 2 AlC grains in different materials as well as on large M 2 3C 6 precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the 13 C isotope, and calculating the concentration of 12 C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) andTi 2 AlC an additional difficulty arises from the overlap between C 2 + ,C 2+ 4 and Ti 2+ at the mass-to-charge 24 Da. © 2010 Elsevier B.V.
|
|
| 3. |
- Hörnqvist Colliander, Magnus, 1979, et al.
(författare)
-
High-temperature crack growth in a Ni-base superalloy during sustained load
- 2014
-
Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 609, s. 131-140
-
Tidskriftsartikel (refereegranskat)abstract
- he high-temperature sustained load crack growth behaviour of a Ni-base superalloy was investigated using a combination of mechanical testing in controlled atmosphere, fractographical and microanalytical investigations, and finite element modelling. The results show that the local crack front geometry is un- even on two scales – jaggedness on the scale of 100 μm was observed in all specimens, whereas mm- scale waviness could occasionally be observed. The jaggedness can be explained by a percolation-type crack growth along weaker grain boundaries, whereas the large-scale waviness is presumably due to larger regions of the material having specific grain texture with high crack growth resistance. The un-even crack front is shown to potentially have considerable effects on the loading conditions at the crack tip, whereas ligaments of un-cracked material in the crack wake are deemed to have less effect on the crack tip loading due to their low area fraction. The ligaments fail intergranularly in the wake as the crack grows in the present case, as opposed to by creep fracture as previously proposed. Finally, the plastically deformed regions about the crack and crack tip are shown not to exhibit any elevated oxygen levels, implying that the damage in these regions is purely mechanical.
|
|
| 4. |
- Hörnqvist, M., et al.
(författare)
-
Hold-time fatigue crack growth of Allvac 718Plus
- 2010
-
Ingår i: 7th International Symposium on Superalloy 718 and Derivatives 2010. Pittsburgh, 10-13 October 2010. - 9781617827709 ; 2, s. 705-717
-
Konferensbidrag (refereegranskat)abstract
- The hold-time fatigue behaviour of Allvac 718Plus was investigated both in the as-heat treated condition and after long-time exposure at temperature close to the maximum operating limit. At temperatures above 450°C the introduction of 90 s hold-time at maximum load significantly increased the fatigue crack growth rate. Both conditions showed purely cycle dependent behavior at 450°C and close to purely time dependent behavior at 700°C. At 600°C intermediate behavior was observed. The long-time exposure had little effect on the cyclic (0.5 Hz) crack growth rates, but the resistance to crack growth with 90 s hold-time decreased. No microstructural effects of long-time exposure (700°C/140 h + 675°C/460 h) could be observed by SEM, but there were indications that the hardness of the material increases somewhat after exposure.
|
|
| 5. |
- Johansson, S., et al.
(författare)
-
Fracture mechanisms during intergranular hold time fatigue crack growth in Inconel 718 superalloy
- 2013
-
Ingår i: 13th International Conference on Fracture 2013, ICF 2013; Beijing; China; 16 June 2013 through 21 June 2013. ; 3, s. 1833-1841
-
Konferensbidrag (refereegranskat)abstract
- Ni-base superalloy IN718 is known to display time-dependent intergranular crack growth under dwell time mechanical loading at high temperature under atmospheric conditions. Oxygen has been pointed out as a cause of the intergranular damage causing embrittled crack growth during both cyclic and hold time loading. Investigation of the mechanisms responsible for the embrittlement should not only focus on the effect of environment but also on the combined action of fatigue, creep, temperature and time. In this work material from experiments with fatigue crack growth in combination with hold times of different length at different temperatures has been investigated. Fractographic studies and metallographic cross sections of fatigued specimens has been subjected to careful analysis using ECCI-imaging in order to shed light on the fracture mechanisms. The results show that the damage is caused by the influence of a combination of environment and severe local damage manifested as a transformation of the microstructure into sub cells, micro twins and recrystallised areas close to the crack tip. The damage mechanism is thus influenced by a combination of oxidation and severe local plastic deformation.
|
|
| 6. |
- Johnson, Lars, et al.
(författare)
-
Blind deconvolution of time-of-flight mass spectra from atom probe tomography
- 2013
-
Ingår i: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 132, s. 60-64
-
Tidskriftsartikel (refereegranskat)abstract
- A major source of uncertainty in compositional measurements in atom probe tomography stems from the uncertainties of assigning peaks or parts of peaks in the mass spectrum to their correct identities. In particular, peak overlap is a limiting factor, whereas an ideal mass spectrum would have peaks at their correct positions with zero broadening. Here, we report a method to deconvolute the experimental mass spectrum into such an ideal spectrum and a system function describing the peak broadening introduced by the held evaporation and detection of each ion. By making the assumption of a linear and time-invariant behavior, a system of equations is derived that describes the peak shape and peak intensities. The model is fitted to the observed spectrum by minimizing the squared residuals, regularized by the maximum entropy method. For synthetic data perfectly obeying the assumptions, the method recovered peak intensities to within +/- 0.33 at%. The application of this model to experimental APT data is exemplified with Fe-Cr data. Knowledge of the peak shape opens up several new possibilities, not just for better overall compositional determination, but, e.g., for the estimation of errors of ranging due to peak overlap or peak separation constrained by isotope abundances. (C) 2013 Elsevier By. All rights reserved.
|
|
| 7. |
- Johnson, Lars, et al.
(författare)
-
Spinodal decomposition of Ti0.33Al0.67N thin films studied by atom probe tomography
- 2012
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 520:13, s. 4362-4368
-
Tidskriftsartikel (refereegranskat)abstract
- Details of the phase decomposition in NaCl-structure Ti0.33Al0.67N thin films deposited by cathodic arc evaporation are studied by atom probe tomography. We demonstrate that as-deposited films are in the earliest stage of decomposition for which electron microscopy and x-ray diffraction indicate a single-phase solid solution. Annealing at 900 degrees C further activates spinodal decomposition of the films, although pockets of unde-composed material remain after 2 h. N preferentially segregates to the AlN and TiN domains, causing the TiAlN matrix to be understoichiometric, by the energetics of N vacancies in TiAlN. The corresponding modulation in stoichiometry implies a Kirkendall effect, caused by different Al and Ti diffusivities.
|
|
| 8. |
- Kadkhodazadeh, S., et al.
(författare)
-
Towards quantitative three-dimensional characterisation of buried InAs quantum dots
- 2011
-
Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 326:1
-
Konferensbidrag (refereegranskat)abstract
- InAs quantum dots grown on InP or InGaAsP are used for optical communication applications operating in the 1.3 - 1.55 μm wavelength range. It is generally understood that the optical properties of such dots are highly dependent on their structural and chemical profiles. However, morphological and compositional measurements of quantum dots using transmission electron microscopy can be ambiguous because the recorded signal is usually a projection through the thickness of the specimen. Here, we discuss the application of scanning transmission electron microscopy tomography to the morphological and chemical characterisation of surface and buried quantum dots. We highlight some of the challenges involved and introduce a new specimen preparation method for creating needle-shaped specimens that each contain multiple dots and are suitable for both scanning transmission electron microscopy tomography and atom probe tomography.
|
|
| 9. |
- Knutsson, Pavleta, 1980, et al.
(författare)
-
A method for investigation of hot corrosion by gaseous Na2SO4
- 2013
-
Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 73, s. 230-236
-
Tidskriftsartikel (refereegranskat)abstract
- A novel in situ exposure method for investigating hot corrosion of aluminde coatings in the absence of an alkali sulphate melt is presented. The samples are exposed to Na2SO4(g) at a temperature above the dew point of the gas. Results from 100 and 500 h exposures at 900 degrees C of Ni-based In792 covered with SIF232 aluminide coating are presented. Already after 100 h of testing, attack characteristic for Type I hot corrosion is observed. The reactions taking place during the testing are discussed and the results are compared with those from the well-known ex situ salt hot corrosion test.
|
|
| 10. |
- Lai, Haiping, 1979, et al.
(författare)
-
High Temperature Corrosion of Ni-Based Alloys SCA425+and IN792
- 2013
-
Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 80:5-6, s. 505-516
-
Tidskriftsartikel (refereegranskat)abstract
- This paper reports the high temperature corrosion of two Ni-base superalloys: a newly developed alloy, SCA425+, and the extensively used IN792. The composition of the two materials is quite similar, but SCA425+ contains more Cr and Al (17.1 and 10 at.% compared to 13.9 and 7.4 at.% in IN792). The results from exposures at 900 A degrees C in SO2 (3,000 ppm) + O-2 (69 vol%) + H2O (31 vol%) mixed gas for 260 h using 65 h/cycle are compared with those obtained from tests in laboratory air. The microstructure of the formed oxide scales was studied using several techniques, such as XRD, SEM, FIB, EDX, STEM and XPS. It is shown that in IN792 severe internal oxidation takes place after both types of exposures. On the other hand, the newly developed SCA425+ has the tendency to form an alumina layer proving that it has more potential to be used in the aggressive environments. Surprisingly the mass gains for SCA425+ alloy exposed in SO2-rich environment are lower than in laboratory air. The reason for this behavior is discussed.
|
|
