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Stekovic, Svjetlana
Publications (10 of 20) Show all publications
Cruchley, S., Sun, J., Taylor, M., Evans, H., Bowen, P., Sumner, J., . . . Stekovic, S. (2014). Cautionary note on use of focused ion beam sectioning as technique for characterising oxidation damage in Ni based superalloys. Materials at High Temperature, 31(1), 27-33
Open this publication in new window or tab >>Cautionary note on use of focused ion beam sectioning as technique for characterising oxidation damage in Ni based superalloys
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2014 (English)In: Materials at High Temperature, ISSN 0960-3409, E-ISSN 1878-6413, Vol. 31, no 1, p. 27-33Article in journal (Refereed) Published
Abstract [en]

Previous observations on Ni based superalloys, obtained through the use of focused ion beam (FIB) sample preparation and imaging, have reported the presence of subsurface voids after oxidation. In this present study, oxidised specimens of the Ni based superalloy, RR1000, were subjected to conventional sample preparation as well as both dual and single beam FIB preparation, with the aim of re-examining the previous observations of subsurface void formation. It is clear from FIB preparations that features previously interpreted as networks of voids have been demonstrated to be internal oxides by varying the sample tilt angles and imaging signal using either secondary electrons (SEs) or secondary ions (SIs). Conventional preparation methods illustrate the presence of subsurface alumina intrusions and the absence of voids, supporting previous evidence. The positive identification of voids and oxides by FIB can be complex and prone to misinterpretation and thus, the use of several imaging conditions and tilt angles must be used, along with conventional preparation methods, to confirm or refute the presence of voids underneath oxides.

Place, publisher, year, edition, pages
Science Reviews 2000 Ltd., 2014
Keywords
Focussed ion beam; Ni based superalloy; Oxidation; Internal oxidation; Voids; SIMS
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-106108 (URN)10.1179/0960340913Z.0000000004 (DOI)000333390000005 ()
Available from: 2014-04-25 Created: 2014-04-24 Last updated: 2017-12-05
Messe, O. M., Stekovic, S., Hardy, M. C. & Rae, C. M. (2014). Characterization of Plastic Deformation Induced by Shot-Peening in a Ni-Base Superalloy. JOM: The Member Journal of TMS, 66(12), 2502-2515
Open this publication in new window or tab >>Characterization of Plastic Deformation Induced by Shot-Peening in a Ni-Base Superalloy
2014 (English)In: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 66, no 12, p. 2502-2515Article in journal (Refereed) Published
Abstract [en]

The shot-peening process is currently employed in most industries to improve the longevity of components by inhibiting crack initiation as well as crack growth at the surface. The protective effect of shot peening has been mainly attributed to compressive stresses within the deformed layer. Intensive research has been carried out to quantify the near-surface residual stresses on entry into service and evolution throughout life. In nickel-base superalloys, the focus of research on the effects of shot-peening has performed using x-rays from either laboratory or synchrotron-based sources. However, this approach cannot evaluate in detail the deformation mechanisms nor the role of the gamma precipitates in a nickel-base superalloy; the latter is responsible for its unique properties. Our study uses a complementary range of techniques to investigate in detail the microstructure and deformation mechanisms associated with shot-peening in a coarse-grained nickel-based superalloy strengthened with coherent gamma precipitates. These include scanning electron microscopy and transmission electron microscopy, nanoindentation and micropillar compression. Accurate mapping of the dislocation structure produced throughout the deformed layers have been performed. Using an unconventional specimen preparation technique, it provides the basis for a more complete interpretation of how shot-peening inhibits fatigue cracking.

Place, publisher, year, edition, pages
Springer Verlag (Germany), 2014
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:liu:diva-113049 (URN)10.1007/s11837-014-1184-8 (DOI)000345397200017 ()
Note

Funding Agencies|EPSRC [EP/H500375/1, EP/H022309]; Rolls-Royce plc. [EP/H500375/1, EP/H022309]; TSB project SILOET TP [AB266C/4]

Available from: 2015-01-09 Created: 2015-01-08 Last updated: 2017-12-05
Cruchley, S., Evans, H. E., Taylor, M. P., Hardy, M. C. & Stekovic, S. (2013). Chromia layer growth on a Ni-based superalloy: Sub-parabolic kinetics and the role of titanium. Corrosion Science, 75, 58-66
Open this publication in new window or tab >>Chromia layer growth on a Ni-based superalloy: Sub-parabolic kinetics and the role of titanium
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2013 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 75, p. 58-66Article in journal (Refereed) Published
Abstract [en]

Oxidation of the Ni-based superalloy RR1000 has been undertaken in air over the temperature range 600-900 degrees C for times up to 5000 h. The surface oxide consisted of a protective Ti-doped chromia layer but with rutile forming on its outer surface. Sub-surface oxidation of Al and Ti also occurred. The thickening kinetics of the chromia layer were sub-parabolic with initial rates around two orders of magnitude higher than expected for Ti-free chromia. This enhancement and the sub-parabolic kinetics are accounted for by Ti-doping of the chromia layer. Over time the enhancement reduced because of Ti-depletion in the alloy.

Place, publisher, year, edition, pages
Elsevier, 2013
Keywords
Superalloys, Oxidation, Selective Oxidation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-97430 (URN)10.1016/j.corsci.2013.05.016 (DOI)000323469800006 ()
Note

Funding Agencies|Engineering and Physical Sciences Research Council||Rolls-Royce plc||

Available from: 2013-09-12 Created: 2013-09-12 Last updated: 2017-12-06
Stekovic, S. & Ericsson, T. (2007). Influence of Long Term Aging on Microstructure and Low Cycle Fatigue Behaviour of Two Coated Nickel-Base Superalloys.
Open this publication in new window or tab >>Influence of Long Term Aging on Microstructure and Low Cycle Fatigue Behaviour of Two Coated Nickel-Base Superalloys
2007 (English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14667 (URN)
Available from: 2007-10-12 Created: 2007-10-12 Last updated: 2013-10-31
Stekovic, S. (2007). Low Cycle Fatigue and Thermo-Mechanical Fatigue of Uncoated and Coated Nickel-Base Superalloys. (Doctoral dissertation). : Institutionen för ekonomisk och industriell utveckling
Open this publication in new window or tab >>Low Cycle Fatigue and Thermo-Mechanical Fatigue of Uncoated and Coated Nickel-Base Superalloys
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High strength nickel-base superalloys have been used in turbine blades for many years because of their superior performance at high temperatures. In such environments superalloys have limited oxidation and corrosion resistance and to solve this problem, protective coatings are deposited on the surface. The positive effect of coatings is based on protecting the surface zone in contact with hot gas atmosphere with a thermodynamically stable oxide layer that acts as a diffusion barrier. During service life, mechanical properties of metallic coatings can be changed due to the significant interdiffusion between substrate and coating. There are also other degradation mechanisms that affect nickel-base superalloys such as low cycle fatigue, thermo-mechanical fatigue and creep.

The focus of this work is on a study of low cycle fatigue and out-of-phase thermo-mechanical fatigue behaviour of three uncoated and coated nickel-base superalloys. Polycrystalline IN792 and two single crystals CMSX-4 and SCB were coated with four different coatings; an overlay coating AMDRY997 (NiCoCrAlYTa), a platinum aluminide modified diffusion coating RT22 and two innovative coatings with a NiW interdiffusion barrier in the interface called IC1 and IC3. A low cycle fatigue and thermo-mechanical fatigue device was designed and set-up to simulate service loading of turbine blades and vanes. The low cycle fatigue tests were run at 500oC and 900oC while the thermo-mechanical fatigue tests were run between 250oC and 900oC.To simulate long service life, some coated specimens were exposed at 1050oC for 2000 h before the tests.

The main conclusions are that the presence of the coatings is, in most cases, detrimental to LCF lives of the superalloys at 500oC while the coatings do improve the LCF lives of the superalloys at 900oC. Under TMF loading conditions, the coatings have negative effect on the lifetime of IN792. On single crystals, they are found to improve TMF life of the superalloys, especially at lower strains. The tests also indicate that long-term aging influences the fatigue and fracture behaviour of coated superalloys by oxidation and diffusion mechanisms when compared to non-aged specimens. The aged specimens exhibit longer life in some cases and shorter life during other test conditions. Fatigue cracks were in most cases initiated at the surface of the coatings, growing transgranularly perpendicular to the load axis.

Place, publisher, year, edition, pages
Institutionen för ekonomisk och industriell utveckling, 2007. p. 44
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1129
Keywords
AMDRY997, CMSX-4, SCB, Thermo-Mechanical Fatigue, Coatings, Gas Turbine, IC1, IC3, IN792, Low Cycle Fatigue, Nickel-Base Superalloys, RT22
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-9820 (URN)978-91-85895-94-6 (ISBN)
Public defence
2007-10-15, A35, A, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2007-10-12 Created: 2007-10-12 Last updated: 2009-05-14
Stekovic, S., Moverare, J. & Ericsson, T. (2007). Low Cycle Fatigue, Thermo-Mechanical Fatigue and Failure of an Uncoated and Coated Polycrystalline Nickel-Base Superalloy IN792. Metallurgical and Materials Transactions. A
Open this publication in new window or tab >>Low Cycle Fatigue, Thermo-Mechanical Fatigue and Failure of an Uncoated and Coated Polycrystalline Nickel-Base Superalloy IN792
2007 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14666 (URN)
Available from: 2007-10-12 Created: 2007-10-12 Last updated: 2017-12-13
Stekovic, S. & Ericsson, T. (2007). Low-Cycle Fatigue and Damage of an Uncoated and Coated Single Crystal Nickel-Bse Superalloy SCB. International Journal of Materials Research, 98(1), 26-32
Open this publication in new window or tab >>Low-Cycle Fatigue and Damage of an Uncoated and Coated Single Crystal Nickel-Bse Superalloy SCB
2007 (English)In: International Journal of Materials Research, ISSN 1862-5282, Vol. 98, no 1, p. 26-32Article in journal (Refereed) Published
Abstract [en]

This paper presents low-cycle fatigue (LCF) behaviour and damage mechanisms of uncoated and coated specimens of a single crystal nickel-base superalloy SCB tested at 500°C and 900°C. Four coatings were deposited on the base material, an overlay coating AMDRY997, a platinum-modified aluminide diffusion coating RT22 and two innovative coatings called IC1 and IC3 with a NiWdiffusion barrier in the interface. AMDRY997 and RT22 were used as reference coatings. The LCF tests were performed at three strain amplitudes, 1.0, 1.2 and 1.4%, with R ¼ % 1, in laboratory air and without any dwell time. The LCF life of the specimens is determined by crack initiation and propagation. Crack data are presented for different classes of crack size in the form of crack density, that is, the number of cracks normalised to the investigated interface length. Micrographs of damage of the coatings are also shown.

The effect of the coatings on the LCF life of the superalloy was dependent on the test temperature and deposited coating. At 500°C all coatings had a detrimental effect on the LCF life of the superalloy. At 900°C both AMDRY997 and IC1 prolonged the fatigue life of the superalloy by factors ranging between 1.5 and 4 while RT22 and IC3 shortened the life of the coating–substrate system. Specimens coated with RT22 exhibited generally more damage than other tested coatings at 9008 C. Most of the cracks observed initiated at the coating surface and a majority were arrested in the interdiffusion zone between the base material and the coating. No topologically close-packed phases were found. Delamination was only found in AMDRY997 at higher strains. Surface roughness or rumpling was found in the overlay coating AMDRY997 with some cracks initiating from the rumples. The failure morphology at 900°C reflected the role of oxidation in the fatigue life, the crack initiation and propagation of the coated specimens. The wake of the cracks grown into the substrate was severely oxidised leading to the loss of Al and Ti to the oxide and resulting in the formation of a ϒ’ depleted zone. The cracks grew more or less perpendicular to the load axis in a Stage II manner.

 

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14664 (URN)
Available from: 2007-10-12 Created: 2007-10-12 Last updated: 2009-05-14
Stekovic, S. & Ericsson, T. (2007). Thermo-Mechanical and Low Cycle Fatigue of Two Uncoated and Coated Single Crystal Nickel-Base Superalloys, CMSX-4 and SCB. In: Fatigue 2007 - The 6th Engineering Integrity Society International Conference on Durability and Fatigue,2007.
Open this publication in new window or tab >>Thermo-Mechanical and Low Cycle Fatigue of Two Uncoated and Coated Single Crystal Nickel-Base Superalloys, CMSX-4 and SCB
2007 (English)In: Fatigue 2007 - The 6th Engineering Integrity Society International Conference on Durability and Fatigue,2007, 2007Conference paper, Published paper (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-34140 (URN)20915 (Local ID)20915 (Archive number)20915 (OAI)
Available from: 2009-10-10 Created: 2009-10-10
Stekovic, S., Ericsson, T. & Johansson, S. (2006). Comparison of Low-Cycle Fatigue Properties of Two Coated Single Crystal Nickel-Base Superalloys, CMSX-4 and SCB. In: 9th International Fatigue Congress: .
Open this publication in new window or tab >>Comparison of Low-Cycle Fatigue Properties of Two Coated Single Crystal Nickel-Base Superalloys, CMSX-4 and SCB
2006 (English)In: 9th International Fatigue Congress, 2006Conference paper, Published paper (Refereed)
Series
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14665 (URN)
Available from: 2007-10-12 Created: 2007-10-12 Last updated: 2009-06-04
Stekovic, S. (2006). Low Cycle Fatigue and Fracture of a Coated Superalloy CMSX-4. In: 16th European Conference of Fracture, ECF16, Greece: .
Open this publication in new window or tab >>Low Cycle Fatigue and Fracture of a Coated Superalloy CMSX-4
2006 (English)In: 16th European Conference of Fracture, ECF16, Greece, 2006Conference paper, Published paper (Other academic)
Series
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14663 (URN)10.1007/1-4020-4972-2_135 (DOI)978-1-4020-4971-2 (Print) 978-1-4020-4972-9 (Online) (ISBN)
Available from: 2007-10-12 Created: 2007-10-12 Last updated: 2009-06-04
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