liu.seSearch for publications in DiVA
Change search
Link to record
Permanent link

Direct link
BETA
Publications (10 of 22) Show all publications
Norman, V., Stekovic, S., Jones, J. & Whittaker, M. (2020). On the Mechanistic Difference Between In-phase and Out-of-phase Thermo-Mechanical Fatigue Crack Growth. International Journal of Fatigue, 135, 1-11, Article ID 105528.
Open this publication in new window or tab >>On the Mechanistic Difference Between In-phase and Out-of-phase Thermo-Mechanical Fatigue Crack Growth
Show others...
2020 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 135, p. 1-11, article id 105528Article in journal (Refereed) Published
Abstract [en]

The crack driving mechanisms in a coarse grained nickel-base superalloy RR1000 when subjected to in- and out of phase thermo mechanical fatigue are investigated. It is found that the difference in fatigue crack growth rate between these two load conditions is accounted for by the different mechanical conditions at the crack tip region, rather than oxidation effects. This is based on digital image correlation and finite element analyses of the mechanical strain field at the crack tip, which demonstrate that in phase leads to larger crack tip deformation and crack opening. Notably, it is demonstrated that in- and out of phase crack growth rates coincide when correlated to the crack tip opening displacement.

Place, publisher, year, edition, pages
Elsevier, 2020
Keywords
Aerospace, superalloys, thermomechanical fatigue, crack growth rate, crack opening
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-164071 (URN)10.1016/j.ijfatigue.2020.105528 (DOI)2-s2.0-85079177535 (Scopus ID)
Available from: 2020-03-03 Created: 2020-03-03 Last updated: 2020-03-19Bibliographically approved
Syväjärvi, M., Stekovic, S. & S. Gustafsson, M. (2019). Using Buurtzorg model as innovation clustering model for regional capacity building in an entrepreneurial context. In: : . Paper presented at Uddevalla Symposium, 27-29 June, L’Aquila, Italy.
Open this publication in new window or tab >>Using Buurtzorg model as innovation clustering model for regional capacity building in an entrepreneurial context
2019 (English)Conference paper, Oral presentation only (Other academic)
Abstract [en]

Smart Specialization (RIS3) is an innovative approach/strategy to bring together local authorities, academia, businesses and society to boost growth and jobs in Europe. It prioritizes domains, areas and economic activities where regions have a competitive advantage.

 

We introduce an innovation model to facilitate translation of ideas and knowledge into regional implementation. Instead of focusing on technology, the innovator / entrepreneur himself is in focus. The proposed model is based on the Buurtzorg model which originally focusses on supporting patients in health care, and which has an onion frame: the patient is surrounded by informal network (family, etc), next is the Buurtzorg support team, and final level is the formal network (society). The individuals need are steering the health care support, rather than adapting it to the social and economic constraints of the health care system. The team consists of specialists who decide how they organize the work, share responsibilities and make decisions. The team is self-managing and with entrepreneurial spirit.

 

We will argue in this paper that a support mechanism as innovation model may be applicable to regional capacity building, and the support acts as a clustering process. Clusters are often limited by geographical constraints, such as having a number of local actors in a certain field. Clustering may be based on other values than given by physical ones.

 

Our model approach is based on that there are innovators who have visionary ideas which are outside their traditional business. They could potentially be of great important for regional growth since they will create value chains, jobs etc, but given the non traditional innovation character they will not be realized unless there is a support mechanism. Similar to the Buurtzorg model, there is a team of specialists that will support the innovator and the innovator informal network. The specialist team has competencies related to smart specialization, regional growth etc (formal network).

 

When RIS3 is applied to broad areas like advanced materials or nanotechnology such a mediation becomes highly complex. Both these areas include an extremely broad range of areas, examples include anything from food (through modifying functional properties by physical and chemical changes), digital communication or connected systems (new materials’ approaches for faster processors or use of higher/faster band frequencies), to construction related materials (buildings, transport, etc). Very likely they do not have actors located geographically close to create a cluster.

 

For synergistic effects, activities in the RIS3 within advanced materials can be linked to other fields, as well as to implementation synergies. If the smart specialization of advanced materials can interact with efforts in building competence, smart industry, sustainable production, automation, digitalization etc, the advanced materials could then find a value chain in specific avenues instead of building ones in its own area. This is a viable route to hold together the fragmented and broad character of advanced materials field, where transferring/translation activities (competence, technical, etc) are a binding agent. Therefore, the specialist team is crucial for such transfer, and a core element in an innovation model.

Keywords
Smart specialization, innovation growth, model
National Category
Political Science (excluding Public Administration Studies and Globalisation Studies)
Identifiers
urn:nbn:se:liu:diva-162395 (URN)
Conference
Uddevalla Symposium, 27-29 June, L’Aquila, Italy
Available from: 2019-11-29 Created: 2019-11-29 Last updated: 2019-12-12Bibliographically approved
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
Show others...
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
Show others...
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: 2020-03-24
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
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-1223-3069

Search in DiVA

Show all publications