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Creep Fracture Mechanism of Polycrystalline Ni-based Superalloy with Diffusion Coatings
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Siemens Industrial Turbomachinery AB, Finspång.
Siemens Industrial Turbomachinery AB, Finspång.
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2013 (English)Conference paper, Published paper (Other academic)
Abstract [en]

Diffusion coatings are widely used to increase oxidation and corrosion resistance of hot superalloy components for gas turbines. The aim of this study is to investigate the effect of coatings (NiAl and PtAl) on the creep fracture mechanism of samples with a substrate of IN792. The samples have been creep tested at two temperatures (850 °C and 950 °C) and different applied tensile stresses, until failure between 205 and 21000 hrs. The observation of cross-sections by SEM shows that the microstructural evolution in the coating is dependent on the diffusion of alloying elements in the sample. Furthermore the time and temperature induced growth of the coating is found to be controlled only by inward diffusion of Al. Grain-boundary cracking is the basic fracture mode in the substrate in all samples irrespective if the crack is initiated from coating or substrate. The analysis of microstructure shows that the diffusion coatings display two types of mechanical behavior - being easily plasticized or cracked - dependent on temperature and type of coating, and therefore can be considered as non-load carrying regions. After recalculating the creep stress to exclude the final effective coating thickness from the total sample thickness, the coated samples showed similar creep rupture behavior as the uncoated samples in the Larson-Miller diagram.

Place, publisher, year, edition, pages
2013.
Keyword [en]
Creep; Ni-based superalloy; Diffusion coating; Crack
National Category
Engineering and Technology Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-89497OAI: oai:DiVA.org:liu-89497DiVA: diva2:608157
Conference
13th International Conference on Fracture ICF13, June 16-21, 2013, Beijing, China
Available from: 2013-02-26 Created: 2013-02-26 Last updated: 2013-03-14Bibliographically approved
In thesis
1. Thermal and Mechanical Behaviors of High Temperature Coatings
Open this publication in new window or tab >>Thermal and Mechanical Behaviors of High Temperature Coatings
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

With superior oxidation and corrosion resistance, metallic coatings (i.e. diffusion coatings and MCrAlX coatings) are widely used to protect hot components made of superalloys in turbine engines. Two issues are critically important for the coating at high temperatures: thermal property related to oxidation/corrosion behavior and microstructure stability, and mechanical properties (e.g. creep and fatigue). The aim of this project is to develop better understanding of the thermal and mechanical behaviors of metallic coatings on superalloys and to improve the accuracy of prediction of their lifetime by thermodynamic modeling. The present work includes an investigation on the oxidation behavior of MCrAlX coating with a new lifetimeprediction model and a study on the influence of diffusion coatings on creep and fatigue behaviors of the superalloy IN792.

Experiments on isothermal and thermal cycling oxidation were designed to investigate the oxidation behavior of a HVOF CoNiCrAlYSi coating on superalloy IN792. It is found that the oxidation behaviors of the coating are related to its thermodynamic property. A diffusion model has been established using the homogenization models in the DICTRA software and taking into consideration of the influence of surface oxidation, coating-substrate interdiffusion and diffusion blocking effect caused by internal voids and oxides. The simulation results show an improved accuracy of lifetime prediction by introducing the diffusion blocking effect.

Microstructural evolution during creep process at high temperatures was studied in different diffusion coatings (NiAl and PtAl). It is found that the inward diffusion of aluminum controls the thickening rate of the diffusion coatings. The developed coatings displayed two types of mechanical behavior - being easily plasticized or cracked - dependent on temperature and type of coating, and therefore could be considered as non-load carrying material during creep test. The influence of cracking of PtAl coating on the high-cycle fatigue (HCF) behavior of the IN792 was also investigated. The results show that precracking of the coating prior to the fatigue test has little influence on the fatigue limit of specimens with thin coating (50 μm) but lowers the fatigue limit of specimens with thick coating (70 μm). The through-coating crack has enough mobility to penetrate into the substrate and causes fatigue failure only when the driving force for crack propagation is increased above a critical value due to a higher applied stress or a larger crack length (thicker coating).

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. 44 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1569
National Category
Engineering and Technology Materials Engineering
Identifiers
urn:nbn:se:liu:diva-89500 (URN)LIU-TEK-LIC-2013:3 (Local ID)978-91-7519-708-1 (ISBN)LIU-TEK-LIC-2013:3 (Archive number)LIU-TEK-LIC-2013:3 (OAI)
Presentation
2013-02-21, A 39, A-huset, Campus Valla, Linköpings universitet, Linköping, 10:00 (English)
Opponent
Supervisors
Note

Som minor changes has been done in the electronic version compared to the printed version such as acknowledgement among others.

Available from: 2013-03-14 Created: 2013-02-26 Last updated: 2013-03-15Bibliographically approved

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Yuan, KangPeng, RuJohansson, Sten

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