13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Creep fracture mechanism of polycrystalline Ni-based superalloy with diffusion coatings Kang Yuan1, *, Ru Lin Peng1, Xin-Hai Li2, Lennart Johansson2, Sten Johansson1, Yan-dong Wang3 1Department of Management and Engineering, Linkoeping University, SE-58183 Linköping, Sweden 2Siemens Industrial Turbomachinery AB, SE-61283 Finspång, Sweden 3Beijing Institute of Technology, Beijing 100081, China *Corresponding author: kang.yuan@liu.se Abstract: 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. Keywords: Creep; Ni-based superalloy; Diffusion coating; Crack 1. Introduction Due to their excellent high-temperature mechanical properties, superalloys are widely used as base material for components in advanced gas turbine operating at high temperatures [1]. Components with Al-rich coatings become much even more sustainable with better resistance against environmental attack from high-temperature oxidation and corrosion. Aluminide diffusion coatings are widely used to protect the superalloys in a gas turbine at temperatures lower than 1000 °C [2]. Due to the difference of mechanical properties compared to the base material, the effect of coatings on the mechanical behavior of the substrate needs to be considered. At higher temperatures creep of the material become an important issue. There have been some investigations on creep mechanism of superalloys with high temperature coatings, which suggest several effects of coatings: a change in microstructure of the base material; a change of load distribution; introduction of cracks [3,4]. Some valuable creep tests (generally < 1000 hrs) of diffusion coating-superalloy system have been made during the past decades [5-8]. However long-term creep test results (> 1000 hrs) are seldom presented in the best literature surveys. Furthermore in some specific situations, the coating has to be deposited onto the thin part of the component, which makes the effect of coating more critical. The industrial background to this research work actually emanates from problems in the walls of the cooling channels which can be as thin as in the order of millimeters. The aim of this paper is to investigate the mechanical effect of diffusion coatings on thin sectioned superalloy specimens (~ 1 mm) subjected to creep at the operating temperatures.
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