Dwell effects on low cycle fatigue behaviour of diffusion coated nickel base superalloy IN 713LC at temperature of 800°C

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Materials used for high temperature applications such as disks, blades and vanes of gas turbine engines require exceptional resistance to mechanical and thermal loading as well as against chemical attacks by oxidation, which significantly increases with operating temperature. First generation Ni-base superalloy IN 713LC developed by the International Nickel Company in the 1950s is the representative of cast materials used for these purposes, especially because of favorable price in conjunction with satisfying high temperature properties. However, the continuous enhancing of operating temperature severely degrades the material surface and adversely affects the strength properties and component life. Protection of the surface and prevention of its degradation is a major goal of today and tomorrow. Diffusion coatings are primarily used to safeguard the material from aggressive environments and to improve corrosion and oxidation stability of surface. Nowadays, superalloy surface is enriched with appropriate oxide formers such as aluminium and other elements (Cr, Si..). The topic of the present paper is focused on the study of low cycle fatigue with tensile dwells of cast nickel-base superalloy IN 713LC with Al-Cr difussion coating at 800 °C. Increased attention is paid to the microstructure observation of the substrate material and Al-Cr diffusion layer. Macrostructure of IN 713LC consists of coarse dendritic grains whose average size was determined to 2.4 mm ± 0.5 mm and some shrinkage pores. Microstructure is formed by γ matrix with γ´ strenghtening precipitates, carbides and eutectics. Polycrystalline diffusion Al-Cr coating was prepared by a Cr modified aluminizing using the chemical vapor deposition (CVD) out of pack process. The coating was deposited in two-steps: 1050 °C/5h and 950 °C/5h. Microstructure of the Al-Cr layer comprises an outer layer and an inner diffusion layer with dispersion of a large amount of complex particles based on Cr, Mo and Nb. For purpose of this work cylindrical specimens were machined from the rods manufactured using investment castings technique. Low cycle fatigue tests and tests with 10 minute tensile dwells included in each cycle were conducted in strain control mode with constant total strain amplitude and strain rate at 800 °C in air. The fatigue behaviour was characterized by cyclic hardening/softening curves, cyclic stress-strain curves, Manson-Coffin curves and Basquin curves. The substrate and the Al-Cr coating were examined in as-received conditions and also after cyclic loading by means of optical microscopy, SEM and energy dispersive spectroscopy (EDS). Micro-hardness depth profile was obtained with the Knoop indenter. The fracture surface observation, changes in microstructure and investigation of polished section parallel to the specimen axis help to discuss the mechanisms operating in fatigue degradation of surface treated superalloy in both regimes of cycling.

Low cycle fatigue, dwell time, Inconel 713LC, diffusion coatings, AlCr coating, high temperature

ŠULÁK, I.; OBRTLÍK, K.; HUTAŘOVÁ, S.; JULIŠ, M.; PODRÁBSKÝ, T.; ČELKO, L.

11. 5. 2015

Karlsruhe

382

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