1. Despite the fact that the first generation nickel base superalloy IN713LC (low carbon) is used more than 50 years it still plays a significant role in the aircraft gas turbine market especially because of its outstanding high temperature properties and also low cost. Due to its high content of strengthening phase γ´ (approximately 60%) there is not necessary to include expensive heat treatment after casting. In contrast, the alloy MAR-M-247 HIP (hot isostatically pressed) is representative of the second generation. It was developed in early 1970s and exhibits higher mechanical properties and improved surface and microstructural stability. Therefore it can replace the IN713LC and become a leading alloy for more efficient aircraft engines in spite of its higher price. The study of mechanisms of crack initiation during a series of forward and reverse loading is nowadays one of the intensively investigated areas. The majority of the fatigue crack initiation is associated with the surface. Within high-temperature low cycle fatigue (HTLCF) plasticity is induced in each cycle. During the accumulated cycles in the fatigue process, dislocations multiply, accumulate, annihilate and rearrange to form persistent slip bands. Their surface slip markings develop in a pronounced surface relief in which fatigue cracks can initiate. Although subsurface initiation can be also found, usually in cast materials with internal defects including pores, shrinkage, inclusions, etc. The fatigue behavior of two representatives of Ni-base superalloys was studied. The first one was IN713LC LG (low grain) which was tested in as received conditions. The second one was MAR-M-247 HIP. This alloy was hot isostatically pressed with subsequent heat treatment. Both materials were supplied by PBS, Velká Bíteš, a.s. in the form of cast rods. HTLCF test were carried out in a computer controlled servo-hydraulic testing machine MTS 810. All LCF tests were performed in a push-pull cycle under total strain control conditions using a fully reversed triangular waveform (R ε =-1) in air at a temperature of 900°C. Total strain rate of 2x10 -3 s -1 and total strain amplitude were kept constant in each test. Both materials are typical of a coarse dendritic structure with carbides, eutectics and shrinkage. Ivo ŠULÁK a, Karel OBRTLÍK b Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, 616 62 Brno, Czech Republic a sulak@ipm.cz, b obrtlik@ipm.czsulak@ipm.czobrtlik@ipm.cz Table 3. Morphology, amount and size of γ´ precipitates Table 1. Chemical composition of IN713 LC LG and MAR-M-247 HIP Table 2. Grain size, spacing of secondary dendrites axes (DAS) and linear dimension of defects Persistent slip markings (surface) Surface (vicinity of casting defects) Crack initiation sites Carbide inclusions (surface) Internal initiation – fish eye (shrinkage) [1] REED, R, STOLOFF, T. N.; HAGEL, W. C. The Superalloys: Fundamentals and Applications: Cambridge: Cambridge University Press, 2008, xiv, 372 s. ISBN 978-052-1070-119. [2] DURAND-CHARRE, M.: The Microstructure of Superalloys. Amsterdam: Gordon and Breach Science Publ.,1997. 124 s. ISBN 90-5699-097-7 [3] POLÁK, J. and MAN, J.: Fatigue crack initiation – The role of point defects. International Journal of Fatigue. 2014, vol. 65, s. 18-27. [4] POLÁK, J.: Cyclic Platic and Low Cycle Fatigue Life of Metals. 1. vyd., Praha: Academia nakladatelství Československé akademie věd, 1991, s.316, ISBN 80-200-0008-9 (Academia), ISBN 0-444-98839-4 (Vol.63) (Elsevier). [5] OBRTLÍK, K., S. POSPÍŠILOVÁ, M. JULIŠ, T. PODRÁBSKÝ a J. POLÁK. Fatigue behavior of coated and uncoated cast Inconel 713LC at 800°C. International Journal of Fatigue. 2012, vol. 41, s [6] OBRTLÍK, K.; HUTAŘOVÁ, S.; ČELKO, L.; JULIŠ, M.; PODRÁBSKÝ, T.; ŠULÁK, I. Effect of thermal barrier coating on low cycle fatigue behavioir of cast Inconel 713LC at 900 oC. Advanced Materials Research, 2014, roč. 891- 892, č. 2014,s. 848-853. ISSN: 1022- 6680. [7] ZÝKA, J., PODHORNÁ, B., HRBÁČEK, K.: Heat Treatment and Properties of Nickel Superalloy MAR-M-247, In: Metal 2011: 20th Anniversary International Conference on Metalurgy and Materials, Brno, 2011, pp. 1033-1038. [8] KAUFMAN, M.: PROPERTIES OF CAST MAR-M-247 FOR TURBINE BLISK APPLICATIONS, Thomson Laboratory General Electric Company Lynn, Massachusetts 01910 log 2N f = 1/b (log σ a – log σ´ f ) Fatigue hardening coefficient - K´ Fatigue hardening exponent -n´ Fatigue ductility coefficient - c Fatigue ductility exponent - ε´ f Fatigue strength coefficient - b Fatigue strength coefficient - σ´ f log σ a = log K´ + n´ log ε ap Basquin fatigue life curves Coffin-Manson fatigue life curves Cyclic stress-strain curves log 2N f = 1/c (log ε ap – log ε´ f )