The effect of frequency on the cyclic strain and fatigue behavior of cast rené at 1600° F

Controlled strain fatigue tests have been carried out for two heat treatments of cast René 80 at 1600°F at several frequencies and strain ranges. Results were fitted to frequency-modified fatigue equations for elastic and plastic strains using regression analysis. Save for a more pronounced cyclic softening, little difference in fatigue behavior was found in the two heat treatments. This material exhibits a normal frequency behavior for the elastic strain component of the total strain, and an inverse frequency effect for the plastic strain component, resulting in a negligible frequency effect on total strain in the high strain, short life regime. This behavior is attributed to the persistence of a transgranular mode of crack propagation in preference to the more usual intergranular mode in all tests including those at frequencies as low as 0.03 cpm. On the other hand, crack nucleation was more characteristic of cast nickel-base superalloys at 1600°F, that is, by oxide ridging at surface grain boundaries. A low transition fatigue life was found, such that for fatigue lives greater than 1000 cycles, elastic behavior essentially prevails.