The relation between high-cycle fatigue behavior and tensile properties was examined at room temperature and 750°C on two kinds of material with different ductility: Ni base superalloy (forging) and Fe base superalloy (casting). The high-cycle fatigue tests were carried out at room temperature and 750°C, and the effects of tensile strength, ductility, stress ratio, notch and frequency on fatigue strength were examined. Moreover, the relation between fatigue behavior and fracture morphology was discussed based on fractographical observations.
The main results obtained are as follows.
(1) The high-cycle fatigue strength of two kinds of material with different ductility was dependent on tensile strength but not on ductility, except for the test result at 750°C under the stress ratio of 0.5.
(2) The influences of the ductility of the materials on the stress ratio effect, notch effect and fatigue deformation were recognized at 750°C under the stress ratio of 0.5.
(3) A good correspondence between the change of fracture morphology and the parameter of fracture mechanics (Kmax/E) was observed independently of test temperature. The effect of the ductility of the materils on fracture morphology was recognized and the fracture surface of Fe base superalloy was influenced by its dendrite structure.
(4) The values of fracture mechanics parameter (Kmax*/E) corresponding to the critical length (as) of fatigue stable crack growth were almost constant for the materials independently of test temperature, if the fatigue deformation was small.
