The cyclic fatigue behavior of direct age 718 at 149, 315, 454 and 538°C

The axial, strain-controlled, cyclic fatigue behavior of direct age alloy 718 is investigated. Ramberg-Osgood (R-O) cyclic stress-cyclic strain relationships were determined at each temperature. The results were used to assess the fatigue life as functions of the R-O alternating plastic strain and R-O alternating stress. As temperature decreased, a departure from Coffin-Manson behavior was observed. The fatigue lives at 149, 315 and 454 degrees C, plotted as a function of the R-O alternating stress, converge when superimposed and a crossover in the responses is observed. Under fixed alternating stress conditions, the fatigue life below the crossover increases as the temperature increases whilst above, a reversion in the fatigue life temperature dependence is seen. A mechanistic model is proposed which seeks to rationalize the increase in fatigue life, as a function of increasing test temperature, to a thermally activated process that mitigates the generation of substructure defects associated with grain boundary stress concentrations. The model activation energy and activation volume are interpreted to substantiate a mechanism whereby a diffusive flux, away from grain boundary stress concentrations, is enabled by the non-conservative motion of grain boundary dislocations in a grain boundary plane containing obstacles. The model predictions are in reasonable agreement with experimental observations. (c) 2006 Elsevier B.V. All rights reserved.