Mechanisms of high-temperature fatigue failure in alloy 800H

The damage mechanisms influencing the axial strain-controlled low-cycle fatigue (LCF) behavior of alloy 800H at 850 degrees C have been evaluated under conditions of equal tension/compression ramp rates (fast-fast (F-F): 4 x 10(-3) s(-1) and slow-slow (S-S): 4 x 10(-5) s(-1)) and asymmetrical ramp rates (fast-slow (F-S): 4 x 10(-3) s(-1) / 4 x 10(-5) s(-1) and slow-fast (S-F): 4 x 10(-5) / 4 x 10(-3) s(-1)) in tension and compression. The fatigue life, cyclic stress response, and fracture modes were significantly influenced by the waveform shape. The fatigue lives displayed by different loading conditions were in the following order: F-F > S-S > F-S > S-F. The fracture mode was dictated by the ramp rate adopted in the tensile direction. The fast ramp rate in the tensile direction led to the occurrence of transgranular crack initiation and propagation, whereas the slow ramp rate caused intergranular initiation and propagation. The time-dependent processes and their synergistic interactions, which were at the basis of observed changes in cyclic stress response and fatigue life, were identified. Oxidation, creep damage, dynamic strain aging, massive carbide precipitation, time-dependent creep deformation, and deformation ratcheting were among the several factors influencing cyclic life. Irrespective of the loading condition, the largest effect on life was exerted by oxidation processes. Deformation ratcheting had its greatest influence on life under asymmetrical loading conditions. Creep damage accumulated the greatest amount during the slow tensile ramp under S-F conditions.