Mechanisms of high-temperature fatigue failure in alloy 800H |
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Authors: | K Bhanu Sankara Rao H Schuster G R Halford |
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Affiliation: | (1) Indira Gandhi Centre for Atomic Research, 603 102 Kalpakkam, India;(2) Institute for Materials in Energy Systems Research Centre, D-52425 Juelich, Germany;(3) Structures Division, NASA-Lewis Research Center, 44135 Cleveland, OH |
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Abstract: | The damage mechanisms influencing the axial strain-controlled low-cycle fatigue (LCF) behavior of alloy 800H at 850 °C have
been evaluated under conditions of equal tension/compression ramp rates (fast-fast (F-F): 4 × 10−3 s−1 and slow-slow (S-S): 4 × 10−5 s−1) and asymmetrical ramp rates (fast-slow (F-S): 4 × 10−3 s−1 / 4 × 10−5 s−1 and slow-fast (S-F): 4 × 10−5 / 4 × 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.
Formerly USA National Research Council Associate, NASA-Lewis Research Center
This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium
as a part of the 1994 Fall meeting of T.S., October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow
and Fracture Committee. |
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