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Conference Article: Fatigue properties of nickel-base high temperature alloys for HTGRs

Tsuji, H.; Nakajima, H. (Japan Atomic Energy Research Inst., Naka, Ibaraki (Japan). Dept. of Fuels and Materials Research)

Abstract

A series of strain controlled low-cycle fatigue tests were conducted at temperatures ranging from room temperature (RT) to 900 deg. C on nickel-base high-temperature alloys, namely Hastelloy X and its modified versions Hastelloy XR and Hastelloy XR-II, which are candidate alloys for HTGR applications. The tests were conducted in the simulated HTGR helium environment with the exception of those conducted in air at RT. In those tests the examination was made on the effects of strain rate, hold time, aging and test temperature on fatigue properties. Decreasing the strain rate led to notable decreases in the fatigue life. Based on the stress-strain curves and the crack morphology, it was suggested that a considerable contribution of creep damage was included at lower strain rates. In the experiments with the trapezoidal strain waveform with different holding types, the fatigue life was found to be reduced most effectively in tensile hold-time experiments. From the observations of the crack morphology the strain holding in the compressive side was suggested to play the role of suppressing the initiation and the growth of internal cracks and/or cavities, and to cause crack branching. The fatigue lives of the aged (900 deg. C, 1000h) specimens were shorter than those of the solution annealed ones at test temperatures ranging from RT to 700 deg. C. The tendency became more pronounced under higher strain range conditions. The reduction in the fatigue life due to aging treatment, observed through the tests conducted at and below 700 deg. C, was closely related with the loss of tensile ductility, which was attributed to the precipitation of M6C carbide along the grain boundaries which occurred during the aging. Fractographic features and crack morphology of the aged specimens fatigued at and below 700 deg. C were of brittle nature relative to those of the solution annealed ones fatigued in the same temperature range. The fatigue lives estimated from the tensile properties using the method proposed by Manson were consistent with the results of fatigue tests under the test conditions employed.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
High temperature metallic materials for gas-cooled reactors. Proceedings of a specialists meeting held in Cracow, 20-23 June 1988
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--18, pp:110-123