Takehiko Itagaki1, Toru Watanabe1 and Renpei Yoda1
1National Research Institute for Metals, Tokyo
The creep rupture life of superalloys at high temperature in air is, sometimes, much longer than expected from its normal dependence on stress.
From a study of the creep rupture on Ni-20Cr-3W alloy, it was found that an extended rupture life took place exclusively in creep testing in an oxdizing atmosphere under a low stress, and that a deceleration of the deformation was observed for these specimens in a stage where an acceleration should be observed in normal cases. Since the acceleration is explained by propagation of cracks, this reverse trend is assumed to be due to the inhibition of crack propagation. A ``pinning'' of cracks by oxide particles formed at the tip of cracks was revealed metallographically for the specimens tested in the oxidizing atmosphere. As no extended rupture life was recognized for the specimens in the oxidizing atmosphere under a high stress, this pinning is thought to have an important role only in the creep under a considerably lower stress.
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