Creep under very small torsional stress has been investigated for 99.95% polycrystalline iron in the temperature range of 350°-525℃. The general features of the creep curves obtained are essentially the same as those previously reported for 99.991% aluminum. The creep is shown to be consisted of two parts, one occurring at lower temperatures or shorter times and the other at higher temperatures or longer times. The first part of the creep is limited in extent and is associated with the viscous behavior of grain boundaries. The activation energy for this grain-boundary creep is 78,000 ± 4,000 calories per mole according to the present creep and stress relaxation measurements. This value is close to the activation energy for self-diffusion of iron, a correlation which indicates that, in so far as to the mechanism of atomic migration is concerned, grain boundaries may not be very different from the interior of the grains.Both parts of the creep are noticably lessened by the addition of carbon in the iron specimen. A striking feature is that the viscous slip along grain boundaries appears to be hindered by carbon at a concentration as low as 0.0004%. These observations provide a guiding clue as to the problem of controlling high-temperature creep of metals and render further experimental support on the hole model previously suggested for the grain boundaries.With regard to the second part of the creep, it is tentatively suggested that its mechanism may be connected with atomic rearrangements of holes inside the grains.
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