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Various techniques employed in studying the factors controlling resistance to creep and creep rupture are reviewed and discussed. The primary modes of deformation that have been identified metallographically on specimen surfaces include coarse and fine slip and grain boundary shearing. More complex modes of deformation have also been found. Measurements of grain distortions well below the specimen surface indicate that grain boundary shearing may be more important than is indicated by surface studies.
Substructure formation is being studied by metallographic methods and by electron transmission and high-resolution X-ray techniques. The latter techniques are suitable for determining rearrangement of dislocations during creep. Initial observations indicate that dislocation pile-ups are probably not involved in the processes controlling primary and secondary creep.
Time laws of creep and dependence of transient and steady-state creep on stress and temperature are discussed briefly. Such information is necessary to isolate the prominent creep processes. Their strong temperature-dependence indicates that creep processes are thermally activated. The similarity between the activation energy for secondary creep and that for self-diffusion is pointed out and the dislocation models based on this fact are discussed. Some of these models postulate dislocation pile-ups which as yet have not been observed. Furthermore, the fact that creep is found at stresses very much lower than the static yield strength indicates that the development of extensive pile-ups is unlikely. Prominent creep processes may involve, therefore, the activation of dislocation sources rather than movement of loops of freshly generated dislocations.
The end of secondary creep may depend in some cases on grain boundary shearing and hence may be affected by precipitation in the boundaries. It is shown that rupture life is proportional to the time at the end of secondary creep. This relationship, which is not strongly dependent on stress and temperature, leads directly to the dependence of rupture life on minimum creep rate suggested by Grant and his co-workers. The interrelationship between intercrystalline crack initiation and grain boundary shearing is briefly discussed for both wedge-type and round-type cracks. Results that have been obtained on stainless steel suggest that for this material crack growth does not depend to any extent on vacancy condensation. In this material the intercrystalline cracks are predominantly of the wedge type.
Edgar C. Bain Laboratory for Foudamental Research, U. S. Steel Corp. Research Center, Monroeville, Pa.