The structures which formed during the creep of single-phase polycrystalline material at low stresses were investigated. It was found that ordinary dislocations with a zig-zag morphology, which formed during primary creep under stresses of 158 or 200MPa (region II), could not be seen in samples which had been crept under a stress of 100MPa (region III). The dislocations were homogeneously distributed at the minimum creep rate. Many super-dislocations were observed at the minimum creep rate in region III, and the dislocation density ratio of super-dislocations to the total number of dislocations was about 0.5. This ratio was equal to only 0.2 in region II. The ratio increased with strain, and reached a maximum value of about 0.6 at a strain of 0.1. The ordinary dislocations could not produce c-component strain and, therefore, the preferential activation of ordinary dislocations in region II led to stress concentrations at the grain boundaries.

Y.Ishikawa, T.Kiyokawa, H.Oikawa: Materials Transactions, 1995, 36[8], 1041-7