MICROSTRUCTURE FORMATION AND ITS ROLE ON YIELD STRENGTH IN AISI-316-SS IRRADIATED BY FISSION AND FUSION NEUTRONS

The microstructure of AISI 316 SS irradiated by fission and fusion neutrons at 90 and 290-degrees-C was observed by weak-beam dark-field stereo technique and compared with yield strength data. In all cases, tiny (about 1 nm on average) interstitial type dislocation loops were formed. At 90-degrees-C, the density of dislocation loops was extremely high and those of fission and fusion irradiations correlate well on the basis of dpa. At 290-degrees-C, interstitial loops are formed by rearrangement of point defects formed in a subcascade. Both the loop density and yield stress change of fission neutron irradiation increased abruptly above 0.003 dpa. This peculiar behavior might be caused by undesired irradiation at lower temperatures during daily heating-up and cooling-down of the samples. Major defects which increase yield strength were the observed tiny interstitial type dislocation loops. The barrier strength, alpha, of the dislocation loops was determined to be 0.2.