MORPHOLOGY OF CAVITIES FORMED ON GRAIN BOUNDARY OF CRACKED ALLOY TT690

摘要

The cross sections and fracture surfaces of cracks inAlloy TT690 specimens were observed from the viewpointof cavity formation as a SCC mechanistic study. Clearevidence of cavity formation was confirmed ahead of SCCin Alloy TT690 (30%CW) tested in simulated PWRprimary water at 360°C for 26,576 h. In one specificgrain boundary, approximately half of the bondingstrength was lost before the SCC propagation. However,the results also implied that the cracks were not alwaysassociated with a large number of cavities.The aggregation of traces of 100-500 nm diametercavities was observed on the fracture surfaces. Thesepatterns could be found in 7 of 12 crack growthspecimens, showing the number of traces of cavitiestended to increase with the cold work and testtemperature. The morphology of the traces of cavities wassimilar to that formed in high temperature air at 450°Cand hydrogen gas at 380°C, and that implied the fracturesurfaces were formed through a similar cracking process.Since some of the test conditions did not result in anytraces of cavities, it was thought that the phenomenon wasnot essential for the crack propagation in PWR primarywater. However, the decrease of bonding strength ofgrain boundaries due to the cavity formation could havean important role for the material aging process,especially for highly cold worked alloys at hightemperature.