MICROSTRUCTURAL OBSERVATION OF HELIUM IMPLANTED AND CREEP RUPTURED FE-25-PERCENT-NI-15-PERCENT-CR ALLOYS CONTAINING VARIOUS MC AND MN FORMERS

Transmission electron microscopic observations have been carried out o n Fe-25%Ni-15%Cr austenitic alloys with various MX (M = V, Ti, Nb, Zr; X = C, N) stabilizers after helium implantation and creep rupture at 923 K. It is shown that suppression of helium embrittlement can be ach ieved through a higher dispersion density of incoherent precipitates b ecause of their high capability of bubble entrapment. A good agreement between the average distance of grain boundary bubbles exceeding the minimum critical size and the spacing of cavity traces on intergranula rly fractured surfaces is obtained. This suggests that the enhancement of grain boundary decohesion by helium is a result of unstable growth of super-critical helium bubbles. (C) 1998 Elsevier Science B.V. All rights reserved.