Effect of implanted helium on the microstructure and creep properties of ordered (Fe049Ni051)3V alloys

A series of long-range ordered alloys with base composition (Fe049Ni051)3V were homogeneously injected with approx. 100 appm helium at 6000C using the cyclotron at the Institut fuer Festkoerperforschung (IFF), KFA Juelich, West Germany. Post-implantation creep testing at 6000C revealed that the time to rupture as well as the total creep strain was reduced drastically compared to unimplanted material. Transmission electron microscopy (TEM) revealed that while the material retained its long range order after implantation, the microstructure in the as-implanted material was characterized by a high density of faulted interstitial dislocation loops as well as a high density of cavities 2.5 to 5.0 nm in diameter. Most of the cavities were preferentially located on the faulted loops or on grain boundaries. The high angle grain boundaries also contained a large number of small vanadium carbide precipitates. It was observed that cavities on grain boundaries were smaller when associated with these precipitates. In addition a number of larger, 7 to 10 nm diameter, faceted cavities were found to be associated with antiphase domain boundaries (APBs) in the ordered microstructure. TEM was also used to investigate the details of the microstructure which developed during creep. The results indicate that the embrittlement is most likely due to the combined effect of helium at the grain boundaries and the increased resistance to deformation in the hardened matrix material