EFFECTS OF SIMULTANEOUS BORON AND NITROGEN IMPLANTATION ON MICROHARDNESS AND FATIGUE PROPERTIES OF FE-13CR-15NI ALLOYS

Eight complex austenitic stainless steel alloys based on the compositi on Fe-13Cr-15Ni-2Mo-2Mn-0.2Ti-0.8Si-0.06C were implanted simultaneousl y with 400-keV B+ and 550-keV N+ ions and were investigated for change s in fatigue properties and surface microhardness. The near-surface ha rdness of all eight alloys improved, but the fatigue life of each decr eased. These findings were contrary to those obtained in an earlier st udy using four simple Fe-13Cr-15Ni alloys, where the dual implantation improved fatigue life by up to 250 pct. While unimplanted specimens f ailed by slip-band crack initiation, it was hypothesized that the dual implantation suppressed slip to the extent that fewer slip-band crack s were initiated and these were subjected to accelerated crack propaga tion. In addition, grain-boundary cracking was promoted, yielding a lo wer fatigue life. Support for this hypothesis was obtained by a study of single crystals of Fe-15Cr-15Ni, which were also implanted with Band N+. The dual implantation caused a lower fatigue life due to conce ntration of slip along a few slip bands to relieve applied stress. Evi dence of grain-boundary cracking was obtained using the four simple al loys, which were subjected to triple ion implantation with B+ N+, and C+. The triple implantation decreased the fatigue life of the alloys a nd caused accelerated growth of fewer slip bands and grain-boundary cr acking due to suppression of surface slip bands. This study thus shows the existence of an optimum level of strengthening when multiple ion implantation is used to improve the fatigue properties of alloys.