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 composition Fe-13Cr-15Ni-2Mo-2Mn-0.2Ti-0.8Si-0.06C were implanted simultaneously with 400-keV B+ and 550-keV N+ ions and were investigated for changes in fatigue properties and surface microhardness. The near-surface hardness of all eight alloys improved, but the fatigue life of each decreased. These findings were contrary to those obtained in an earlier study using four simple Fe-13Cr-15Ni alloys, where the dual implantation improved fatigue life by up to 250 pct. While unimplanted specimens failed by slip-band crack initiation, it was hypothesized that the dual implantation suppressed slip to the extent that fewer slip-band cracks were initiated and these were subjected to accelerated crack propagation. In addition, grain-boundary cracking was promoted, yielding a lower fatigue life. Support for this hypothesis was obtained by a study of single crystals of Fe-15Cr-15Ni, which were also implanted with B+ and N+. The dual implantation caused a lower fatigue life due to concentration of slip along a few slip bands to relieve applied stress. Evidence of grain-boundary cracking was obtained using the four simple alloys, which were subjected to triple ion implantation with B+, N+, and C+