Retarding the precipitation of η phase in Fe-Ni based alloy through grain boundary engineering

It is important to inhibit the precipitation of η phases in precipitation strengthened Fe-Ni based alloys, as they will deteriorate not only the mechanical property but also the hydrogen resistance. The present investigation shows that grain boundary engineering (GBE) can retard the formation and growth of η phase in J75 alloy. After GBE treatment with 5% cold rolling followed by annealing at 1000 °C for 1 h, the fraction of special boundaries (SBs) increases from 38.4% in conventional alloy to 77.2% and the fraction of special triple junctions increases from 10% to 74%. During 800 °C aging treatment, quite amount of cellular η phases adjacent to random grain boundary (RGB) will be found in conventional alloy, and only a few small η phases have been observed in GBE treatment alloy subjected to the same aging treatment for long time. The reason for GBE in inhibiting precipitation of η phase can be attributed to not only introducing high fraction of SBs but also breaking the connectivity of RGB networks. As nucleation and growth of η phases on SBs are difficult due to their lower Ti concentration and diffusion rate, and the disruption of RGB networks reduces supply of Ti atoms to the η phases significantly, which impedes their growth at RGB.