The s phase, one of the TCP phases with crystal structure of D8b (tP30), is often formed massively at grain boundaries in conventional Fe-Cr-Ni based austenitic heat resistant steels after long-term exposure at elevated temperatures. This phase is regarded as detrimental phase to deteriorate the creep rupture strength. However, the s phase is thermodynamically equilibrium phase, so that it could be a potential strengthener for long-term creep strength if the phase stability and precipitation morphology are appropriately controlled, just like Fe2Nb Laves phase. In fact, we could successfully change the morphology of grain-boundary s phase by alloying. In this study, thus, design approaches and morphology control of the s phase will be presented, based on the phase equilibria, phase stability and precipitation kinetics in Fe-Cr-Ni-M (M: transition metals) ternary systems.
This research is partly supported by the Grants-in-Aid for Science Research Program (14205102), JSPS.