Formation of the Fe2Hf Laves Phase Along the Eutectoid-type Reaction Path of δ-Fe→γ-Fe+Fe2Hf in an Fe-9Cr Based Alloy

This paper reports the precipitation kinetics along the eutectoid-type reaction path of delta-Fe ->gamma-Fe+Fe2Hf in an Fe-9Cr-0.3Hf (wt.%) alloy. Three types of precipitation modes were observed in the reaction path: (1) interphase precipitation forming periodically arrayed rows of fine Fe2Hf particles, (2) the precipitation of Fe2Hf phase in the delta phase followed by a phase transformation from the delta phase to the gamma phase and (3) the precipitation of Fe2Hf phase in the gamma phase matrix preceded by alpha -> gamma phase transformation. The nose for the interphase precipitation in time-temperature- transformation diagram is located at a few second and at 1100 degrees C. The interphase precipitation is expected to be used as a tool for strengthening heat resistant ferritic steels, but the heat treatment window for the precipitation mode is narrow in the ternary alloy.