Experimental determination of (BCC plus FCC) phase fields in the quaternary isotherm of Fe-Ni-Co-Cu at 950°C

Quaternary isotherm of Fe-Ni-Co-Cu at 950 degrees C is determined by multiphase diffusion couple experiments, focused on establishing the (BCC + FCC) two-phase regions. The present findings, combined with our previous study, discover the presence of five separate two-phase fields in this isotherm namely (BCC + Cu-rich FCC), (BCC + Cu-lean FCC richer in Fe), (BCC + Cu-lean FCC richer in Co), (Cu-rich FCC + Fe-rich FCC), and (Cu-rich FCC + Co-rich FCC). This also indicates the existence of two three-phase fields. Based on diffusion couples exhibiting planar interfaces between BCC and FCC phases, three tie lines in (BCC + Cu-rich FCC) two-phase field were also determined. It is observed that Fe-Co-Cu ternary isotherm at 950 degrees C has a wider BCC region (up to 8 wt% Cu) than the commonly accepted phase diagram. A qualitative representation of the entire quaternary isotherm is proposed in the form of multiple iso-Ni concentration sections.