Revealing solidification behaviour and phase transformations in Fe$_{60}$Co$_{25}$Ni$_{10}$Mo$_5$ and Fe$_{63}$Co$_{26}$Ni$_{11}$ alloys by using high-speed video and in situ X-ray diffraction

A combination of time-resolved synchrotron X-ray diffraction (XRD), high-speed video and electromagnetic levitation (EML) enabled revealing the solidification path and phase transformations in Fe$_{60}$Co$_{25}$Ni$_{10}$Mo$_5$ and Fe$_{63}$Co$_{26}$Ni$_{11}$ alloys during heating and cooling. High-speed XRD provided direct evidence for nucleation of metastable δ-ferrite in undercooled liquid state and its quick transformation to γ-austenite. Addition of Mo clearly favours nucleation of $δ$-ferrite in Fe$_{60}$Co$_{25}$Ni$_{10}$Mo$_5$ melts undercooled by $Δ$T of about 100 K and more. In contrast, primary crystallisation of any of $δ$ and γ phases appears to be possible in Fe$_{63}$Co$_{26}$Ni$_{11}$ melts at all levels of undercooling. Neither Ni nor Mo exhibit any notable effect on the growth behaviour of $δ$ and $γ$ phases, which remains close to the one in the Fe$_{60}$Co$_{40}$ base alloy. Alloying with Mo has a strong impact on the microstructure and solid state transformations in the Fe$_{60}$Co$_{25}$Ni$_{10}$Mo$_5$ samples such as grain refinement at $Δ$T < 200 K, incomplete transformation of $γ$-austenite to $α$-ferrite, and formation of martensite at $Δ$T ≥ 330 K.