Characterization of the FFC Cambridge process for NiTi production using in situ X-Ray synchrotron diffraction

Abstract

To date, the characterization of the reduction pathway for the Fray Farthing Chen (FFC) Cambridge process has been achieved through ex situ studies, leading to some ambiguities. This study employs a synchrotron X-ray diffraction technique to monitor in situ the FFC reduction of NiTiO3 to NiTi, yielding an unmatched level of detail on the electrochemical and chemical reactions involved. The reduction pathway consists of rapid initial reduction of NiTiO3 to form CaTiO3 and Ni, then the transformation of Ni to Ni3Ti, and finally the consumption of CaTiO3 and Ni3Ti to produce NiTi. The phases observed agree with thermodynamic predictions [J. Electrochem. Soc., 155, E171 (2008)] and allow the mapping of the reduction pathway on an electrochemical predominance diagram. Ni3Ti is a short-lived transient phase in the reduction pathway. Ni3Ti was found in significant quantities in ex situ studies [J. Electrochem. Soc., 155, E171 (2008); Chin. Sci. Bull., 51, 2535 (2006)]. The authors propose that Ni3Ti forms during furnace cooling to ambient temperature if extracted before a complete reduction. Neither Ni2Ti4O nor CaO was observed. The progression of the reduction front is clearly in line with existing observations and models [Metall. Mater. Trans., B, Process Metall. Mater. Proc. Sci., 35, 223 (2004); J. Phys. Chem. B, 109, 14043 (2005)]