Microstructural Characterisation of Ti and V-containing Synthetic Slags Mimicking the Chemical Composition of the Slag Inclusions of Ferrous Artefacts Produced in the Iron Factory of São João de Ipanema (XIX century, Brazil)

In the present work, synthetic slags (FeO-SiO₂-CaO-P₂O₅-Al₂O₃-TiO₂-V₂O₅ system) mimicking the composition of the slag inclusions of Ipanema were solidified at different cooling rates. FactSage software was used to calculate the slag’s phase equilibria from 500ºC to 1600°C to guide the microstructural characterisation. The samples solidified with slower cooling rates featured wüstite dendrites and a eutectic-like matrix, with intradendritic (Al,V,Ti)-containing iron spinel precipitates in the wüstite. The rapidly solidified slag showed refined wüstite dendrites surrounded by a Fe-O-Si-Ca-containing amorphous single-phase matrix without intradendritic (Al,V,Ti)-containing iron spinel precipitation. The comparison of the calculated phase diagram with the as-cast microstructures showed a few discrepancies. For instance, the FactSage’s database must be updated to consider the simultaneous solubility of Al, V and Ti in the thermodynamic description of the spinel phases, such as FToxid-SP-V, FToxid-SPINA, FToxid-TiSp and (FeO)2(TiO2)(s). Finally, the intradendritic precipitation of (Al,V,Ti)-containing iron spinel phases in the wüstite of the slag inclusions indicated that an Ipanema’s iron artefact was exposed to high service temperatures between 750 and 1050ºC.

Keywords:
Archaeometallurgy; microstructure; synthetic slag; Iron Factory of São João de Ipanema