Transmission electron microscopy studies of mechanical alloying in the immiscible a-Fe2O3-SnO2 system

Microstructural development and nanoscale compositional variations in mechanically alloyed Fe2O3-SnO2 powders have been examined by transmission electron microscopy and energy dispersive X-ray spectrometry. The mean grain size was found to stabilize around 10 nm after 19 h milling time, in close agreement with that estimated form X-ray diffraction line broadening measurements, whereas dissolution of SnO2 grains was incomplete even efter 110 h. Isolated grains with the SnO2 cassiterite structure, of diameter >10 nm, persisted up to the maximum milling time. These observations are discussed in relation to precious measurements in the same system by X-ray diffraction and Mossbauer spectroscopy, which suggested that alloying on the atomic scale occurred after 110 h milling. The present studies confirm that the amount of Sn dissolved in the Fe2O3 hematite lattice increases with longer milling times, indicating that a supersaturated solid solution is formed, but that mixing may be locally inhomogeneous at the atomic level. Similar conclusions have been reported for studies of mechanical alloying in immiscible metallic systems. The tendency for SnO2 grains above a certain critical size to remain undissolved, while smaller grains can more easily enter into solid solution with Fe2O3, is consistent with the expected behaviour due to the increased chemical contribution to the interfacial energy with decreasing grain size. Mossbauer results also showed that some of the SnO2 had not reacted with Fe2O3 after 110 h milling.