High Voltage Microscopy of the Reduction of Hematite to Magnetite.

摘要

The microstructural changes associated with the formation of magnetite in hematite have been studied in specimens which have been partially reduced outside the microscope, thinned until electron transparent, and then examined in the normal way. Three types of structure have been observed in varying proportions which depend on the reduction temperature. At low temperatures, magnetite grows by the propagation of a cellular interface, the gas phase being transported to the cell boundary by a network of tunnels. At intermediate temperatures, magnetite plates are formed, whereas at high temperatures, both plate magnetite and blocky magnetite appear. It is proposed that the factor controlling the morphology which develops is the ratio of the cell boundary to volume diffusivity of ferrous ions. It is noted that the decomposition morphologies of hematite and austenite have many similarities. The basic reason for this similarity is that both transformations involve substantial redistribution of elements in the solid state and the microstructures which develop are those that perform this redistribution the most efficiently at the temperatures involved.