TEM investigation of heterogeneous nuclei for graphite spheroids in ductile cast iron

In ductile cast iron with magnesium additions the graphite phase forms spherical particles embedded in either a ferritic or a pearlitic matrix. It generally accepted that the spheroids are heterogeneously nucleated at small particles which are formed in the melt [1,2]. It has been suggested by Jacobs et al. [2] that the nuclei are spinel type oxides containing Mg, Al and Si, having an FCC unit cell with lattice parameter (0.77 -± 0,05) nm. In the present investigation the crystal lattice of the nuclei has been the subject of closer examination using a ductile cast iron contained 3.6%C, 2.4%Si and 0.05% Mg. Small specimens were taken from the melt after the magnesium treatment and quenched in a copper die. In this way the formation of graphite spheroids on the nuclei was avoided. Nuclei were extracted on to carbon replicas by standard techniques. The replicas were mounted on copper grids for TE M investigations which were done in a JEOL JEM-2010 attached with a Link EDX analyzer.