Nuclei for heterogeneous formation of graphite spheroids in ductile cast iron

The crystal structure and composition of nuclei for graphite spheroids in a ductile cast iron containing small amounts of magnesium and traces of aluminium have been studied in a transmission electron microscope equipped with EDX and parallel electron energy loss spectrometer and in an electron microprobe analyser. The particles were identified as Al-Mg-Si nitrides, having a trigonal superlattice crystal structure derived from a hexagonal AIN type fundamental cell. The superlattice can be indexed according to a hexagonal Bravais lattice, with parameters a = 0.544 nm and c = 0.482 nm. The parameters of the fundamental cell are a(f) = 0.314 nm and c(f) = 0.482 nm, deviating only 1-3% from the parameters of hexagonal AIN. Based on the compositional analysis, the chemical formula of the nitride particles is suggested to be AlMg2.5Si2.5N6. (C) 2001 IoM Communications Ltd.