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 mag esium and traces of aluminium have been studied in a transmission electron microscope equipped with EDX and PEELS spectrometers and in an electron microprobe analyser. The particles were identified as Al-Mg-Si nitrides having a trigonal superlattice cr stal structure derived from a hexagonal AlN type fundamental cell. The superlattice can be indexed according to a hexagonal Bravais lattice with parameters a = 0.544nm and c = 0.482nm. The parameters of the fundamental cell are af = 0.314nm and cf = 0. 82nm, deviating only 1-3% from the parameters of hexagonal AlN. Based on the compositional analysis, the chemical formula of the nitride particles is suggested to be AlMg2.5Si2.5N6.