Structural characterisation of iron aluminide coatings prepared by chemical vapour deposition

The present paper presents the results of iron aluminide coatings formed on pure iron by low pressure chemical vapour deposition. The coatings were characterised using X-ray diffraction, optical microscopy, TEM and electron microprobe analyser. Three distinct layers were seen in the optical micrographs in the coatings where deposition was carried out above the α–γ transformation temperature. The smaller thickness of the top layer compared to that of the second layer has been attributed to the smaller diffusivity of Fe compared to Al in FeAl. The formation of the third layer has been rationalised based on the fact that at deposition temperatures >1185 K, solid solution with Al above 1·5 at-% transforms to α with a definite boundary, while solid solution with less that 1·5 at-%Al will remain as γ close to the substrate. Those deposited below this temperature will remain in α at all compositions, and no boundary is formed.