Contribution of advanced microscopy techniques to nanoprecipitates characterization: case of AIN precipitation in low-carbon steel

Aluminum nitride (AIN) precipitation in a Fe-Al-N alloy was studied by Transmission Electron Microscopy (TEM). Crystallographic and chemical properties of precipitates were investigated using electron diffraction, High Resolution TEM, EDX microanalysis, EELS spectroscopy and Energy-Filtered TEM. Observations were carried out on extraction replicas and thin foils. Principally, two varieties of AIN precipitates have been observed: fine cubic (NaCl-type) precipitates with a platelet-like morphology exhibiting a Bain orientation relationship with respect to the alpha-iron matrix in the case of 3 h annealing at 923 K, and coarse cuboidal-shape hexagonal (wurtzite) precipitates having a particular orientation relationship with respect to the ferrite in the case of 5 It annealing at 973 K. A cubic-hexagonal transformation of AIN nitrides activated by the heterogeneous germination on iron sulfide (FeS) was established. The two AIN varieties were clearly recognized by EELS spectroscopy and the quantification of both populations of precipitates from EFTEM images is discussed. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.