Nitride precipitation and coarsening in Fe-2.23 at.% V alloys: XRD and (HR)TEM study of coherent and incoherent diffraction effects caused by misfitting nitride precipitates in a ferrite matrix

Specimens of Fe-2.23 at.% V alloy were nitrided in a NH3/H-2 gas mixture at 580 degrees C. The nitrided microstructure was investigated by X-ray diffraction (XRD), and by conventional and high-resolution transmission electron microscopy ((HR)TEM). For specimens homogeneously nitrided for relatively short times no separate VN reflections developed but instead sidebands associated with ferrite reflections, most pronouncedly for the alpha-Fe-200 reflection, appeared. The diffractograms measured for the different specimens were interpreted as the result of coherent diffraction of the nitride platelets with the surrounding ferrite matrix, which is tetragonally distorted: the distorted ferrite matrix and the nitride platelets are represented by a single body-centered tetragonal lattice, whereas the remaining part of the ferrite is described by a body-centered cubic lattice. Analysis of the microstructure of the nitrided specimens using (HR)TEM confirmed the existence of very tiny VN platelets, coherent with the surrounding matrix. Annealing at elevated temperatures (up to 750 C) after nitriding led to (moderate) coarsening of the nitride precipitates. The coarsening is associated with the occurrence of local disruptions/bending of lattice planes in the VN platelet. This effect causes the VN platelets to appear segmented in the diffraction-contrast images. The specific changes in the X-ray diffractograms, as function of the stage of aging, could be consistently described as consequences of the transition from coherent to incoherent diffraction of the nitride platelets with reference to the surrounding ferrite matrix. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.