X-ray and transmission electron microscopy investigation of strain in a nitrided steel: No evidence of plastic deformation

Abstract

In order to characterize the nitrided layers of steels, the X-ray diffraction (XRD) technique has been used coupled with transmission electron microscopy (TEM) observations. The TEM observations of the nitrided layers were made using a cross-sectional specimen preparation technique. Using the Warren-Averbach approach to the analysis of the diffraction peak broadening and TEM investigations, the existence of several precipitates of nitrides was brought to the fore, but no evidence for dislocations was found. To improve the calculation of the Fourier coefficient, the diffraction peaks were described in reciprocal lattice by a mixed Cauchy-Gauss curve. In fact, our investigations show that the broadening of the diffraction peaks of nitrided layers is strongly dependent on the size of the coherently diffracting domains. For nitriding, this result confirms that the residual stresses are only attributable to the precipitation of nitrides. In contrast, the microstructural state obtained after mechanical surface treatments, such as shot peening, is very different; the incompatibility of the plastic strains is the origin of the stress field.