In-situ X-ray investigations of phase formation and diffusion during low energy nitrogen ion implantation into austenitic stainless steel
Zusammenfassung der Projektergebnisse
Reliable and fast in situ XRD measurements during low energy nitrogen ion nitriding of austenitic stainless steel were performed after extensively modifying an existing high vacuum XRD chamber. A redesigned sample holder allows very precise temperature measurements almost on the surface of the sample. A broad beam ion source with an electronic beam switch for controlled variations of the ion current density was attached to the equipment and was characterized before use for implantation for a broad range of beam parameters using a Faraday cup and a special designed method for the restricted geometry of the whole in situ system. The phase formation kinetics during low energy nitrogen ion implantation into austenitic stainless steels 1.4301 and 1.4571 using in situ x-ray diffraction techniques was investigated. While no influence of the ion energy on the diffusion was observed, two separate regimes could be identified for varying the current density, i.e. nitrogen supply at the surface. The inverse parabolic law for nitrogen diffusion in austenitic stainless steel – known from ex-situ measurements – is confirmed only for a sufficiently high current density, leading to a diffusion controlled layer growth with a thermal activation energy of around 0.8 eV. At lower current densities, a linear growth was observed, transitioning to the inverse parabolic law at a certain time, i.e. later for lower current densities. There, a supply limited growth is occurring. No influence of grain orientation on the diffusion was observed for these experiments. When comparing the intensity evolution of the substrate peaks and the peaks arising from the expanded austenitic phase, similar diffusion rates were observed for certain process parameters allowing a simple layer growth model for analysis. However, different structure factors for the base material and expanded phase are found, thus a direct calculation of the layer thickness from the relative intensities is leading to false results. The lattice expansion itself increases gradually with time with the absolute values and the peak shape and width following a complex time-temperature behaviour. A direct correlation of the nitrogen content with the peak position and width is not possible, indicating a large influence of surface stress and stress gradients within the surface on the peak evolution. At the same time, higher temperatures lead to a stress reduction. Effects of grain rotation or plastic deformation as well as defect annealing on the peak intensities may be present but are not significant. In contrast, the decay of metastable expanded austenite into CrN was found indirectly from the decreasing peak intensity of the expanded austenite before CrN or ferrite was visible in the XRD spectra. Ex situ SEM analysis shows a complex growth starting as a 3D nucleation process and continuing as an inhomogenous 2D layer growth, complicating a direct analysis. When comparing FeCrNi (steel) with CoCr and NiCr alloys, the former two alloy groups show a strong tendency to form expanded austenite (similar to NiFe alloys known from literature), the NiCr reverts to a more traditional phase formation of nitrides. The origin of the expanded austenite and its peculiarities is still unknown. In summary, the results presented in this report demonstrate the advantages of in-situ XRD investigations in comparison to ex situ investigations yielding detailed insight into processes of phase formation and nitrogen diffusion during nitriding of fcc alloys.
Projektbezogene Publikationen (Auswahl)
- , Influence of Temperature on Layer Growth as Measured by in-situ XRD Observation of Nitriding Austenitic Stainless Steel, Nucl. Instrum. Meth. B
D. Manova, C. Günther, A. Bergmann, S. Mändl, H. Neumann, B. Rauschenbach
(Siehe online unter https://dx.doi.org/10.1016/j.nimb.2012.11.056) - Integration of a broad ion source with a high-temperature vacuum XRD chamber, Rev. Sci. Instrum. 83, 113901 (2012)
D. Manova, A. Bergmann, S. Mändl, H. Neumann, S. Rauschenbach
