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Lecture

WEB New insights into the structure of expanded austenite



Expanded austenite is a well-established metastable phase exhibiting excellent hardness and wear resistance yet retaining the corrosion resistance of the austenitic stainless steel substrate. However, the insertion of nitrogen up to 20 – 30 at.% into the fcc lattice leads to stress accumulation, lattice expansion and plastic deformation introducing additional dislocations. Consequently, the probability of stress corrosion cracking may increase after formation of expanded austenite. In situ XRD measurements allow further insight into the peculiarities of this phase.

XRD analysis of expanded austenite formed in steel 1.4571 (316 Ti) yields the identification of an “anisotropic lattice expansion” with the (200) reflection indicating a larger expansion normal to the surface than the (111) reflection. Two differently oriented types of crystallites are probed and only one lattice orientation of each group is actually measured. Additionally, the (200) peak of the expanded phase is very broad. For layered and strongly textured systems with a thickness of 1–5 µm – compared to an information depth of about 2.5 µm for Cu K radiation – conventional XRD provides only limited information.

In situ XRD investigations were performed in combination with ion beam sputtering for surface etching: on the one hand, depth-dependent information using differential XRD spectra was obtained. On the other hand, in situ XRD with controlled removal of nitrided surface layers allows us to investigate the artificially obtained surfaces, otherwise not accessible with more comprehensive ex situ examination. For expanded austenite formed by energetic ion insertion with either plasma immersion ion implantation or low energy ion implantation, depth-resolved XRD measurements indicate that the {200} oriented fcc grains are representing two separate expanded phases: the phase with a larger expansion perpendicular to the surface is on top of the phase with a smaller expansion. The ex situ measurements show a detailed comparison of these two phases while additional in situ investigations during the growth by low energy implantation demonstrate the observation of the transition between these two phases as function of the thickness of the expanded austenite layer.

 

Speaker:
Dr. Stephan Mändl
Leibniz Institute of Surface Engineering (IOM)
Additional Authors:
  • Patrick Schlenz
    Leibniz Institute of Surface Engineering (IOM)
  • Dr. Jürgen W. Gerlach
    Leibniz Institute of Surface Engineering (IOM)
  • Dr. Darina Manova
    Leibniz Institute of Surface Engineering (IOM)