Nitridation during oxidation as a challenge for Cr-based alloys and its mitigation by alloying
July 17-21, 2016
The high temperature behavior of pure chromium above 1000°C shows very interesting features such as vaporization, scale spallation and worst of all extreme nitridation when exposed to air. A detailed investigation of the different mechanisms can explain the wide range and controversial results given in the literature when the oxidation and its oxidation rates are reported for pure Chromium. Especially, the attack by nitridation deteriorates the mechanical properties by severe embrittlement. It also has a huge impact on the mass change during oxidation. To mitigate this embrittlement an alloying strategy is developed by the addition of silicon and germanium. The oxidation behavior of binary Cr-Si and ternary Cr-Ge-Si alloys at ultra-high temperatures (T=1200°C-1350°C) is reported (see also Figure 1) and the morphological evolution of the oxide scale and the metal subsurface zone was investigated using scanning electron microscopy, electron probe micro-analysis, and X-ray diffraction techniques. The A15-phase Cr3Si is shown to have a crucial influence on prevention of nitridation. During oxidation of a two phase Crss-Cr3Si system an A15 barrier develops in form of a continuous intermetallic layer underneath the surface. The in-situ formed barrier layers shown to be able to successfully prevent nitridation and the same time to improve the oxidation kinetics. Beyond that, ternary additions of Ge to the Cr-Si system strengthens this effect even more and significantly improves the oxidation kinetics of the chromium alloys at ultra-high temperatures to a level comparable to alumina formers (Figure 2).
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Mathias C. Galetz and Ali Soleimani Dorchev, "Nitridation during oxidation as a challenge for Cr-based alloys and its mitigation by alloying" in "Beyond Nickel-Based Superalloys II", Chair: Dr Howard J. Stone, University of Cambridge, United Kingdom Co-Chairs: Prof Bernard P. Bewlay, General Electric Global Research, USA Prof Lesley A. Cornish, University of the Witwatersrand, South Africa Eds, ECI Symposium Series, (2016). https://dc.engconfintl.org/superalloys_ii/26