Challenging Nitridation at High Temperatures: the role of A15-IntermetallicsThursday (29.09.2016) 10:00 - 10:15 Part of:
Nitridation is a well-known phenomenon which deteriorates the mechanical properties of structural materials via embrittlement during high temperature service. In case of chromium, nitridation severely embrittles the material at high temperature which hinders the development of Cr-based alloys. To mitigate this embrittlement, an alloying strategy is developed by the addition of silicon and germanium. In the present paper, oxidation and nitridation behavior of binary Cr-Si and ternary Cr-Ge-Si alloys at ultra-high temperatures (T>1200°C) is investigated and the morphological evolution of the oxide scale and the metal subsurface zone is studied 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 eutectic Crss-Cr3Si alloy, an A15 barrier develops in form of a continuous intermetallic layer underneath the surface. The in-situ formed barrier layers successfully prevent nitridation and simultaneously improved 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.