A.S. Ulrich
Schriftenreihe des DECHEMA-Forschungsinstituts, Universität Bayreuth, Shaker Verlag, 2020, ISBN 978-3-8440-7322-5
Heat-treatable Cr-Si-based alloys for high temperature applications with Cr ≥ 89 at.% were developed and optimized with respect to their oxidation and nitridation resistance as well as their microstructure. Therefore Ge, Mo and Pt were chosen as additional alloying elements.
All investigated alloys developed a two-phase microstructure consisting of a chromium solid solution, Crss, matrix and A15 phase precipitates, which was proven using EPMA, SEM and XRD. Depending on alloy composition and heat treatment, the fraction of A15 precipitates and thereby the alloy properties can be adjusted. Annealing at 1200°C led to a maximum A15 phase fraction of 37% with respect to the investigated alloy compositions. Si, Ge, and Pt are A15 phase formers while Mo was found in comparable concentrations in both phases. The precipitates formed in Mo-containing alloys were smaller compared to the other systems.
Oxidation tests at temperatures of 1050 - 1350°C were conducted for up to 200 h in synthetic air to investigate the alloys’ oxidation and especially nitridation resistance. For an improved description of oxidation kinetics of Cr-based alloys, a kp-kv-P-model was developed to analyse TGA measurements with mass discontinuities due to oxide scale cracking. It was found that by additional alloying with Ge, Mo and Pt both oxidation and nitridation resistance were improved in comparison to the binary system. Cr3PtN forms in the presence of Pt instead of detrimental Cr2N. Additionally, Ge decreases the effect of local scale failure on the oxidation kinetics and improves scale adhesion. Alloying with Mo enhances SiO2 formation and reduces the evaporation rate constant, most likely due to a morphological change in the Cr2O3 scale.
