(Invited) Oxidation, Nitridation and Volatilisation of Noval Chromium-Based Alloys Above 1000°C
Authors: Galetz Mathias C. , Ulrich Anke Silvia 
Published: December 2020

Cr-Si-based alloys are promising candidates for high temperature materials which can resist temperatures beyond the commonly used Ni-based superalloys (> 1150°C). Thereby, higher working temperatures can be used in high temperature applications, which increases the process efficiency. The main drawback of Cr-based alloys is their nitridation and embrittlement at high temperatures during exposure in air. Additionally, at temperatures above 1000°C, Cr-based alloys are prone to spallation of the protective oxide scale and to the formation of volatile oxides.

The aim of this work was to develop heat-treatable Cr-Si-based alloys with Cr ≥ 89 at. % and to optimize their oxidation and nitridation resistance. Ge, Mo, and Pt were chosen as additional alloying elements and the microstructure as well as the oxidation resistance of the alloys were investigated.

All investigated binary, ternary, quaternary, and quinary alloys developed a two-phase structure consisting of a chromium solid solution Crss matrix and A15 phase precipitates, which was proven using EPMA, SEM, and XRD.

Depending on the alloy composition and on the heat treatment, the fraction of A15 precipitates and thereby the alloy's properties can be adjusted. Annealing at 1200°C led to a maximum A15 phase fraction of 37% with respect to the investigated alloy compositions. Increasing the A15 phase fraction goes along with an increase in hardness. 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 are smaller compared to the other systems, which is important in terms of maintaining the material's properties during high temperature exposure.

Additionally, Mo and Pt were found to decrease the lattice misfit between A15 phase and Crss matrix. However, in contrast to Mo, Pt promotes coarsening of the A15 precipitates during annealing.

Thermogravimetric analysis and isothermal oxidation tests at temperatures of 1050 -1350°C for up to 200 hours in synthetic air were conducted to investigate the alloy's oxidation and especially nitridation resistance. It was found that, in comparison to the binary system, both properties were improved by additional alloying with Ge, Mo, and Pt.

During oxidation Cr-Si-alloys form SiO2 (interface metal/oxide) next to Cr2O3 (interface oxide/gas). This leads to a decrease in oxide growth and in nitridation attack. Alloying with Ge and Pt supports the formation of a continuous, nitridation-resistant A15 phase layer in the subsurface area which further reduces nitridation. Cr3PtN forms in the presence of Pt instead of detrimental Cr2N. Additionally, Ge decreases the effect of local scale failure of the protective oxide scale 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. In the Cr-Si-Mo alloy nitridation is further hindered as a Mo-enriched Crss phase forms in the subsurface scale which is resistant against nitridation.

To describe the oxidation in more detail, a kinetic model (kp-kv-P model) was developed. This model includes for the first time the impact of local oxide scale failure in oxidation kinetics. Thereby, the formation of local defects and scale adhesion depending on the alloy composition can be investigated using thermogravimetric measurements.

The influence of the alloying elements on the Cr2O3 morphology was also investigated. Combining both, the assumption is made that the oxide scale is intrinsically protective against nitridation and Cr2N formation is solely caused by local scale failure. Therefore, the improvement of nitridation resistance by alloying is mainly attributed to a change in oxide morphology, induced growth stresses, and scale adhesion.

In this work it was shown that appropriate alloying of Cr-Si further reduces the parabolic oxidation constant (kp) and the evaporation rate (kv) by one order of magnitude. Additionally, nitridation was reduced by 98%. In microstructural investigations and oxidation tests the effects of the different alloying elements Ge, Mo, and Pt were found to be combinable. All in all, the Cr-Si-Ge-Mo-Pt-system shows high potential for the future development of refractory-based high-temperature alloys.