Heat-treatable Chromium-based alloys for extrem temperatures

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This study aims to develop new heat treatable Cr-Si-Ge alloys with Cr > 90 at%, which show complete solubility at high temperatures. The heat treatment cycle allows control of the microstructure. The desired microstructure consists of a Cr solid solution matrix and intermetallic A15-phase precipitates. The precipitation behaviour and A15/Matrix interface as well as mechanical and physical properties (especially creep and oxidation resistance) will be investigated with regard to microstructure. The heat treatment parameters will be optimized in order to obtain a high A15/matrix phase ratio and enhanced properties. In the second step, the alloying potential of the Cr-Si-Ge system will be evaluated using two main approaches. First is the addition of a substitutional alloying element (in both A15 and Cr) which provides solid solution strengthening as well as substituting chromium in the A15 phase. Molybdenum shows a high alloying potential with pure chromium as well as in the A15-Cr3Si phase. Mo-alloyed chromium (with up to 9 at% Mo) shows complete solubility in all temperature ranges up to the melting point. Alloying with up to 0.2 at% Mo improves the creep resistance of pure chromium to an order of magnitude and increases the room temperature ductility. In addition, it is shown that alloying Cr3Si with Mo (up to 30 at.%) enhances the oxidation resistance in a broad temperature range by improved protection through SiO2 formation. Also, concerning the oxidation resistance, Mo is interesting because in comparison to Cr, Mo shows oxide pesting during oxidation which can locally increase the Si or Ge activity. This is to be investigated. The second approach is the addition of an element that forms the A15 phase, which can also substitute Si and Cr, and shows a high temperature dependent solubility in chromium solid solution. The Cr-Pt system shows an analogous binary composition as the Cr-Si and Cr-Ge systems composed of a solid solution and an A15-Cr3Pt phase field. The system provides the required steep solvus line in the Cr-rich region. It is aimed to study how soluble to A15 precipitates are and how their volume fraction can be increased and eventually stabilized through the addition of platinum. Therefore, it is necessary to study the interface properties between the A15 precipitates and the solid solution matrix. Potential applications of this system are directly as an optimized alloy or as a metal matrix, which might be further enhanced via oxide dispersed strengthening and used for turbine applications as well as applications in the chemical industry, where extrem corrosion resistance is required at elevated temperatures.