## 1 Motivation and Introduction  
In recent years, environmental restrictions on CO\({}_{2}\) emission have demanded advanced power plants with higher efficiency. For further efficiency improvement, an increase of steam conditions to approximately 650 \({}^{\circ}\)C (300 bar) would be required. The present study is part of a project that aims to develop high-chromium fully ferritic stainless steels for application in ultra-super-critical power plants, with an operation temperature of 650 \({}^{\circ}\)C. The goal of this project was to achieve a stable ferritic, heat-resistant microstructure by combining solid solution and Laves phase precipitation strengthening. The steels currently used in power plants provide certain disadvantages with respect to the high operation temperature of 650 \({}^{\circ}\)C. The conventional alloy concept of the 9 wt. % chromium ferritic-martensitic steels is challenged regarding the required steam oxidation resistance. These steels are below the limit for forming protective chromium oxide layers in steam at a temperature range of 550 to 650 \({}^{\circ}\)C due to their low chromium content [1]. As a result, their resistance to steam oxidation shows high fluctuation ranges and depends on many parameters, which means great uncertainty for stable use [1; 2; 3]. For example, the application of the common ferritic-martensitic steel P92 is limited to a maximum operation temperature of 620 \({}^{\circ}\)C due to the restricted thermodynamic stability of the strengthening precipitates in combination with the limited steam oxidation resistance [4]. Chromiumcontents higher than 15 wt. % are necessary to reliably reduce steam oxidation at 650 \({}^{\circ}\)C [5]. In general, the creep resistance of ferritic-martensitic steels is based on microstructural stabilization by finely precipitated carbides and nitrides [6]. However, due to the higher chromium content (11-12 wt. %), ferritic-martensitic steels tend to the precipitation of Z-phase (Cr(V,Nb)N), which replaces nitrides as a more thermodynamically stable phase and therefore causes a reduction in creep resistance during long term operation [7; 8]. An improvement in the creep resistance of ferritic-martensitic steels has been achieved with higher chromium contents of up to 14 wt. % in combination with controlled precipitation of the intermetallic Laves phase [9]. The formation of the Z-phase has not been observed in these steels. Furthermore, it has been attempted to use the Z-phase for strengthening [10; 11]. Consequently, the maximum operation temperature of all currently available ferritic-martensitic steels is about 610 to 630 \({}^{\circ}\)C [4; 12; 13]. The maximum chromium content of ferritic-martensitic steels is controlled by the availability of a high-temperature austenitic phase field, which is about 14 wt. % Cr if a martensitic microstructure has to be achieved [14]. In most cases this value is even lower due to the avoidance of the undesirable \(\delta\)-ferrite formation during solidification. Considering the low chromium content and the resulting limited steam oxidation resistance, the concept of high-temperature ferritic-martensitic steels appears to be exhausted for the current required highly efficient power plants. Full ferritic steels with a higher chromium content than 15 wt. %, without martensitic transformation and strengthened by Laves phase precipitation, provide potentially sufficient steam oxidation resistance of up to 650 \({}^{\circ}\)C [5]. The main task of alloy design is to maximize the volume fraction of the strengthening Laves phase and to avoid the formation of the brittle \(\sigma\)-phase. A high chromium content of over 15 wt. % provides in principle a sufficient steam oxidation resistance of up to 650 \({}^{\circ}\)C [15]. However, the intermetallic Laves phase precipitation binds chromium from the matrix, which reduces the chromium fraction in solid solution to only 12.5 wt. % [16].