This paper reviews our current research activities on developing new multiphase metallic materials for structural applications with a temperature capability beyond 1,200ºC. Two promising material systems have been chosen: fi rst, alloys in the system Mo-Si-B which have demonstrated potential due to their high melting point of around 2,000ºC and due to the formation of a protecting borosilicate glass layer on the surface at temperatures exceeding 900ºC; and second, novel Co-Re-based alloys which have been chosen as a model system for complete miscibility between the elements cobalt and rhenium, offering the possibility of continuous increases of the melting point of the alloy through rhenium additions.

Mo-Si-B alloys are promising candidates for structural high-temperature applications due to their excellent high-temperature mechanical properties along with high melting temperatures and oxidation resistance. After an initial period with high weight loss rates as a consequence of the volatilization of Mo-oxide, a protective borosilica (glass) layer develops on the alloy surface and steady-state oxidation is achieved. Aiming at improved mechanical properties of Mo-Si-B alloys which exhibit a continuous Mo solid solution matrix as a consequence of a powder metallurgical production route, small amounts of Zr were added. The presence of oxygen in the alloy leads to the formation of thermodynamically very stable Zr-oxide precipitates in the bulk alloy causing an enhancement of its mechanical properties. It was observed that the addition of Zr (distributed in the alloy matrix) also has significant influence on the oxidation behaviour of Mo-Si-B alloys by reducing the period for the formation of the protective and stable silica scale. Furthermore, the weight loss due to vaporization of Mo-oxides is consequently reduced. Besides this beneficial effect, Zr is harmful for the oxidation resistance at temperatures beyond 1,200°C. This is mainly due to the increased oxygen transport through defects in the silica scale.

Nowadays, even fourth generation nickel base superalloys are approaching their fundamental limitation, the melting point. Hence, a further increase in efficiency, i.e. of jet engines, can only be realised by developing new materials for the use at temperatures beyond 1200uC. A new alloy concept using the Mo-Si-B system for ultrahigh temperature applications is discussed. Those alloys have melting points y2000uC, while retaining good mechanical properties and oxidation resistance in the desired temperature range. A three phase Mo-9Si-8B alloy (composition in at.-%) consisting of a-Mo, Mo 3 Si and Mo 5 SiB 2 (T2) was produced by powder metallurgical processing route. At temperatures higher than 1000uC in laboratory air, a protective SiO 2 /B 2 O 3 glass layer develops on the alloy surface giving excellent oxidation resistance. However, in the temperature range between 700 and 900uC, non-protective and highly volatile molybdenum oxide cause the disintegration of the material (the so called pesting phenomenon). Additions of Zr and La 2 O 3 to the Mo-Si-B alloy systems were investigated to improve the performance of the alloys in the pesting temperature range. The oxidation kinetics was determined by means of thermogravimetric analysis and discontinuous oxidation experiments. Microstructural examinations were performed by means of optical and scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. The microstructural observations were compared with the theoretical prediction of phase stability using computational thermodynamic calculations. A significant improvement of the alloys during oxidation in the pesting temperature range was found. The rate of formation of molybdenum oxides could be drastically reduced at intermediate temperature range. At high temperatures (.1000uC), a homogeneous and protective SiO 2 oxide layer was formed on the alloy surface leading to a slow growing oxide scale.