High temperature oxidation of mechanically alloyed Mo–Si–B alloys

Abstract

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 1200°C. A new alloy concept using the Mo–Si–B system for ultrahigh temperature applications is discussed. Those alloys have melting points ∼2000°C, 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 α-Mo, Mo3Si and Mo5SiB2 (T2) was produced by powder metallurgical processing route. At temperatures higher than 1000°C in laboratory air, a protective SiO2/B2O3 glass layer develops on the alloy surface giving excellent oxidation resistance. However, in the temperature range between 700 and 900°C, non-protective and highly volatile molybdenum oxide cause the disintegration of the material (the so called pesting phenomenon). Additions of Zr and La2O3 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 (>1000°C), a homogeneous and protective SiO2 oxide layer was formed on the alloy surface leading to a slow growing oxide scale.