S. Burk

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.

Effect of Si addition on the oxidation resistance of Co–Re–Cr-alloys: Recent attainments in the development of novel alloys

Abstract The influence of silicon on the oxidation behaviour of Co—Re—Cr-alloys has been studied at 1 000°C and 1 100°C. Consideration was given to the synergetic effects between chromium and silicon with respect to the development of a protective Cr2O3 layer. The Si addition to the Co—Re-alloys produces a significant decrease in the evaporation rate of Re oxides. Moreover, the beneficial influence in the transient oxidation period results in a rapid formation of Cr2O3 scale. While the addition of 1 and 2 at.% Si to the ternary Co-17Re-23Cr alloy was insufficient to form a continuous Cr2O3 scale, the addition of 3 at.% silicon caused a change in the oxidation mode resulting in the formation of a nearly continuous Cr2O3 scale. On the oxide/alloy interface of the alloy Co-17Re-30Cr-2Si, a continuous and dense Cr2O3 scale was observed, which remained stable after 100 h exposure protecting the metallic substrate.

High-Temperature Oxidation Performance of Mo-Si-B Alloys: Current Results, Developments and Opportunities

Ni-base superalloys are approaching the melting point as their fundamental limitation. For high-temperature components one possibility aiming at a further increase of efficiency, e.g. of jet turbines, is the use of refractory metals. Mo as base material is suitable for operating temperatures far beyond 1200°C. As a consequence of the formation of volatile Mo-oxides, it exhibits no intrinsic oxidation resistance when exceeding 700°C. Mo-Si-B alloys have melting points around 2000°C and retain good mechanical properties and oxidation resistance at very high temperatures. In air, the three-phase Mo-Si-B alloy dealt with in this paper shows excellent oxidation behaviour between 900°C-1300°C as a consequence of the formation of a protective silica scale. Below 900°C, alloys of this class suffer from catastrophic oxidation, leading to an evaporation of Mo-oxide and giving rise to a linear rate law of the weight loss. A protective oxide layer is not formed as a consequence of simultaneous and competitive Mo- and Si-oxide formation. Several approaches are possible to improve the oxidation performance of Mo-Si-B alloys, especially in this moderate temperature range. These include classical alloying, e.g. with Cr aiming for protective Cr-oxide scales, addition of small amounts of reactive elements for microstructure-refinement as well as selective oxidation of silica in oxygen-deficient atmospheres prior to operation in air. The results presented show promising opportunities and indicate that an oxidation protection from room temperature up to 1300°C requires a combination of the suggested approaches.

Oxidation behaviour of model Co–Re alloys during exposure to laboratory air at 1000°C

Abstract The oxidation behaviour of model Co–Re based alloys at 1000°C in air has been investigated. Thermogravimetric studies in combination with microstructural examinations of Co–Re alloys with different compositions showed the negative influence on the oxidation resistance of Co based alloys due to the evaporation of rhenium oxide(s). Oxidation at 1000°C in air yielded an oxide scale, which consists of a Co oxide outer layer on a thick and porous mixed Co–Cr oxide and a semicontinuous and therefore non-protective Cr oxide film on the base metal substrate. This allows the vaporisation of rhenium oxide formed during oxidation and leads to a loss of Re. In addition, computer aided thermodynamic calculations were carried out to supplement the experimental analyses.