Development of density reduced Mo-Si-B alloys

Abstract

Improving the efficiency of turbines for power plants and aircraft engines is an increasingly important research subject. Ternary Mo-Si-B alloys, consisting of a molybdenum solid solution (Moss) phase and two intermetallic phases Mo5SiB2 (T2) and Mo3Si, are able to combine balanced room temperature fracture toughness, high temperature creep strength and good oxidation performance. However, the high density (>9 g/cm3 ) of this class of alloys is a drawback when used as a turbine blade material. Therefore, the present thesis deals with vanadium as a potential alloying partner for density optimized Mo-based alloys. In order to identify the role of vanadium in terms of strengthening the solid solution phase, different Mo-5X (X=Ti, V, Zr) alloys were produced via arc melting and evaluated by means of RT microhardness measurements, three-point bending as well as compression tests up to elevated temperatures (1100 C). Additionally, quantitative values for solid solution hardening were determined by the approach of Labusch. Those initial characterization of the binary systems confirmed vanadium to be a competitive alloying partner compared with Ti and Zr, due to its positive effect on the strength and the ductility of Mo-based alloys at the same time. In a next step, potential Mo-V-Si-B materials which provide a reduced density by about 20% as compared with the reference alloy Mo-9Si-8B were investigated. Different alloy compositions Mo-XV-Si-8B (X=10, 20, 30, 40 at.%) were produced by powder metallurgy, including mechanically alloying and a thermal treatment, to observe the effects of V as a solute in the respective phases. X-ray diffraction showed that V can entirely be solved in the Moss phase as well as in the Mo3Si and Mo5SiB2 phases. From Rietveld refinements the preferred V sites in the lattices of the present phases were derived and could be explained by density functional theory calculations. The thermomechanical characterization was carried out on sintered (FAST) and arc-melted Mo-40V-9Si-8B alloys. Three point-bending with notched samples as well as compressive creep tests reveal a high fracture toughness and acceptable creep strength of this new type of alloys. Furthermore, the effect of minor additions of Fe on the oxidation resistance was investigated by cyclic oxidation tests.