Microstructural and mechanical properties of ternary Mo-Si-B alloys resulting from different processing routes
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Microstructural and mechanical properties of ternary Mo-Si-B alloys resulting from different processing routes Manja Krüger1, Martin Heilmaier2, Veronika Shyrska3, Petr I. Loboda3 1
Otto von Guericke University Magdeburg, Institute for Materials and Joining Technology, P.O. Box 4120, D-39016 Magdeburg, Germany 2 Technical University Darmstadt, Dept. Materials Science, D-64287 Darmstadt, Germany 3 National Technical University of Ukraine “Kiyv Polytechnic Institute”, Kiew, Ukraine ABSTRACT Mo-base silicide alloys take advantage of their outstanding intrinsic properties, notably the high melting point and, thus, their excellent mechanical and creep strength. We demonstrate how the processing route influences the microstructure and consequently the mechanical and oxidation behaviour. Therefore two fabrication routes, a powder metallurgical (PM) and a zone melting (ZM) process, both starting from elemental powders, were used to prepare several Mo-Si-B alloys with varying chemical compositions. While PM processing leads to an ultrafine microstructure with a continuous Mo solid solution (“α-Mo”) matrix and embedded particles of the two intermetallic compounds Mo3Si and Mo5SiB2, the directionally solidified (ZM) materials possess a coarse grained structure composed of an intermetallic matrix with dendritic islands of α-Mo. A comparative assessment of the mechanical behaviour of the alloys utilizing both the Vickers indentation fracture (VIF) technique and three-point bending tests emphasizes the beneficial effect of a continuous Mo matrix resulting in increased room temperature fracture toughness and a reduction of the brittle-to-ductile-transition-temperature (BDTT). Likewise, the positive effect of the fine grained and homogeneous microstructure on oxidation performance is shown by the evaluation of mass change during heat treatment at 1100°C. INTRODUCTION The development of alloys offering increased temperature capability is of paramount importance for applications in the aerospace and power generation industry. While nickel-base superalloy turbine blade materials already operate at very high homologous temperatures, new metallic materials that can withstand surface temperatures higher than 1100°C would be desirable in order to increase the thermodynamic efficiency of a gas turbine. Mo-Si-B alloys have been subject to intensive research because of both promising mechanical properties and oxidation resistance. According to the approach of Berczik [1] the target for development of those alloys is to produce a composite material consisting of a molybdenum matrix with fine and homogeneously dispersed intermetallic particles. This is motivated by assuming that the mechanical properties (especially fracture toughness) are mainly determined by the molybdenum matrix and the oxidation resistance is provided by the intermetallic phases acting as reservoirs for the formation of a dense borosilicate glass layer at the surface [2, 3]. Thereby an increased amount of Si and B leads to enhanced oxidation resistance of Mo-base alloys [
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