Current Status of Mo-Si-B Silicide Alloys for Ultra-high Temperature Applications
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1128-U07-07
Current Status of Mo-Si-B Silicide Alloys for Ultra-high Temperature Applications Martin Heilmaier1, Holger Saage2, Manja Krüger2, Pascal Jehanno3, Mike Böning3 and Heinrich Kestler3 1
Materials Science Department, TU Darmstadt, Petersenstr. 23, D-64287 Darmstadt, Germany Institute for Materials & Joining Technology, Otto-von-Guericke University Magdeburg, D-39106 Magdeburg, Germany 3 Technology Centre, Plansee SE, A-6600 Reutte/Tyrolia, Austria 2
ABSTRACT We review the current development status of molybdenum borosilicide (Mo-Si-B) alloys for ultra-high temperature applications in excess of 1100°C in air. The assessment of several ingot and powder metallurgy approaches revealed that (i) the presence of a continuous Mo solid solution matrix is mandatory for adequate low temperature toughness and (ii) wrought processing of such alloys at temperatures established for refractory metals requires the presence of an ultrafine (sub-micron) microstructure. Both the prerequisites could be fulfilled using mechanical alloying (MA) as the crucial processing step , however, values for the ductile-tobrittle transition temperature (DBTT) below 800°C could not be obtained due to grain boundary embrittlement by Si segregation. First results on the effect of different microalloying additions (e.g. Zr) on the reduction of this segregation will be presented and discussed. INTRODUCTION The development of structural materials for ultra-high temperature applications is a major challenge for the materials science and engineering community. In the case of energy producing gas turbines the establishment of Nickel based superalloys in the 1950’s opened an era of 50 years of continuous growth and development. Today’s engines expose these superalloys to temperatures approaching 1150°C, corresponding to homologous temperatures above 0.8 of the melting point, while the turbine inlet temperature was raised to about 1500°C. However, when looking at the maximum service temperature of superalloys over the last 20 years, only a few major improvements, i.e. the introduction of single-crystals (including the addition of Re) and the application of thermal barrier coatings, respectively, have been achieved [1,2]. New metallic materials which could withstand surface temperatures higher than 1200°C would be desirable in order to increase the (thermodynamic) efficiency. Amongst other metallic materials systems such as alloys based on precious metals [3] which will be disregarded here for weight reasons, multiphase silicide alloys with Nb or Mo as a base metal have shown promise as novel structural materials for applications in excess of 1100°C in air [1,4-6]. In this overview we report on the current status of Mo-Si-B alloys consisting of Mo solid solution (Moss) and of the intermetallic phases Mo3Si and Mo5SiB2 which could take advantage of (i) the beneficial oxidation resistance of the silicide phases and (ii) the outstanding mechanical properties of molybdenum. A typical alloy composition that showed balanced properties with respect to oxid
