Transition Metal Alloying and Phase Stability in the Mo-Si-B System
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Transition Metal Alloying and Phase Stability in the Mo-Si-B System R. Sakidja, S. Kim, J. S. Park and J. H. Perepezko University of Wisconsin-Madison, Materials Science & Engineering 1509 University Avenue, Madison, WI 53706, USA ABSTRACT The effect of transition metal substitution for Mo on the phase stability and multi-phase microstructures in the Mo-Si-B ternary system has been examined. The metal-rich portion of the ternary Mo-Si-B system at equilibrium is comprised of thermally stable BCC Mo(ss) phase, a ternary-based Mo5SiB2 (T2 phase), binary-based metal-rich silicides (Mo3Si [the A15 phase] and Mo5Si3 [the T1 phase]) and borides (Mo2B and MoB phases). Systematic alloying with selected transition metals which are substitutional in both Mo(ss) and T2 phases such as Cr, V, Nb, W , Ti and Hf, has been performed to elucidate the roles of the substitution on the stability of the three phase fields of Mo(ss) + T2 + A15 and T2+ T1 + A15. The potential of the alloying effects on the microstructure design and control of the solidification pathways is further detailed. INTRODUCTION The challenges of a high temperature environment (T>1400°C) impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. A number of ceramic materials, intermetallic compounds and refractory metals with high melting temperature are available as material choices. However, in a single component form, these materials rarely satisfy all the above requirements because of the brittleness of ceramic materials and intermetallic compounds at low temperatures and the oxidation problems and poor creep resistance of refractory metals at high temperatures. In this respect, the multiphase microstructures that can be developed in the Mo-Si-B system offer useful options for high temperature applications [1]. Figure 1 shows the 1600oC isotherm in the metal-rich portion of the system. Two phase alloys based upon the coexistence of the high melting (>2100°C) and creep resistant ternary intermetallic Mo5SiB2 (the T2 phase) with the Mo solid solution (BCC phase) allow for in-situ toughening and a further possibility for strengthening through a precipitation of Mo within the T2 phase [2]. Three phase alloys comprised of Mo, T2 and Mo3Si offer balance of oxidation resistance and mechanical properties [3-4]. There is also the highmelting Mo5Si3(B) (the T1 phase) which has been reported to possess attractive materials properties such as high oxidation resistance (comparable to that of MoSi2) and superb creep resistance[5]. The T1 phase however is not in equilibrium with the BCC phase in the ternary MoSi-B system even though it does form a stable two-phase field with the T2 phase. In other systems of RM-Si-B (RM= Nb, Cr, W), the T2 phase has been shown to develop thermally stable multi-phase equilibria involving the BCC as well as the silicide phases. Thus, the focal point of further microstructural design is the elucidation of the role of alloying additions, in particular the refractory metals
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