Phase stability and alloying behavior in the Mo-Si-B system
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I. 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. In this respect, the multiphase microstructures developed in the Mo-Si-B system offer some attractive options.[1,2] Figure 1 shows the 1600 deg isotherm in the metal-rich portion of the system.[1] Two-phase alloys based upon the coexistence of the high melting (2100 °C) and creep-resistant ternary intermetallic Mo5SiB2 or 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.[3] Three phase alloys comprised of Mo, T2, and Mo3Si offer a promising balance of oxidation resistance and mechanical properties.[4] Hence, a focal point of microstructural design to optimize the materials properties has been the elucidation of the alloying behavior, in particular of the transition metals (TMs), on the stability of the two-phase field of bcc T2 and the structural stability of the T2 phase. In previous works,[1,2,] the incorporation of selected TMs (W, Nb, V, and Cr) has been demonstrated to form a continuous T2 phase and the two-phase field of bcc T2 in their respective quaternary systems (Figure 2). The observed alloying trends also appear to highlight the fundamental geometrical factor that influences the relative stability of the T2 phase and provides a basis to develop a multiphase design.[1] In the present work, the alloying evaluation is extended further to include a wide range of refractory metal additions R. SAKIDJA, Postdoctoral Fellow, and J.H. PEREPEZKO, Professor, are with the Department of Materials Science and Engineering, University of Wisconsin–Madison, Madison WI 53706. Contact e-mail: sakidja@cae. wisc.edu This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
(group IV to VII B metals). The main focus of this investigation is on the alloying extension of group IVB (such as Ti, Zr, and Hf) and group VIIB (such as Re) metals. Titanium and hafnium are the major additives to the commercial Mo-based alloys such as “TZM,” which is composed of a bcc phase and dispersoids of TM carbides to enhance the high-temperature strength. However, there is no known ternary-based T2 phase reported in TM-Si-B systems (TM Ti, Zr, and Hf).[5,6,7] The addition of rhenium (Re) into Mo bcc solid solution has been well known to markedly lower the ductile to brittle transition temperature of the bcc phase, resulting in a higher ductility and toughness.[8,9] The stability of the T2 crystal structure can be analyzed further not on
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