Phase stability and transition metal addition in molybdenum-silicon-boron alloys

Mo-Si-B alloys have been identified as materials candidates for high-temperature structural materials due to the high-temperature oxidation resistance and mechanical properties of the high melting temperature three phase constituents, namely BCC, Mo₅SiB₂ (T₂) and Mo₃Si phases. Transition metal alloying has been shown to critical for effective processing of the ternary alloys. While there has been a great deal of data available with respect to the TM alloying behavior in the BCC Mo-solid solution [Mo(ss)] phase and in the A15 phase, there have been no systematic alloying studies performed on the ternary-based Mo₅SiB₂ (T ₂ phase). A systematic study on the phase stability and alloying behavior of the transition metals in the T₂ phase has been conducted in the current study. The majority of the TM elements establish an extensive and often continuous solid solution with Molybdenum in the T₂ phase. The extent of solid solution in large part mimics the well-known extended alloying of the TM elements in the BCC phase. The role of several factors that may be responsible to the observed extensive alloying behavior and the stability of the T₂ crystal structure have been examined. The similarity of the alloying behavior between the BCC and T₂ phase can be attributed primarily to the presence of a dense network of the BCC-like metal arrangement embedded within the T₂ crystal structure. Further analysis has also been performed to elucidate some of the geometric regularities observed in the T₂ phase and to relate them to the stability criteria of the crystal structure. The contributions of chemical bonding factors to the phase stability are reflected by the valence electron concentration (e/a) as well as the optimum valence electron concentration as well as the position of Fermi level close to the pseudo gap separating the bonding and anti-bonding states. There is a strong complementary nature between the drive to optimize the packing density and the chemical bonding in defining the stability criteria. A combined geometric and electronic structure criteria has been proposed to serve as the basis for the T₂ phase stability and its alloying extension.