Phase stability and alloying behavior in the Mo-Si-B system


The effect of transition metal (TM) substitution for Mo has been examined in terms of the phase stability and multiphase microstructures in the Mo-Si-B ternary system. 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, land a binary-based metal-rich silicide (Mo3Si [A15]). The structures that are developed by following systematic alloying with a wide range of TMs, which are substitutional in both Mo(ss) and T2 phases (group IVB, VB, and VIB metals), have been analyzed to elucidate the roles of the substitution on the stability of the three phase fields of Mo(ss) + T2 + Mo3Si. In particular, the borosilicide ternary-based T2 phase shows an extended solid solution with a wide range of TMs. The extended solubility in the T2 phase essentially mimics the alloying behavior of the TM-based bcc phase. The critical factor for the phase stability appears to be the existence of a unique feature of bcc-like TM clusters within the T2 lattice structure. The combined criteria of atomic size factor and the valence electron concentration per atom (e/a) have been used to elucidate the observed alloying behavior.

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Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science