Solidification of Bcc/T₁/T₂ three-phase microstructure in Mo-Nb-Si-B alloys

The three-phase, (Mo,Nb)_ss/(Mo,Nb)₅Si₃/(Mo,Nb)₅SiB₂, Bcc/T₁/T₂ microstructures that develop in Mo-Nb-Si-B alloys have been examined in arc cast and directionally solidified samples to identify the phase sequencing during solidification. A Mo-32.6Nb-19.5Si-4.7B (at.%) alloy was directionally solidified using an optical floating zone (OFZ) furnace in a flowing Ar gas atmosphere at a constant growth rate of 10 mm/h. The microstructure of the directionally solidified alloy is characterized by an elongated T₂ phase surrounded by bcc and T₁ phases with an interwoven morphology From the evaluation of the microstructures in arc cast ingots of several alloys at a constant 32.6 at%Nb composition, the path of the liquidus valleys with decreasing temperature has been determined to intersect at a common point that establishes the solidification reaction as a eutectic. The incorporation of the experimental results into a computational thermodynamic analysis provides insight on the partitioning of components within each phase during solidification.