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.