Phase formation and solidification routes near Mo-Mo₅SiB ₂ eutectic point in Mo-Si-B system

The phase formation behavior during solidification in the Mo-Si-B system was experimentally examined around the triple junction point (T) of the Mo solid solution (Mo_ss), Mo₅SiB₂ (T₂) and Mo2B primary phases in the Mo-Si-B liquidus projection suggested by Yang and Chang through thermodynamic calculations, and the solidification routes of the Mo-Si-B alloys were reconsidered based on the obtained results. Alloys of four different compositions around the T point were produced by arc-melting. The primary phases observed in these alloys were in excellent agreement with the liquidus projection proposed by Yang and Chang. The solidification routes were basically Mo_ss (primary) → Mo_ss + Mo₂B → Mo_ss + T₂ eutectic -+ Mo_ss + T ₂ + Mo₃Si (A15), T₂ (primary) → Mo _ss + T₂ eutectic → Mo_ss + T₂ + A15, or Mo₂B (primary) → Mo_ss + T₂ eutectic → Mo_ss + T₂ + A15, depending on the composition. However, a quantitative EPMA analysis indicated the compositions of the Mo_ss + T₂ eutectic and Mo_ss + T ₂ + A15 phases differed from the reported liquidus projection. This is due to the difficulties interpreting the unusual solidification routes in the Mo-Si-B system and their as-cast microstructures.