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

Abstract

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 (Moss), 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 Moss (primary) → Moss + Mo2B → Moss + T2 eutectic -+ Moss + T 2 + Mo3Si (A15), T2 (primary) → Mo ss + T2 eutectic → Moss + T2 + A15, or Mo2B (primary) → Moss + T2 eutectic → Moss + T2 + A15, depending on the composition. However, a quantitative EPMA analysis indicated the compositions of the Moss + T2 eutectic and Moss + 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. © 2010 The Japan Institute of Metals.