Study of solidification pathway of a MoSiBTiC alloy by optical thermal analysis and in-situ observation with electromagnetic levitation

Abstract

MoSiBTiC alloys are promising candidates for next-generation ultrahigh-temperature materials. However, the phase diagram of these alloys has been unknown. We have developed an ultrahigh-temperature thermal analyser based on blackbody radiation that can be used to analyse the melting and solidification of the alloy 67.5Mo-5Si-10B-8.75Ti-8.75 C (mol%). Furthermore, electromagnetic levitation (EML) was used for in-situ observation of solidification and microstructural study of the alloy. On the basis of the results, the following solidification pathway is proposed: Mo solid solution (Mo_ss) begins to crystallize out as a primary phase at 1955 °C (2228 K) from a liquid state, which is followed by a (Mo_ss+TiC) eutectic reaction starting at 1900 °C (2173 K). Molybdenum boride (Mo₂B) phase precipitates from the liquid after the eutectic reaction; however, the Mo₂B phase may react with the remaining liquid to form Mo_ss and Mo₅SiB₂ (T₂) as solidification proceeds. In addition, T₂ also precipitates as a single phase from the liquid. The remaining liquid reaches the (Mo_ss + T₂ + TiC) ternary eutectic point at 1880 °C (2153 K), and the (Mo_ss + T₂ + Mo₂C) eutectic reaction finally occurs at 1720 °C (1993 K). This completes the solidification of the MoSiBTiC alloy.