Microstructure and phase stability of Ti and Al co-added MoSiBTiC alloys

Microstructure and phase equilibria and stability of Ti and Al co-added MoSiBTiC alloys were investigated for Mo-xTi-5Si-3Al-10C- 10B(in mol%) alloys (xTi-3Al alloys) and Mo-xTi-5Si-5Al-10C-10B(in mol) alloys(xTi-5Al alloys)(x=15-30). In the as-cast state after arc-melting, lower-Ti-content alloys had the constituent phases of Moss, Mo5SiB2 (T2), TiC and Mo3 (Al,Si). After primary TiC crystallization, Moss+TiC, Moss+T2+TiC and Mo3 (Al,Si)+TiC eutectic phases were crystallized out in stages during solidification. At and above the Ti concentration of 20 mol%, Moss+Ti5Si3 eutectic was also observed to form in finally solidified regions. On the other hand, no Mo3 (Al,Si) was crystallized in the 25Ti and 30Ti-3Al alloys, and the 30Ti-5Al alloy. After heat treatment at 1600°c for 24 h, the Moss+Ti5Si3 eutectic phase observed in finally-solidified regions disappeared. The volume fraction of Mo3 (Al,Si) decreased with increasing Ti content, and in the 25Ti and 30Ti-3Al alloys and the 30Ti-5Al alloy, no Mo3 (Al,Si) appeared. As a result, the constituent phases were Moss, T2, and TiC in the 25Ti and 30Ti-3Al alloys and the 30Ti-5Al alloy as same as those of the first generation MoSiBTiC alloy. For all the alloys, the partitioning ratio of Ti for Moss/Mo3 (Al,Si) was over 1, meaning Ti stabilizes Moss against Mo3 (Al,Si). Accordingly, it is concluded that Ti expands the solubility limit of Al in Moss and suppresses Mo3 (Al,Si) formation in the MoSiBTiC system.