Multiphase equilibria in the metal-rich region of the Mo-Ti-Si-B system: Thermodynamic prediction and experimental validation

Multiphase composite alloys based on the phases of Mo solid solution (Bcc), Mo₅SiB₂(T2), Mo₅Si₃(T1), Ti ₅Si₃ (D88), and Mo₃Si (A15) in the Mo-Ti-Si-B system are candidate materials for ultra-high temperature applications. Determination of the phase relationship in a quaternary system exclusively from experiments is very time-consuming and expensive. A strategy using thermodynamic modeling to aid the selection of key alloy compositions was used in this study. The key alloys selected with the guidance of the calculated phase diagram were prepared by arc-melting and then subjected to homogenization at 1600°C for 150 h and 1200°C for 1200 h. The microstructure of these alloys was characterized by means of scanning electron microcopy, electron probe microanalysis, X-ray diffraction and electron backscatter diffraction. The experimental results obtained from this study were then used to validate the thermodynamic modeling. The phase relationship among Bcc, T2, T1, A15 and D88 at 1600 and 1200°C is clearly defined, which can be used as a road map for alloy design and processing control.