Evolution of the O phase in SiCf/Ti2AlNb composites fabricated by hot isostatic pressing

The matrix microstructure of continuous SiC fiber-reinforced Ti₂AlNb composites (SiC_f/Ti₂AlNb) plays a critical role in determining their mechanical properties. Understanding the mechanisms behind the precipitation and evolution of the O phase in the matrix is crucial for optimizing processing parameters and enhancing composite performance. In this study, SiC_f/Ti₂AlNb composites were fabricated by preparing precursor wires from matrix-coated SiC fibers, followed by consolidation via hot isostatic pressing (HIP). The microstructure of both the precursor wires and the composite matrix including morphology, content, grain size, and phase evolution were systematically characterized. Results show that the precursor wire matrix contained nanoscale O, B2, and α₂ phases, where the O phase had the highest relative percentage. The magnetron sputtering process induced a specific orientation relationship between the nanoscale O, α₂, and B2 phases. After HIP processing in the three-phase region (O + B2 + α₂), microscale O, B2, and α₂ phases were observed in the composite matrix. The O phase exhibited the lowest relative percentage. The O phase in the composites undergoes transformation through the following three processes: (1) The α₂ phase transforms from α₂ → α₂ (Nb-lean) + O (Nb-rich) due to niobium diffusion; (2) the B2 phase transforms into the O phase through elemental diffusion at high temperatures, although some regions of the B2 phase do not precipitate into the O phase; (3) the O phase in the precursor wires grows and transforms into α₂ and B2 phases, with some untransformed O phase being retained. The O phase is a key reinforcing phase in the Ti₂AlNb matrix. However, the content of the O phase in the composite matrix is relatively low, making the regulation of its content through HIP processing or heat treatment a potential method for enhancing the composite's performance.