Nonequilibrium solidification behavior and microstructural evolution of SiC fiber/TC17 composites under rapid solidification conditions

Abstract

Hybrid additive manufacturing is an efficient method for fabricating complicated structures on fiber-reinforced titanium matrix composite workpieces. However, the laser thermal input can remelt the involved substrate, causing considerable interactions between the Ti-alloy matrix and SiC fibers (SiC_fs). Therefore, it is difficult to characterize nonequilibrium-solidification behaviors and the corresponding microstructural evolution under extreme rapid-solidification conditions. Herein, in-situ synchrotron radiation X-ray diffraction was employed to investigate the microstructural evolution mechanisms of SiC_f/TC17 composites in real-time under different laser remelting conditions. Results indicated that in different regions within the melt pool, the phase precipitation behaviors were different, which were influenced by the Si and C concentrations in the melt due to the decomposition of fibers. Solidification begins near the melt pool boundaries, where low Si and C concentrations results in the β-Ti phase precipitating first, followed by the precipitation of TiC_x phases in the dendritic regions between the β-Ti phases. In the middle region of the melt pool, increasing solidification time causes decomposition of more SiC_fs. Increasing the Si and C concentrations in the melt enhances TiC_x precipitation, which should be prioritized, followed by the formation of Ti₅Si₃ phases and (β-Ti + Ti₅Si₃) eutectic phases. Further solidification induces Ti₃SiC₂ precipitation. The top region of the melt pool solidifies during the last solidification stage. High Si and C concentrations promote the preferential precipitation of Ti₅Si₃ dendrites and (Ti₅Si₃ + TiSi₂) eutectic phases, forming a considerably textured microstructure. The Ti₃SiC₂ and TiSi₂ phases primarily precipitate between the Ti₅Si₃ dendrites, and TiSi₂ phases are the last phases to precipitate.