Microstructure evolution and mechanical properties in laser directed energy deposited Ti65 matrix composites with varying B/C ratios

Non-equilibrium solidification calculations reveal that the boron/carbon (B/C) mass ratio is a critical factor in tailoring the microstructure of titanium matrix composites (TMCs) in laser directed energy deposition (LDED). In this work, three kinds of TMCs, i.e., TMC1 (B/C mass ratio of 11:1), TMC2 (B/C mass ratio of 2.5:1), and TMC3 (B/C mass ratio of 1:1.4) were prepared, in which a transition from quasi-continuous ceramic network into continuous ceramic network along β grain boundaries was achieved due to the differences in the precipitation sequence and growth kinetics of the ceramic phases in LDED. TiBw in TMCs could directly coalesce to form whiskers bundle via low angle grain boundaries when their misorientation was below critical value, and misorientation hindered the interconnection of independent TiBw once beyond the critical threshold. The numerous primary TiCp in the TMC3 can act as heterogeneous nucleation sites for TiBw, forming a continuous TiBw and TiCp network. The elastic modulus increased significantly as the connectivity of the ceramic phase distribution increased. However, the reduction in ductility was associated with the formation of a continuous TiBw and TiCp network, which intensifies strain constraint and promotes preferential crack propagation along ceramic pathways. The present work can provide guidance for designing advanced TMCs by tailoring the ceramic phase distributions during LDED.