Microstructure evolution of Ti64-TiBw composites via electron beam powder bed fusion using as-prepared composite powder as feedstock

A self-designed spherical Ti64 powder containing nanosized TiB whisker(TiBw) was developed by electrode induction melting gas atomization (EIGA), which was used as feedstock to prepare Ti64-TiBw composites by electron beam melting (EBM). Nano-scaled TiBw distributed along the grain boundaries, forming a discontinuous network structure in EBM-Ti64-TiBw composites. In order to elucidate the effect of nano-sized TiBw on microstructure and properties of EBM-Ti64-TiBw composites, heat treatment of as-fabricated EBM-Ti64-TiBw is performed from 800 to 1200 °C with interval of 100 °C, to regulate the size characteristics (diameter, aspect ratio) of TiBw, the revolution of microstructure and interface were studied in detail. The results indicate that Ti64/TiBw interface structure transforms from a semi-coherent interface to a coherent interface with increasing heat treatment temperature (HTT). Furthermore, TiBw begin to coarsen after the heat treatment above the β-transus temperature, leading to a weakening of its pinning effect on Ti64 matrix, and the discontinuous network structure disappears at 1000 °C. The growth and coarsening of TiBw are closely related to the diffusion rate of B atom in α-Ti and β-Ti phases. This work provides a theoretical foundation for size control of TiBw reinforcement and the elucidation of its evolution effect on microstructure in EBM-Ti64-TiBw composites.