GROWTH MECHANISM OF PRIMARY HOLLOW TiB IN Ti-Al-B ALLOY

Ti-17Al-1.5B alloy was produced by an in-situ method. The microstructure and the phase constitution of this alloy were investigated by XRD and SEM. The results show that this alloy is composed of Ti₃Al and TiB. The coarser and longer primary TiB is highly faceted and exhibits a hollow needle morphology, and the eutectic TiB is fine needle shape. Based on the stability theory of crystal growth, the propagation of the steps on the center of the solid-liquid interface of TiB crystal is stopped due to very large diffusional under-cooling when the solid-liquid interface grows to a critical dimension at 010 direction; therefore, the TiB crystal grows preferentially from the edge of the crystal. Additionally, the growth rate at 010 direction of TiB crystal is very fast because of its B27 crystal structure. So it is considered that the inhomogeneity in supersaturation of crystal-liquid interface and B27 crystal structure of TiB resulted its growing with hollow shape when the (010) facet reaches a critical dimension. The eutectic needle TiB is difficult to grow with hollow morphology because of its much smaller cross section.