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.