MICROSTRUCTURE OF Ti-40Al-2B AND GROWTH CHARACTERISTIC OF PRIMARY TiB₂

Ti-40Al-2B(wt%) alloy was produced successfully by an in-situ method. The microstructure and the phase constitution, especially the morphology of the primary TiB₂ of this alloy were investigated by XRD and SEM. The results show that this alloy is composed of TiAl and TiB₂. TiB₂ mainly exists in the matrix with block primary TiB₂ and plate or fine bar shape eutectic TiB₂.The primary TiB₂ is hexagonal prism morphology. Clear growth steps were observed at the top of the primary TiB₂, and the crystal faces were determined to be (0001) and ｛101 - 0｝. Based on the crystal growth theory, it was considered that the growth element of the primary TiB₂ is the trigonal prism of six Ti atoms with one B atom at the center, and the primary TiB₂ grows mainly in a step growth method. The growth rate of ｛112 - 1｝ faces is the fastest and that of ｛101 - 0｝ is the slowest, which results in a hexagonal prism TiB₂ with ｛101 - 0｝ as prismatic faces.