Modification of the α-Ti laths to near equiaxed α-Ti grains in as-sintered titanium and titanium alloys by a small addition of boron

摘要

A small addition of boron (B) changes the morphology of the α-Ti laths in as-sintered Ti-6A1-4V and Ti-10 V-2Fe-3Al to near equiaxed α-Ti grains and increases the number density of the resulting α-Ti grains by up to six folds. TiB forms at about 700 °C during heating to the isothermal sintering temperature and more than 90% of the TiB particles were found inside the α-Ti grains. Transmission electron microscopy (TEM) was used to identify the orientation relationships (ORs) between the TiB and α-Ti phases. Their exact ORs are affected by both the chemistry of the alloy and the processing conditions. The modification of the α-Ti laths and the substantial increase in the number density of the α-Ti grains are attributed to the enhanced heterogeneous nucleation of α-Ti on TiB due to the identified specific ORs and excellent lattice matches between these two phases. In addition, there exists a unique peritectoid reaction between p-Ti and TiB during the subsequent cooling after isothermal sintering by which β-Ti + TiB -> α-Ti, which may have contributed to the enhanced heterogeneous nucleation of α-Ti on TiB.