Sequence of phase evolution during mechanically induced self-propagating reaction synthesis of TiB and TiB2 via magnetically controlled ball milling of titanium and boron powders
Mechanically induced self-propagating reaction synthesis of titanium boride and diboride was investigated for elemental mixtures of titanium and amorphous boron in starting atomic ratios of Ti50B50 and Ti34B66. Samples were milled for different periods leading up to and after an ignition point which was determined by in-situ monitoring of the mill temperature. Methods of X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy were used to determine product evolution. For both compositions, a partial crystallisation of amorphous boron commences during the early stages of milling and progresses through to the ignition point. At later stages and prior to ignition, HRTEM combined with electron diffraction revealed the formation of nanocrystalline titanium diboride, which was confirmed by Raman spectroscopy to be of an off-stoichiometric, boron-lean composition; TiB2-x. This phase is believed to have formed by solid-state reaction at the interface of the heavily deformed Ti and amorphous B via a mechanism which parallels the formation of nanocrystalline off-stoichiometric Ti1+xC1-x prior to the self-propagating synthesis of TiC from Ti and graphite. After ignition, the product for Ti34B66 comprised relatively large facetted grains of TiB2, liquid phase sintered by thin regions of unreacted Ti which had melted due to high heat of reaction. In contrast, the product of Ti50B50 comprised platelike TiB particles, and spheroidal dispersions of TiB2 in unreacted Ti and B. Likely mechanisms for the different product evolution routes for the two compositions are proposed and discussed.