Stacking faults and mechanisms strain-induced transformations of hcp metals (Ti, Mg) during mechanical activation in liquid hydrocarbons

The evolution of the structure and substructure of metals Ti and Mg with hexagonal close-packed (hcp) lattice is studied during their mechanical activation in a planetary ball mill in liquid hydrocarbons (toluene, $n$-heptane) and with additions of carbon materials (graphite, fullerite, nanotubes) by X-ray diffraction, scanning electron microscopy, and chemical analysis. The temperature behavior and hydrogen-accumulating properties of mechanocomposites are studied. During mechanical activation of Ti and Mg, liquid hydrocarbons decay, metastable nanocrystalline titanium carbohydride Ti(C,H)$_x$ and magnesium hydride $\beta$-MgH$_2$ are formed, respectively. The Ti(C,H)$_x$ and MgH$_2$ formation mechanisms during mechanical activation are deformation ones and are associated with stacking faults accumulation, and the formation of face-centered cubic (fcc) packing of atoms. Metastable Ti(C,H)$_x$ decays at a temperature of 550$^\circ$C, the partial reverse transformation fcc $\to$ hcp occurs. The crystalline defect accumulation (nanograin boundaries, stacking faults), hydrocarbon destruction, and mechanocomposite formation leads to the enhancement of subsequent magnesium hydrogenation in the Sieverts reactor.