Microstructure and hydrogenation thermokinetics of ZrTi_0.2V _1.8 alloy

The microstructures and phase composition of the pseudobinary ZrTi _0.2V_1.8 alloy were examined by scan electron microscope (SEM) and X-ray diffraction (XRD). Before hydrogenation, the hypoeutectic structure accompanied with ZrV₂ + (ZrV₂ + Zr) spherical-like texture has been observed in ZrTi_0.2V_1.8 and the dominant phase could be ascribed to the C15 Laves phase. Hydrogen absorption pressure-composition isotherms (P-C isotherms) and hydriding kinetics of ZrTi_0.2V_1.8 were investigated by pressure reduction method using Sievert apparatus from 673 to 823 K. At hydrogen concentration 0.65 (H/A), the relative partial molar enthalpy and entropy calculated by Van't Hoff equation are -60 ± 1 kJ mol^-1 and -119 ± 1 J mol ^-1 K^-1, respectively. In addition, two stages in the hydrogen absorption reaction between 673 and 823 K could be attributed to the different hydrogen absorption mechanisms including redistribution of the hydrogen atoms in the hydride phase and the diffusion of hydrogen in the β-phase. The activation energy E_a of the alloy is ∼3.6 kJ mol^-1 for the first absorption stage and ∼61.9 kJ mol ^-1 for the second one.