Dependence and mechanism of hydrogenation behavior on absorption conditions in hypo-eutectic Mg-Ni-Cu alloy

In this article, the isothermal hydrogenation curves of hypo-eutectic Mg-6Ni-3Cu (at. %) alloy under various temperatures and applied hydrogen pressures are fitted by Johnson-Mehl-Avrami equation. The hydrogenation behavior and its dependence on absorption conditions are investigated, as different from that of pure Mg. A three-stage hydrogenation behavior is illustrated and an extra absorption process (Stage III) resulted from H dissolution or the hydride formation in boundaries appears when the hydrogen pressure reaches a critical value. Due to the same role of increased Delta P and decreased Delta T on hydride nucleation driving force and their opposite impacts on H diffusion, temperature and hydrogen pressure are influencing the absorption process differently. As the great H permeability in Mg-Mg2Ni-Mg2Cu eutectic, more hydrogen is absorbed under larger hydrogenation driving force, rather than hydrogen uptake deficit. It is revealed that the reaction order eta corresponding to H diffusing in thin MgH2 layer is larger than that in thick MgH2 shell, which can preliminarily identify the hydrogenation behavior after MgH2 impinging around primary Mg (Stage II). (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.