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 ΔP and decreased ΔT 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 η 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).