Mg3Cd: A model alloy for studying the destabilization of magnesium hydride

We prepared an ordered Mg3Cd alloy by high energy ball milling of elemental powders. The synthesized alloy exhibited good hydrogenation kinetics and reversibly absorbed about 2.8 wt. % of hydrogen. The temperature dependence of hydrogenation kinetics of the alloy measured in the range of temperatures covering the order-disorder phase transformations in the Mg3Cd and MgCd phases did not exhibit any anomalies and could be fitted with a single Arrhenius line. The measured apparent activation energy (69 ± 2 kJ/mol) hinted that hydrogenation process was controlled by diffusion of Cd in metallic phase. The pressure-composition isotherms exhibited negligible pressure hysteresis and sloping pressure plateau. Based on microstructural evidence obtained with the aid of X-ray diffraction and scanning electron microscopy, we built a thermodynamic model predicting the plateau hydrogen pressure for partially hydrogenated alloy. The predictions of the model were in a good agreement with the experimental data. Finally, we discussed the origins and the growth mechanisms of Cd whiskers observed in the alloys after full hydrogenation cycle.

► We synthesized ordered Mg3Cd alloy by high energy ball milling.
► The alloy reversibly absorbs 2.8 wt.% of hydrogen and exhibits fast kinetics.
► Order-disorder transformation does not affect kinetics of hydrogenation.
► Cd is rejected from the growing magnesium hydride into the metal phase.
► Thermodynamic model without any adjustable parameters fits experimental data.