Effect of substitution of Fe and Ni for Co in the synthesis of Mg₂Co compound using the mechanical alloying method

The influence of the elaboration route of Mg₂Co_1-xM_x (M = Fe or Ni and x≤0.3) hydrogen storage alloy was investigated. When Co was replaced by Fe, the formation of `Mg<sub>2</sub>Co' (containing a very small amount of Fe) was observed after about 60 h of ball milling, but the yield of formation was lower. Increasing milling time just led to an amorphization of the `Mg₂Co' compound. We also observed, that as expected, no new compound was formed between Mg and Fe. Annealing of unmilled products did not lead to any new compounds and annealing of milled products just led to the crystallization of the existing compounds. When cobalt was replaced by nickel, the major difference was the formation, after annealing, of the Mg₂Ni compound. This difference is attributed to the higher stability of Mg₂Ni compared to that of `Mg<sub>2</sub>Co'. The yield of the `Mg₂Co' phase was higher than that with Fe and that without any adding element.