Solid state synthesis of non-equilibrium phase in Mg-Co and Mg-Fe systems via bulk mechanical alloying

Mg-Co and Mg-Fe systems were employed as a candidate hydrogen storage alloy. Different from Mg-Ni system, there exist no line compounds of Mg₂Co and Mg₂Fe. Non-equilibration of these compounds is indispensable to make solid state synthesis. Bulk mechanical alloying was applied to this non-equilibration of Mg₂Co with success. Planetary ball milling was also utilized to discuss the process efficiency of bulk mechanical alloying. In particular, the on-line monitored energy density was used to describe the homogeneous refining and solid-state reaction with increasing the number of cycles. Through SEM observation of intermediate phase change, the solid-state reaction commences when the total energy density exceeds the critical limit. SEM/EDX and XRD analyses assured that the synthesized non-equilibrium phase should be Mg₂Co. The Goldschmidt-factor analysis was used to determine that the synthesized Mg₂Co has mainly fcc-structure. No significant change of XRD profiles was observed even when increasing the holding temperature. This Mg₂Co is quasi-stable, non-equilibrium phase even at the elevated temperature. In case of Mg-Fe system, the initial elemental particle mixture was homogeneously refined. Under the similar condition to the solid-state synthesis of Mg₂Co, however, Mg₂Fe was not synthesized even via bulk mechanical alloying. Through precise analysis, non-equilibrium phase with high iron content was recognized, so that non-equilibration via the bulk mechanical alloying might well be effective to investigate the solid state synthesis of binary compounds even in Mg-Fe system.