Solid State Synthesis of Non-Equilibrium Phase in Mg-Co and Mg-Fe Systems via Bulk Mechanical Alloying

Abstract

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, nonequilibrium 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.