Nanostructured Mg-Mm-Ni hydrogen storage alloy: Structure-properties relationship

Kinetics of H uptake/release in Mg can be improved by alloying with Ni and RE, as well as by reducing the grain size of the Mg alloy. Both these approaches were applied in the present work for the alloy 72 wt.% Mg-20 wt.% Ni-8 wt.% Mm (ternary eutectic Mg-Mg2Ni-MmMg12). The alloy was processed by the equal channel angular pressing (ECAP) technique. In ECAP the sample is subjected to heavy plastic strain by pressing it through a die with an angle of 90°. ECAP treatment resulted in a fine microstructure compared to the rather heterogeneous as-cast material. Hydrogenated and non-hydrogenated samples were investigated using SEM and XRD. Hydrogenation properties were studied by TDS and PCT. Hydrogenated samples consist of MgH2, Mg2NiH4 and MmH2+x and exhibit a maximum H-storage capacity of ∼5.5 wt.%. To initiate the first hydrogenation, the alloy needs to be activated at ∼300 °C. However, already after one hydrogenation cycle its H-absorption becomes quite fast: 4.5 wt.% H is absorbed in just 15 min. Vacuum TDS (heating rate 0.5 °C/min) shows that desorption starts at low temperature, ∼135 °C, with a peak at ∼210 °C. The alloy was also subjected to high energy ball milling (HEBM) in Ar or H2 to yield further refinement of the microstructure. ∼1 wt.% Nb2O5 was added to facilitate H exchange. The H sorption characteristics of the alloy treated by ECAP and HEBM have been compared with the ones for the as-cast material.