Hydrogen storage performance of TiFe after processing by ball milling

Activation of TiFe for hydrogen storage by severe plastic deformation (SPD) through ball milling technique and the effect of microstructure on this activation have been investigated. TiFe becomes activated after the ball milling and is not deactivated after exposure to air, similar to TiFe activated by high-pressure torsion (HPT). The hydrogenation capacity reaches 1.3-1.5 wt.% at 303 K for the first to third cycles and the hydrogen absorption plateau pressure decreases to ∼1 MPa for any hydrogenation cycles. Observation of the microstructure after ball milling shows that the average grain size and crystallite size are as small as ∼7 and ∼11 nm, respectively (smaller than that after HPT or rolling), but few dislocations are detected within the detection limit of high-resolution transmission electron microscopy. This study shows clearly that there is a strong relation between the grain size of TiFe and its activation for hydrogen absorption: the activation is easier and the hydrogen pressure for activation is smaller, when the grain size is smaller.