The metastable Mg∼6Ni phase-Thermal behaviour, crystal structure and hydrogen reactivity of the rapidly quenched alloy

The thermal behaviour of melt-spun and of electrochemically H2 pre-charged Mg85.7Ni14.3 ribbons was investigated under 0.05 MPa He atmosphere and under 0.5 MPa H2 gas. The crystallisation of the Mg∼6Ni phase from the melt-spun amorphous pre-cursor is diffusion-controlled under the different atmospheres. Under He the Mg∼6Ni phase is observed between 140 °C ≤ TA ≤ 310 °C (TA = annealing temperature) with an average crystallite size of D ≈ 53 ± 12 nm. The Mg∼6Ni phase shows the cubic Mg6Pd-type-structure with 84.8-86.6 at.% Mg in the unit cell and 20.012 ± 0.002 Å ≤ a ≤ 20.037 ± 0.002 Å in dependence on TA and the annealing time tA. The lattice parameters of Mg∼6Ni were estimated using the Rietveld refinement of the XRD pattern after annealing at TA = 140 °C and tA = 100 h. Under H2 atmosphere and also after an electrochemical pre-charging the Mg concentration decreases drastically to 80-83 at.% in the Mg∼6Ni unit cell. A lattice constant of a = 20.000 ± 0.002 Å and a grain size of D ≈ 20 ± 2 nm were found. A chemical disorder in the unit cell is assumed. Interstitial H2 absorption is not observed. The phase reaction of Mg∼6Ni with H2 starts already at TA = 180° with the formation of the orthorhombic high-pressure modification γ-MgH2. The transformation enthalpy of γ-MgH2 into β-MgH2 was determined with ΔH = 46.33 kJ/mol and the desorption enthalpy of β-MgH2 with ΔH = 74.56 kJ/mol. The electrochemical charging of the melt-spun ribbons yields a maximum H2 content of 0.33 wt.% and leads to the same thermal behaviour, that was observed under the H2 gas.