The effect of ball milling and equal channel angular pressing on the hydrogen absorption/desorption properties of Mg-4.95 wt% Zn-0.71 wt% Zr (ZK60) alloy

The microstructure of commercial alloy ZK60 (Mg-4.95 wt% Zn-0.71 wt% Zr) was refined by equal channel angular pressing (ECAP), high energy ball milling (HEBM) and by a combination of both methods. The effect of microstructure refinement on the hydrogenation properties of ZK60 was studied. The alloy processed by a combination of ECAP and HEBM was found to exhibit the highest rate of hydrogen desorption at 300 degreesC and the final pressure of 2 atm in the desorption volume. Similar, but slightly slower, desorption kinetics was observed for the alloy processed by ECAP alone. The desorption times for 5.2 wt% of H-2 were 20 and 26 min for the alloys processed by combined ECAP/HEBM and by ECAP alone, respectively. The hydrogen absorption capacity at 300 degreesC of the alloy processed by combined ECAP/HEBM was 6.2 wt% of H-2. The pressure-composition-temperature diagrams of this alloy in the temperature range of 280-350 degreesC were determined. The enthalpy and entropy of hydrogen absorption/desorption were 76.8 kJ/mol and 141.8 J/mol K, respectively. No pressure hysteresis with regard to hydrogen absorption/desorption was found for the alloys processed by combined ECAP and HEBM. This hydrogenation behavior was correlated with the microstructure of bulk samples and the morphology of dehydrogenated powders observed by transmission electron microscopy and high resolution scanning electron microscopy. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.