Influence of hydrogen absorption-desorption on structural properties of Dy<inf>1-x</inf>Mm<inf>x</inf>Co<inf>2</inf> alloys

The effect of hydrogen absorption-desorption on the structural properties of Laves phase Dy1-xMmxCo2 (x = 0.1, 0.3 and 0.5; Mm = mischmetal, a natural mixture of the light rare earth metals containing 50 wt% Ce, 35 wt% La, 8 wt% Pr, 5 wt% Nd and 1.5 wt% of other rare earth elements and 0.5 wt% Fe) alloys has been investigated by means of hydrogen absorption-desorption pressure-composition (PC) isotherms, kinetics of hydrogen absorption and powder x-ray diffraction (XRD). The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.001-1 bar and temperature range 50-200 °C using Sieverts-type apparatus. The experimental results of the kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions have been identified from the different slope regions of the PC isotherms and first-order type kinetic plots. The dependence of the reaction rate parameter upon hydriding pressure and temperature in the (α+β)-phase region has been discussed. The effect of hydrogenation pressure, temperature and Mm concentration on the hydrogen-induced transformation from crystalline Dy1-xMm xCo2-H to amorphous Dy1-xMmxCo 2-H and decomposition into crystalline (Dy, Mm)H2 and Co have been discussed in detail. Further, the effect of dehydrogenation on the recovery of the crystalline Laves phase structure of Dy1-xMm xCo2 from its decomposed state is presented. This hydrogenation-disproportionation-desorption-recombination (HDDR) process can be conveniently used in powder metallurgy. © 2008 IOP Publishing Ltd.