Influence of hydrogen absorption–desorption on structural properties ofDy1−xMmxCo2 alloys

Abstract

The effect of hydrogen absorption–desorption on the structural properties of Laves phaseDy1−xMmxCo2 (x = 0.1, 0.3and 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–desorptionpressure-composition (PC) isotherms, kinetics of hydrogen absorption and powderx-ray diffraction (XRD). The PC isotherms and kinetics of hydrogen absorptionhave been studied in the pressure range 0.001–1 bar and temperature range50–200 °C using Sieverts-type apparatus. The experimental results of the kineticcurves are interpreted using the Johnson–Mehl–Avrami (JMA) modeland the reaction order and reaction rate have been determined. Theα-,(α+β)- and β-phase regions have been identified from the different slope regions of thePC isotherms and first-order type kinetic plots. The dependence of thereaction rate parameter upon hydriding pressure and temperature in the(α+β)-phase region has been discussed. The effect of hydrogenation pressure, temperatureand Mm concentration on the hydrogen-induced transformation from crystallineDy1−xMmxCo2–H toamorphous Dy1−xMmxCo2–H and decomposition into crystalline (Dy,Mm)H2 and Co have been discussed in detail. Further, the effect ofdehydrogenation on the recovery of the crystalline Laves phase structure ofDy1−xMmxCo2 fromits decomposed state is presented. This hydrogenation–disproportionation–desorption–recombination(HDDR) process can be conveniently used in powder metallurgy.