THE RELATIONS BETWEEN THE MICROSTRUCTURE AND THE CAPACITY DECAY-RATE OF THE MLNI3.8CO0.6MN0.55TI0.05 ALLOY II - MICROSTRUCTURE INVESTIGATIONS AND DISCUSSIONS

The microstructure of the MLNi3.8Co0.6Mn0.55Ti0.05 alloys prepared wit h different methods has been investigated before and after electrochem ically charge/discharge cycling using SEM, TEM, EDXRD and SAD techniqu es. A strong correlation between the microstructure of the alloy inclu ding segregation, grain size, crystallization status and precipitates and the cycle stability was observed. Much segregation in the interior of the as-cast alloy leads to pulverization and fast oxidation. The s egregation could be avoided by quenching at a high rate of mure than 1 0 ms(-1) and the grain size was very small in the rapidly quenched all oy, which decreased the pulverization and the oxidation of the alloy p articles. Furthermore, there was also an amorphous phase in the as-que nched alloy which could further depress the surface oxidation of the a lloy. TEM studies showed that no amorphous phase was found in the allo ys annealed at a temperature higher than 873 K and the grain size of t he alloy became larger than 2 mu m Moreover, a precipitated phase occu rred in the high-temperature-annealed sample. All of these changes in microstructure due to high-temperature annealing could account fur the deterioration of the cycle stability. There are mainly two kinds of m echanism for the capacity decay of the AB(5)-type hydrogen storage all oy due to repeated cycling. One is the pulverization-oxidation process which decreases the active material and, the other is the surface pas sivation process which decreases the reaction kinetics. It was found t hat the two mechanisms play different roles in different alloys prepar ed with different processing methods. (C) 1998 Elsevier Science S.A.