The relations between the microstructure and the capacity decay rate of the MLNi 3.8Co 0.6Mn 0.55Ti 0.05 alloy : II. Microstructure investigations and discussions

Abstract

The microstructure of the MLNi 3.8Co O.6Mn 0.55Ti 0.05 alloys prepared with different methods has been investigated before and after electrochemically charge/discharge cycling using SEM, TEM, EDXRD and SAD techniques. A strong correlation between the microstructure of the alloy including 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 segregation could be avoided by quenching at a high rate of more than 10 ms − 1 and the grain size was very small in the rapidly quenched alloy, which decreased the pulverization and the oxidation of the alloy particles. Furthermore, there was also an amorphous phase in the as-quenched alloy which could further depress the surface oxidation of the alloy. TEM studies showed that no amorphous phase was found in the alloys annealed at a temperature higher than 873 K and the grain size of the alloy became larger than 2 μm. Moreover, a precipitated phase occurred in the high-temperature-annealed sample. All of these changes in microstructure due to high-temperature annealing could account for the deterioration of the cycle stability. There are mainly two kinds of mechanism for the capacity decay of the AB 5-type hydrogen storage alloy due to repeated cycling. One is the pulverization–oxidation process which decreases the active material and, the other is the surface passivation process which decreases the reaction kinetics. It was found that the two mechanisms play different roles in different alloys prepared with different processing methods.