Investigations on the cycle stability and the structure of the MmNi3.6Co0.75Mn0.55Al0.1 hydrogen storage alloy -: II.: Microstructure investigations and discussions

The microstructure of as-cast and melt-spun MmNi(3.6)Co(0.75)Mn(0.55)Al(0.1) alloys before and after electrochemical cycling and its influence on the cycle stability have been investigated. There was much segregation with large dimensions within the as-cast alloy and the grain size of the as-cast alloy was larger than 50 mu m, which could cause rapid pulverization and oxidation. No evident segregation was found by SEM in all the investigated melt-spun alloys and the grain size was very small, which could suppress the pulverization of the alloy particles and hence prohibit the alloy from oxidizing. The higher the quenching rate, the smaller the grain size. A mixture of a microcrystalline, nanocrystalline and amorphous phase was present in the high-rate quenched alloys (e.g. 27 ms(-1)) which was proven to be in favor of depressing the oxidation of the alloy. For the first time, composition fluctuations were detected in regions where the microcrystalline, nanocrystalline and amorphous phases co-exist. The composition fluctuation lowered the increase of the cycle stability with increasing quenching rate. The capacity also decreased with increasing quenching rate due to occurrence of the amorphous phase because the higher the quenching rate the larger the amount of the amorphous phase. Too high quenching rates are not necessary and are disadvantageous to the overall properties of the alloy. However, melt-spinning can effectively increase the cycle stability, like substitution of cobalt for nickel, and makes it possible to produce low-cobalt or no-cobalt alloys with good cycle stability with low cost of raw materials. (C) 1999 Published by Elsevier Science S.A. All rights reserved.