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

摘要

The microstructure of as-cast and melt-spun MnNi3.6Co0.75Mn0.55Al0.1 alloys before and afterelectrochemical cycling and its influence on the cyclestability have been investigated. There was muchsegregation with large dimensions within the as-castalloy and the grain size of the as-cast alloy waslarger than 50μm, which could cause rapidpulverization and oxidation. No evident segregationwas found by SEM in all the investigated melt-spunalloys and the grain size was very small, which couldsuppress the pulverization of the alloy particles andhence prohibit the alloy from oxidizing. The higherthe quenching rate, the smaller the grain size. Amixture of a microcrystalline, nanocrystalline andamorphous phase was present in the high-rate quenchedalloys 9e.g. 27 ms-1) which was proven to be in favorof depressing the oxidation of the alloy. For the firsttime, composition fluctuation were detected inregions where the microcrystalline, nanocrystallineand amorphous phases co-exist. The compositionfluctuation lowered the increase of the cyclestability with increasing quenching rate. Thecapacity also decreased with increasing quenchingrate due to occurrence of the amorphous phase becausethe higher the quenching rate the larger the amountof the amorphous phase. Too high quenching rates arenot necessary and are disadvantageous to the overallproperties of the alloy. However, melt-spinning caneffectively increase the cycle stability, likesubstitution of cobalt for nickel, and makes itpossible to produce low-cobalt or no-cobalt alloyswith good cycle stability with low cost of raw ofcobalt for nickel, and makes it possible to producelow-cobalt or no-cobalt alloys with good cyclestability with low cost of raw materials.