THERMAL-STABILITY AND HYDRIDING PROPERTIES OF NANOCRYSTALLINE MELT-SPUN MG63NI30Y7 ALLOY

Nanocrystalline Mg-2(Ni,Y) hydrogen storage alloy (with exact composit ion Mg63Ni30Y7) was prepared by rapid solidification, using a melt-spi nning technique. Thermal stability and phase transition in the as-quen ched alloy were studied by TEM, DSC, X-ray and electron diffraction. I t was found that the as-cast material consists of mainly hexagonal :Mg ,(Ni;Y) nanocrystals with an average size of 2-3 nm and a significant amount of amorphous phase with similar composition located between the m. During heating the alloy crystallizes completely by three dimension al nanocrystal growth, with an activation energy of 140 +/- 7 kJ mol(- 1). The hydriding properties of the as-quenched nanocrystalline alloy were studied as well. The maximum hydrogen absorption capacity (about 3.0 wt.%) and hydrogenation kinetics of the melt-spun Mg-2(Ni,Y) were found to exceed those of the conventionally prepared polycrystalline M g2Ni alloys and to be comparable to the hydrogen absorption characteri stics of nanocrystalline ball-milled Mg2Ni. Hydrogenation of the as-ca st alloy causes a change in the crystallization mechanism during annea ling, as the microstructure remains nanocrystalline (15-20 nm) even af ter complete crystallization of the alloy. (C) 1998 Elsevier Science S .A. All rights reserved.