Increase in Thermal Stability of Mg<SUB>62</SUB>Ni<SUB>33</SUB>Ca<SUB>5</SUB> Amorphous Alloy by Absorption of Hydrogen

It was found that an amorphous Mg62Ni33Ca5 alloy absorbs a large amount of hydrogen at 323 K and the hydrogen content is much larger than that of the corresponding crystalline alloy. The maximum absorption concentration of hydrogen at 323 K is 2.3 mass% in the amorphous phase and 1.3 mass% in the crystalline state. The Mg-based amorphous alloy with 2.3 mass%H2 crystallizes through the process of Am → Am′+Mg2Ni → Mg2Ni+Mg2Ca+MgNi2+Mg2NiH4. The crystallization process is different from that (Am → Mg2Ni+Mg2Ca+MgNi2) of the as-quenched amorphous phase. The onset temperature and the completed temperature for crystallization is 453 and 532 K, respectively, for the as-quenched amorphous alloy and 475 and 572 K, respectively, for the amorphous phase containing 2.3 to 3.0 mass%H2. The absorption of hydrogen causes a significant increase in the thermal stability of the amorphous phase, presumably because of the necessity of a larger amount of hydrogen for the crystallization of the remaining amorphous phase which is coexistent with Mg2Ni. The retardation of the crystallization reaction of the Mg-based amorphous alloy by absorption of hydrogen is encouraging for future application to hydrogen-storage materials. It is concluded that the hydrogen can be used to control the thermal stability and crystallization process of Mg-based amorphous alloys.