Primary crystallization kinetics in rapidly quenched Mg-based Mg–Ni–Y alloys

Abstract

The primary crystallization kinetics of two magnesium based Mg–Ni–Y amorphous alloys (Mg 76Ni 19Y 5, Mg 78Ni 18Y 4) with attractive hydrogen storage characteristics were studied. Although the compositional difference of the alloys is very small they show essentially different devitrification behaviour. In Mg 76Ni 19Y 5 the crystallization starts with formation of the equilibrium hexagonal Mg 2Ni phase, obeying a continuous nucleation and parabolic growth kinetics law during the whole transformation. Mg 78Ni 18Y 4 alloy forms a nanocrystalline metastable phase during primary crystallization, which was previously identified by us as fcc Mg 6Ni and was found to be responsible for the improved hydrogenation/dehydrogenation kinetics [1,2]. The kinetics of the nanocrystal formation in the last alloy were found to deviate from the JMKA model, including nucleation and diffusion controlled growth, due to diffusion field impingements in the advanced stage of the primary crystallization reaction. On the basis of the experimental data on the transformed fraction and grain size as a function of time, important information about the mechanism of these transformations was obtained.