Hydrogen-induced amorphization of YNi 2 enhanced by mechanical grinding

Abstract

The C15 Laves phase YNi 2, which transforms into amorphous phase by hydrogenation, was mechanically ground under various hydrogen partial pressures up to 1.0 MPa, to investigate the effect of mechanical grinding on hydrogen-induced amorphization (HIA) processes. The thermal stability of the amorphous phase and hydrogen contents dissolved in the compound were also examined. Under an initial hydrogen pressure of 1.0 MPa, the HIA processes are remarkably enhanced by grinding. Under an initial partial hydrogen pressure of 0.2 MPa, on the other hand, the YNi 2-H system decomposes into two phases, the α- and α'-phase by grinding, because the amount of hydrogen in the grinding vial is limited. Further grinding leads not only to an amorphization of the α'-phase but also to a transfer of hydrogen from the α'-phase to an amorphous phase and to a phase transformation of YNi 2 into YNi 5. Excess Y left in the phase transformation is dissolved into the amorphous phase. The difference between the above two processes depends on the initial hydrogen pressure and is understood by considering the free energy variation of the YNi 2-H system during mechanical grinding.