In situ x-ray studies of rapid crystallization of amorphous NiZr2

Abstract

We have studied the crystallization of amorphous NiZr2, using time-resolved x-ray scattering, for crystallization times varying from several minutes to tens of milliseconds. At relatively low temperatures, where the kinetics of transformation are slow, crystallization of the equilibrium NiZr2 phase occurs by time-dependent nucleation and growth. As the transformation temperature is increased, an ever greater fraction of a transient metastable crystal phase is formed, before the equilibrium phase is reached. We believe that the transient phase is in fact a precursor to the formation of the equilibrium phase at all temperatures. By rapidly quenching samples containing this transient phase, we have, by x-ray and electron diffraction, identified the crystal structure as belonging to the space group Oh7, with a lattice parameter of 12.61±0.01. The transient phase is believed to be formed because it is structurally closer to the amorphous than the equilibrium phase, thereby requiring less atomic rearrangements to form. By considering the formation of the Oh7 structure in similar (transition-metal)-Zr2 alloys, we deduce that the stability of the transient phase is closely related to the filling of the electronic d band. © 1992 The American Physical Society.