In situx-ray studies of rapid crystallization of amorphousNiZr2

We have studied the crystallization of amorphous ${\mathrm{NiZr}}_2$, 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 ${\mathrm{NiZr}}_2$ 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 ${\mathit{O}}_{\mathit{h}}^7$, with a lattice parameter of 12.61±0.01 Å. 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 ${\mathit{O}}_{\mathit{h}}^7$ structure in similar (transition-metal)-${\mathrm{Zr}}_2$ alloys, we deduce that the stability of the transient phase is closely related to the filling of the electronic d band.