Structural characterization of NiV amorphous alloys prepared by mechanical alloying

Partial NiNi atomic structures were directly observed by neutron diffraction in the amorphization process of Ni_100-xV_x powders during mechanical alloying (MA). The partial radial distribution function RDF_NiNi(r) shows that peaks of the f.c.c.-Ni crystal gradually broaden with increasing MA time. The peaks of the second and fifth NiNi neighbor positions observed within 7 A dramatically decrease and disappear almost completely after 400 h of milling. This means that a tetrahedral unit characteristic of an amorphous structure increases at the expense of the octahedral unit, because the second and fifth NiNi neighbors contribute exclusively to the formation of the octahedral units in the f.c.c. structure. The coordination number of nickel atoms around a nickel atom calculated from the area below the first peak decreases with increasing vanadium concentration. From this concentration dependence it is concluded that the chemical short range order in the NiV amorphous powders is analogous to that of the σ-phase existing as a stable phase in the phase diagram.