A NEUTRON-SCATTERING STUDY OF THE AMORPHIZATION REACTION IN A NI50MO50 ALLOY

The solid state amorphization reaction in Ni50Mo50 alloys has been investigated by neutron diffraction and neutron small-angle scattering. A series of specimens has been mechanically alloyed for 2, 4, 8, 16 and 32 hours in a high-energy ball mill. The initial stages of the reaction are characterized by the deformation of the parent crystalline phases. After 8 hours of treatment a change in the samples is observed and those treated for 16 and 32 hours consist of an amorphous NiMo phase with some free molybdenum. The intensity of the small-angle scattering is much greater for the 2 hour specimen than for the parent, but decreases regularly for longer milling times. The Guinier plots show considerable curvature indicative of particles with a range of sizes. Graphs of log I (Q) versus log Q show a sequence of changes which correspond to the sequence of changes seen in the diffraction data. The graphs for the 2 and 4 hour samples are linear over an extended range of scattering vector Q. The slope of these graphs is about -3.4, which is indicative of a rough surface having the fractal dimension of 2.6. We tentatively associate this with the convoluted surfaces between the nickel and molybdenum particles which are drawn into an interpenetrating filamentary structure in the early stages of the mechanical alloying process.