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

The solid state amorphization reaction in Ni50Mo50 alloys has been inv estigated 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 reacti on are characterized by the deformation of the parent crystalline phas es. 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 decr eases regularly for longer milling times. The Guinier plots show consi derable curvature indicative of particles with a range of sizes. Graph s of log I (Q) versus log Q show a sequence of changes which correspon d 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 scat tering vector Q. The slope of these graphs is about -3.4, which is ind icative of a rough surface having the fractal dimension of 2.6. We ten tatively associate this with the convoluted surfaces between the nicke l and molybdenum particles which are drawn into an interpenetrating fi lamentary structure in the early stages of the mechanical alloying pro cess.