Effects of Si and Ni powder refining on the formation mechanism of nickel silicides induced by mechanical alloying

Abstract

The synthesis of the Ni2Si, Ni5Si2, and NiSi phases has been investigated by mechanical alloying (MA) of Ni-33.3 at. pct Si, Ni-28.6 at. pct Si, and Ni-50 at pct Si powder mixtures. As-received and 60-minute premilled elemental powders were subjected to MA. The average surface area of the premilled Ni powder particles, which had a flaky shape, was 3.5 times larger than that of the asreceived Ni powder particles, which had a spherical shape. The as-received Si powder was angular in shape and the mean particle size was 19.1 μm, whereas the mean particle size of the premilled Si powder was 10 μm. A self-propagating high-temperature synthesis (SHS) reaction, followed by a slow solid-state diffusion reaction, was observed to produce Ni silicide phases during MA of the elemental powders. The reactants and the product, however, coexisted for a long period of MA time. On the other hand only the SHS reaction was observed to produce Ni silicides during MA of the premilled elemental powders, indicating that Ni silicides formed rather abruptly in a short period of MA time. The mechanisms and reaction rates for the formation of Ni silicidesvia MA appeared to be influenced by the elemental powder particle size and shape as well as the heat of formation of the products.