Effect of Stable Phase Formation from Unstable Phase on Densification Acceleration of Amorphous Powder Produced by Mechanical Alloying

The effect of phase formation from unstable to stable phase on densification process of pressure sintering was studied using mechanically alloyed powders. Powder mixture of Ti and Si having a composition of Ti-37.5mol%Si was milled for 360ks and 3600ks. The 3600ks milled powder showed homogeneous microstructure and amorphous-like XRD profile, although the 360ks milled showed (Ti+Si) lamellar microstructure. Milled powders were vacuum hot pressed at a heating rate of 20K/min up to 1273K with applying constant pressure from 10 to 200MPa, then kept for 10.8ks. It was observed that the density of the compact suddenly increased at the temperature range between 870K and 900K for 3600ks milled powder and about 820K and 910K for 360ks milled powder. This temperature showed good agreement with the exothermic reaction of DTA run, which corresponded to the phase formation from amorphous to Ti 5 Si 3 and from elemental phases to Ti-silicides. It was assumed that the reasons of this extraordinary densification behavior were temperature increase, volume change, rearrangement and acceleration of plastic deformation during phase formation. From experimental results to examine these factors, with the exception of plastic deformation, other factors did not show any effect in increasing density. It is concluded that the phase formation from unstable (amorphous) to stable (Ti 5 Si 3 ) phase accelerates plastic deformation of MA powder, and therefore it leads to increase the density. After the formation of stable phase, densification was stopped for a while then restarted at 1050K (3600ks milled powder) and (360ks milled powder). These temperatures seem to be a softening temperature of MA powders.