Influence of dynamic densification on nanostructure formation in Ti5Si3 intermetallic alloy and its bulk properties

Dynamic densification was used to consolidate mechanically amorphized Ti-Si alloy powders, using a three-capsule, plate-impact, gas-gull loading syste m at velocities of 300, 500, and 700 m s(-1). The dense compacts were subse quently crystallized at annealing temperatures in the range of 800-1200 deg rees C, for time periods of 1-12 h. The compacts were observed to crystalli ze to a typically single-phase Ti5Si3 compound and an ultra-fine grain micr ostructure, based on TEM and XRD analysis. The average grain size changed f rom similar to 50 nm upon heat treatment at 800 degrees C for 1 h, to simil ar to 160 nm at 1200 degrees C for 3 h, however, it remained stable (simila r to 115-125 nm) during annealing at a constant temperature of 1000 degrees C and increasing heat treatment time from 1 to 12 h. In-situ crystallizati on studies performed by heating the dynamically-densified samples in the TE M at temperatures up to 900 degrees C, revealed that the increase in fracti on of amorphous to crystalline phase was occurring by an increase in the nu mber density of nucleating crystallites, and not via significant growth of existing crystallites since their growth is inhibited by the impingement of the crystals. Vickers microhardness measurements showed values of 1200-140 0 kg mm(-2) for grain size ranging from similar to 60 to 160 nm. While thes e microhardness values are similar to 80% higher than those for microcrysta lline shock-densified Ti5Si3 alloy, the fracture toughness values measured using the indentation method were observed to be similar to 2-4 MPa root m, which is typical of that of brittle ceramics. (C) 1999 Published by Elsevi er Science S.A. All rights reserved.