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 system at velocities of 300, 500, and 700 m s(-1). The dense compacts were subsequently crystallized at annealing temperatures in the range of 800-1200 degrees C, for time periods of 1-12 h. The compacts were observed to crystallize to a typically single-phase Ti(5)Si(3) compound and an ultra-fine grain microstructure, based on TEM and XRD analysis. The average grain size changed from similar to 50 nm upon heat treatment at 800 degrees C for 1 h, to similar to 160 nm at 1200 degrees C for 3 h, however, it remained stable (similar 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 crystallization studies performed by heating the dynamically-densified samples in the TEM at temperatures up to 900 degrees C, revealed that the increase in fraction of amorphous to crystalline phase was occurring by an increase in the number 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 1200-1400 kg mm(-2) for grain size ranging from similar to 60 to 160 nm. While these microhardness values are similar to 80% higher than those for microcrystalline shock-densified Ti(5)Si(3) 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 Elsevier Science S.A. All rights reserved.