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–gun 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°C, for time periods of 1–12 h. The compacts were observed to crystallize to a typically single-phase Ti5Si3 compound and an ultra-fine grain microstructure, based on TEM and XRD analysis. The average grain size changed from ∼50 nm upon heat treatment at 800°C for 1 h, to ∼160 nm at 1200°C for 3 h, however, it remained stable (∼115–125 nm) during annealing at a constant temperature of 1000°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°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 ∼60 to 160 nm. While these microhardness values are ∼80% higher than those for microcrystalline shock-densified Ti5Si3 alloy, the fracture toughness values measured using the indentation method were observed to be ∼2–4 MPa√m, which is typical of that of brittle ceramics.