MICROSTRUCTURE AND MECHANICAL-BEHAVIOR OF REACTION HOT-PRESSED TITANIUM SILICIDE AND TITANIUM SILICIDE-BASED ALLOYS AND COMPOSITES

Titanium silicide (Ti5Si3) and its composites show promise for applica tions at temperature higher than 1000 degrees C. Dense Ti5Si3 was proc essed by reaction hot pressing of a TiH2/Si powder mixture, which invo lved decomposition of TiH2 into Ti and H-2 at around 800 degrees C, a chemical reaction between Ti and liquid Si at 1500 degrees C forming T i5Si3 in situ, and densification under pressure. The use of fine TiH2 particles led to the formation of a relatively fine-grained microstruc ture with fewer microcracks and higher hardness and fracture toughness values than those expected for a coarse-grained Ti5Si3. The addition of 8 wt pct Al as an alloying element led to the formation of Al0.67Si 0.08Ti0.25 and Al2O3 in situ and a solid solution of Al in Ti5Si3. Bot h alloying with Al and the addition of TiC as a reinforcement phase im proved the room-temperature facture toughness. Fracture toughness meas urements were performed by three-point bend testing of single-edge not ch bend (SENB) specimens, as well as by indentation techniques using d ifferent models, and the data have been compared. The role of differen t operating toughening mechanisms such as crack deflection, bridging, branching, and energy dissipation through microcracking have been exam ined. The investigation has also shown that Ti5Si3 maintains a high yi eld strength value up to 1200 degrees C.