In situ formation of titanium silicides-reinforced TiAl-based composites

A new method for preparing Ti5Si3/TiAl in situ composite has been tried. TiAlSi-based metastable precursors were first synthesized by mechanical alloying, and blended with Ti-50Al powder matrix. The composite mix was cold-pressed to form pellets, followed by heat treatment to initiate reactive sintering that led to the formation of Ti5Si3/TiAl composites. The resultant in situ formed Ti5Si3/TiAl composites exhibited multiplex structures containing alpha(2) and gamma phases, extra Ti and small silicide precipitates, and showed some morphological traces of liquid phase sintering. The microstructural evolutions during the formation of metastable precursors and the subsequent phase reactions/transitions involved in the formation of composites have been investigated. A phase hierarchy has been observed during in situ formation of the reinforcement, i.e. TiSi2 --> Ti5Si4 --> Ti5Si3. Such a phase selection route, though intrinsically forwarded by thermodynamics, is strongly biased by kinetic factors such as diffusion. Nearly fully monolithic Ti,Si, reinforcements can be obtained by phase selection for metastable precursors with stoichiometry of Ti-14Al-21Si but not Ti-11Al-21Si, though both were subjected to excess Ti as kinetic bias. (C) 2000 Elsevier Science S.A. All rights reserved.