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

A new method for preparing Ti5Si3/TiAl in situ composite has been tried. Ti AlSi-based metastable precursors were first synthesized by mechanical alloy ing, and blended with Ti-50Al powder matrix. The composite mix was cold-pre ssed to form pellets, followed by heat treatment to initiate reactive sinte ring that led to the formation of Ti5Si3/TiAl composites. The resultant in situ formed Ti5Si3/TiAl composites exhibited multiplex structures containin g alpha(2) and gamma phases, extra Ti and small silicide precipitates, and showed some morphological traces of liquid phase sintering. The microstruct ural evolutions during the formation of metastable precursors and the subse quent phase reactions/transitions involved in the formation of composites h ave been investigated. A phase hierarchy has been observed during in situ f ormation of the reinforcement, i.e. TiSi2 --> Ti5Si4 --> Ti5Si3. Such a pha se selection route, though intrinsically forwarded by thermodynamics, is st rongly biased by kinetic factors such as diffusion. Nearly fully monolithic Ti,Si, reinforcements can be obtained by phase selection for metastable pr ecursors with stoichiometry of Ti-14Al-21Si but not Ti-11Al-21Si, though bo th were subjected to excess Ti as kinetic bias. (C) 2000 Elsevier Science S .A. All rights reserved.