Thermal stability and phase formation of mechanically alloyed Ti-Al-Si-C powders

Two quarternary Ti-Al-Si-C powder mixtures, 55Ti-27Al-12Si-6C and 55Yi-36Al-6Sl-3C, were mechanically alloyed. The as-alloyed and heated powders have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). XRD patterns showed diffuse halos of amorphous like phase for 20-40 h milled powders, but TEM examinations demonstrated that the 40 h milled powders were mainly composed of Ti solid solutions, with some amount of amorphous phase. SEM observations displayed that the lamellar structures of Ti and Al formed at the early stage of milling process subsequently led to the formation of nano- or sub-micrometer particles of homogeneous composition after prolonged milling to 40 h. It is deduced that the solid-stated reaction by inter-diffusion of components should be responsible for phase formation during mechanical alloying. DSC curves of the 40 h milled powders exhibited two sharp exothermal peaks, and the investigation on thermal stability of the 40 h milled powders indicated that Ti₅Si₃ was first formed at lower temperature, followed by Al₂Ti₂and TIC at intermediate temperature (820°C), and these phases were stable at elevated temperatures. These results raise the possibility of synthesizing TiAl-based composite with titanium silicides and titanium carbides as reinforcements by proper selection of powder compositions. © 2004 Kluwer Academic Publishers.