Characterization of mechanically alloyed Ti-Al-Si powder blends and their subsequent thermal stability

With the aim of producing metastable precursors for tailoring TiAl based composite, five compositional powder blends of Ti-Al-Si system, i.e. Ti65Al17Si18, Ti65Al21Si14, Ti72Al14Si14, Ti65Al14Si21 and Ti58Al21Si21, were prepared by mechanical alloying (MA). The structural and compositional evolution during MA has been investigated using X-ray diffraction method (XRD), energy-dispersive spectrometer (EDS) and transmission electron microscope (TEM). MA achieved a significant increase in solid solubility limits. Supersaturated solid solution of Ti(Al,Si) was produced with prolonged milling time which was strongly dependent on the starting compositions of blends. No fcc phase or ordered and disordered intermetallics were produced during MA process. With higher and balanced Al and Si contents, for example Ti58Al21Si21 blend, the time required for solid solution formation was reduced drastically. Thermal stability of the mechanically alloyed powders was systematically examined by subsequent annealing process according to the results of differential thermal analyzer (DTA) curves. From XRD patterns of the annealed blends, it was observed that annealing at temperatures just below 600 degreesC leads to crystalline phases of mainly Al3Ti and TiSi2, while more stable Ti-5(Al,Si)(3), (Ti,Si)Al and (Ti,Si)(3)Al phases were formed at higher annealing temperatures of above 750 degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.