Phase transitions in the formation of Ti5Si3/TiAl in situ composites and their multiplex structures

A mixture of elemental Ti, Al, and Si powders with a stoichiometry of Ti-14Al-21Si has been mechanically alloyed (MA) for producing metastable phases. Differential thermal analysis (DTA) and x-ray diffraction (XRD) have been used to identify the phase reactions and transitions that occurred in the metastable phases during heating. It has been found that Ti₅Si₃ is in situ formed through a phase transition chain, i.e., TiSi₂→Ti₅Si₄→Ti₅Si₃. The metastable phases that serve as reinforcement-precursors were incorporated into Ti-Al powder matrix, and a final Ti₅Si₃/TiAl composite was obtained when the constituent powders underwent a reactive sintering (RS) process followed by hot isostatic pressing (HIP). Scanning electron microscope (SEM) observations revealed a hierarchical structure including phases α₂, γ and titanium silicides, and, more interestingly, an evident peripheral zone around the in situ silicide was found to have formed during the composite formation. It is expected that this zone may take a different role in improving the composite properties (e.g., ductility) than the interfacial zone commonly formed in composite preparation using other methods. In addition, elemental Si powders of two different starting sizes (71 μm and 50 nm) were tried to evaluate the effect of the size of Si on the composite formation in this manner.