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Abstract: Mechanical alloying of Al60−x Ti40Si x (x =10,20,30) by high-energy ball milling yields a composite microstructure comprising varying proportions of amorphous and nanocrystalline phases. The microstructural evolution of these alloys at different stages of milling was monitored by X-ray diffraction, high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Furthermore, the role/influence of atomic defect and its density/distribution on partial solid state amorphization of the present alloys was investigated by positron annihilation spectroscopy. The results indicate that positron lifetime, contrary to the anticipated trend, initially decreases with the progress of partial amorphization, and subsequently shows a marginal increase with continued milling when strain induced nucleation of nanocrystalline aluminides occurs in the microstructure. We propose a possible mechanism responsible for reduction in the positron lifetime with the progress of partial amorphization from a nanocrystalline aggregate, and an increase in the lifetime when a reverse transformation (amorphous to nanocrystalline) takes place. [Copyright &y& Elsevier]