A refining mechanism of primary Al₃Ti intermetallic particles by ultrasonic treatment in the liquid state

The mechanism underlying the considerable refinement of primary Al₃Ti intermetallic particles induced by ultrasonic treatment (UST) in an Al-0.4 wt% Ti alloy in the fully liquid state was investigated. Scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam 3D tomography and transmission electron microscopy were used to clearly identify that α-Al₂O₃ particles were located at or near the centres of primary Al₃Ti particles in the samples solidified with and without UST. Crystallographic evaluation using the edge-to-edge matching model and experimental determination of orientation relationships between the α-Al₂O₃ and primary Al₃Ti particles using the convergent beam Kikuchi line diffraction patterns confirmed the high potency of α-Al₂O₃ particles as nucleation sites for the Al₃Ti phase. Based on the experimental results, the refining mechanism is discussed in terms of proposed hypotheses in the literature. It is suggested that the significant refinement of primary Al₃Ti particles upon UST is due to the cavitation-induced deagglomeration and distribution of the α-Al₂O₃ particles and the cavitation-enhanced wetting of the α-Al₂O₃ particles by liquid aluminium.