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 alpha-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 alpha-Al2O3 and primary Al3Ti particles using the convergent beam Kikuchi line diffraction patterns confirmed the high potency of alpha-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 alpha-Al2O3 particles and the cavitation-enhanced wetting of the alpha-Al2O3 particles by liquid aluminium. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd.