Ultrasound induced fragmentation of primary Al₃Zr crystals

¹ Faculty of Technology, Design and Environment, Oxford Brookes University, Oxford OX33 1HX, United Kingdom
² Brunel Centre for Advance Solidification Technology (BCAST), Brunel University London, Uxbridge UB8 3PH, United Kingdom
³ Anton Paar TriTec SA, Vernets 6, 2035 Corcelles, Switzerland
⁴ Cavitation Laboratory, School of Engineering, University of Glasgow, Glasgow G12 8QQ, United Kingdom
⁵ Computational Science and Engineering Group (CSEG), Department of Mathematics, University of Greenwich, London SE10 9LS, United Kingdom
⁶ Tomsk State University, Tomsk 634050, Russia
⁷ Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom

^* Corresponding author: abhinav.priyadarshi-2018@brookes.ac.uk

Abstract

Ultrasonic cavitation melt treatment (UST) of aluminium alloys has received considerable attention in the metal industry due to its simple and effective processing response. The refined primary intermetallic phases formed in the treated alloys during controlled solidification, govern alloy structural and mechanical properties for applications in the automotive and aerospace industries. Since the UST is performed close to the liquidus temperatures of the alloys, understanding the refinement mechanism of the primary intermetallic phases has been beset by difficulties in imaging and handling of liquid metals. In this paper, the sonofragmentation behaviour of primary intermetallic Al₃Zr crystals extracted from the matrix of an Al-3 wt% Zr alloy and fixed on a solid substrate was investigated. The intermetallics were exposed to cavitation action in deionized water at 24 kHz of ultrasound frequency. The fragmentation mechanism from the nearby collapsing cavitation bubbles was studied with in-situ high speed imaging. Results revealed that the main fragmentation mechanism is associated with the propagation of shock wave emissions from the collapsing bubble clouds in the vicinity of the crystal. The mechanical properties of the Al₃Zr phase determined previously were used for the fracture analysis. It was found that an Al₃Zr intermetallic undergoes low cycle fatigue fracture due to the continuous interaction with the shock wave pressure. The magnitude of the resulting shear stress that leads to intermetallic fragmentation was found to be in the range of 0.6 – 1 MPa.