Ultrasound induced fragmentation of primary Al3Zr crystals

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.

Evaluating the Growth Restriction Factor in Stable and Metastable Al-Si-Ti Alloy Solidification

Evaluation concepts for theGrowth Restriction Factor, Q, in multicomponent alloys are discussed and illustrated for ternary Al-Si-Ti alloys involving precipitation of primary intermetallic phases.

Amorphous phase formation in Al70Si17Fe13 alloy

The alloy Al70Si17Fe13 was subjected to a range of rapid solidification conditions and the resulting microstructures were evaluated. It was found that when solidification was sufficiently rapid to bypass the formation of primary intermetallic phases, the alloy consisted of spherical regions of amorphous (or microquasicrystalline) material surrounded by a crystalline phase(s). This microstructure is interpreted as the result of solidification of the amorphous phase from the melt by a first-order transformation. The structure of the amorphous phase is different from that of a liquid (or usual metallic glass).