Enhanced heterogeneous nucleation on oxides in Al alloys by intensive melt shearing

Abstract

Aluminium alloys, including both foundry and wrought alloys, have been extensively used for light-weight structural and functional applications. A grain refined as-cast microstructure is generally highly desirable for either subsequent processing ability or mechanical properties of the finished components. In this thesis, the grain refined microstructures in Al alloys have been achieved by intensive melt shearing using the melt conditioning by advanced shearing technology (MCAST) without deliberate grain refiner additions. Such grain refinement has been attributed to the enhanced heterogeneous nucleation on the dispersed oxide particles. It has been established that the naturally occurring oxides in molten Al alloys normally have a good crystallographic match with the a-Al phase, indicating the high potency of oxide particles as the nucleation sites of the a-Al phase. The governing factors for these oxide particles to be effective grain refiners in Al alloys have been proposed, including the achievement of good wetting between oxide particles and liquid aluminium, a sufficient number density and uniform spatial distribution of the dispersed oxide particles, and near equilibrium kinetic conditions in liquid alloys. In the present study, near equilibrium kinetic conditions can be achieved by intensive melt shearing using a twin screw mechanism, which has been confirmed by the observed equilibrium a-AlFeSi phase in a cast Al alloy and the transformation from g- to a-Al2O3 at 740±20oC under intensive shearing. For different alloy systems, depending on the alloy system, and melting conditions, due to the particular types of oxide formed and its crystallographic and chemical characteristics, the nucleation site of the nucleated phase is different. Specifically, MgAl2O4 relative to MgO, and a-Al2O3 relative to g-Al2O3, have higher potency as heterogeneous nucleation sites of a-Al phase in Al alloys. In future, the modification of the crystallographic match, and of the other surface characteristics related to the interfacial energy between the specific oxides and nucleated phase by trace alloying addition through segregation to the interface between oxides and nucleated phases combined with physical melt processing (such as intensive shearing in the present study) should be investigated in more detail.