Microstructure control of Mg alloys for structural component applications

Abstract

Low-density Magnesium alloys are increasing in importance to address emissions in transportation systems. Alloying with Aluminium improves mechanical properties though has the tendency to form the microstructural Mg17Al12 phase, limiting use at high temperatures due to poor creep properties. With further introduction of Lanthanum, Mg17Al12 is able to be supressed with preferential formation of intermetallic phases. This has increased interest in the Mg-Al-La ternary system and motivated this work to refine the transition between phase fields in the Mg-Al-La system and concurrently characterise morphology. This work can be used as a foundation to optimise the selection of alloys in order to achieve desired material properties. The purpose of this work is to identify solidification pathways and fundamental mechanisms of eutectic solidification, of the scoped Mg-rich portion of the Mg-Al-La system, including the effect of differing composition on the morphology of the eutectic phases. With this, refinement between the phase fields and characterisation of phase morphology, with differing compositions, is established. Currently, there is no extensive documentation of systematic research with regard to the effects of composition on cast microstructures and their relevant mechanical properties; this work aims at filling this gap in knowledge. Research and optimisation of the Mg-Al-La system has suggested a variety of phases form in the scoped Mg-rich portion of the phase diagram, with different phases forming according to alloy composition. To investigate this, eleven alloys were casted in the Mg-rich portion, their composition based on Scheilein solidification parameters. Following this, there was examination of microstructures and identification of phases using SEM/EDS and ImageJ. The findings indicate that the impact of composition on the formation of phases and their morphology is more complex than previously assumed. The findings show solidification pathways are dependent on regions of the Mg-rich portion of the phase diagram in which the alloys are casted. Additionally, by differing composition, the morphology of the eutectic phases alters and intermetallic phases form around primary Mg grain boundaries with preferential formation due to localised reactions. The major significance in findings is reliable prediction of microstructures and morphology of phases, depending on alloy composition. Overall, the relationship between composition and phase properties is due to the regions in the Mg-rich portion of the Mg-Al-La phase diagram which the compositions of alloy correspond. The implications of such is accurate prediction of microstructure, morphology and respective properties. However, limitations exist with the amount of data analysed, experimental testing of mechanical properties, lack of thermal analysis and assumptions in modelling regions.