Mg17Al12 phase refinement and the improved mechanical performance of Mg–6Al alloy with trace erbium addition

Abstract

With an improvement in room temperature ductility, the automotive use of cast Mg–Al alloys can be extended to structural body components. This is challenging since Al readily forms brittle intermetallics with the alloying additions such as rare earths and limits their solid solubility largely, nullifying the potential beneficial effects of rare earth elements on plasticity e.g. on stacking fault energy. In this study, significant improvement of mechanical properties in Mg-6wt% Al (Mg–6Al) casting alloy was seen with low levels of Er additions (60 ppm and 880 ppm) that was related to the modifying effect of Er on the Mg17Al12 precipitates and the α-Mg. Thermodynamic simulations, atom probe tomography, scanning electron microscopy, X-rays diffraction and mechanical testing were employed to understand the modification mechanism in Mg–6Al. According to Scheil solidification simulations, Al2Er Laves co-precipitates at the same temperature as Mg17Al12 exerting an inoculating effect when Er is at trace levels in the bulk alloy; however, when Er in the alloy increases, Al2Er forms at higher temperatures much before Mg17Al12 losing its inoculant potential. It was also seen that Er is soluble in Mg17Al12 causing changes to its lattice parameter and cell volume. Tensile strength, ductility, compressive strain to fracture and hardness are influenced by microstructural refinement. Yield strength at the higher level of Er (880 ppm) can be explained by the existence of Er-rich intermetallics while the yield strength at low levels of Er (60 ppm) can more likely be explained by the nature of β-phase.