Strontium-modification in the stepwise solidification of A319 Al alloy: An in-situ neutron diffraction study

The modification of cast Al-Si alloys with trace Sr is common practice for the improvement of material properties. In addition to the typical morphological transformation of the eutectic Si particles, research has shown that Sr can significantly influence the evolution of multiple other phases that are observed in complex industrial Al alloys. In this study, the solidification sequence and characteristics of unmodified and Sr-modified A319 Al alloy was investigated using in-situ neutron diffraction to obtain a novel perspective of the mechanism of its phase transformations. Neutron diffraction intensity data was collected during stepwise alloy solidification from 630 to 200 °C. This enabled the production of fraction solid curves, isolated for the individual Al, Si, Al15(Fe,Mn)3Si2, Al5Mg8Cu2Si6, and Al2Cu phases in the alloy, which corresponded well with the curves simulated using FactSage™ software. The solidification analysis was complimented with optical and scanning electron microscopy, which revealed the production of a unique and fascinating microstructure in the Sr-modified alloy. As a result of the combined action of Sr-induced undercooling and long holding times during stepwise cooling, the Al15(Fe,Mn)3Si2 phase was completely and significantly transformed into numerous, fine, uniformly-dispersed polyhedrons. The unmodified, acicular growth of the Si particles was then enabled, by using the polyhedral phase as substrates for nucleation. Hence, no undercooling of the Al-Si eutectic reaction was observed for the Sr-modified alloy.