Enhanced age-hardening in automotive AlMgSi(Sn) alloys led by Sn-atomic-pillar-based precipitates

Abstract

Sn-atoms, as microalloying element, may significantly change the precipitation behaviors of automotive AlMgSi(Sn) alloys, but their microscopic mechanism and maximum impact on modifying the alloys' properties and microstructures are still unclear. Here, we report the distinct leading roles of Sn-element at the atomic-scale in determining the major precipitation behaviors of an AlMgSi(Sn) alloy upon thermal aging. Using atomic-resolution electron microscopy, our study reveals that upon aging Sn-atoms can quickly form structurally-well-defined Sn-atomic-pillar-based precursors or zones, prior to the formation of commonly known β″-precipitates in the alloy. These Sn-zones then serve as the major nucleation sites for the formation of Sn-atomic-pillar-based zone/β″ composite precipitates hardening the alloy. In the later stage of aging, these composite precipitates directly evolve to β′_Sn/β″ composite precipitates and then evolve further to β′_Sn/β'precipitates, i.e., the hexagonal β′-phase that heavily contains Sn-atoms replacing Si-atoms. Our findings demonstrate that Sn-addition can trigger and lead to a very different precipitation pathway, significantly tailoring the alloys’ properties and microstructures.