Precipitation hardening: Unravelling mechanisms of trade-off between strength and ductility/formability of Al-Mg-Si alloys

Abstract

Heat treatable aluminium alloys have widespread applications in various industries owing to their significant strengthening capability through certain heating and cooling processes. However, these processes result in a trade-off between strength and ductility/formability, and the underlying fundamental mechanisms remain poorly understood. In this study, a multi-scale investigation was conducted on a heat treatable Al-Mg-Si alloy to explore the mechanisms underlying the trade-off between strength and ductility/formability. Three different tempers T6, T4, and O were produced, each leading to the formation of different second-phase precipitates. Uniaxial tensile tests and Nakajima punch tests were carried out to quantify the alloy’s ductility and formability, respectively. These mechanical properties were subsequently linked to the microstructure of the alloy by conducting micro uniaxial tensile tests alongside in-situ microstructural characterisations. The results indicate that the trade-off mechanisms arise from the competition between deformation occurring within the grain interiors and deformation near the grain boundaries. In the T6 temper alloy, precipitates significantly impede slip within the grain interiors, leading to strain concentration near the grain boundaries and the occurrence of intergranular fractures. Consequently, the T6 temper alloy exhibits the lowest ductility and formability. In contrast, slip occurs throughout all grains in the O temper alloy, thereby accommodating more deformation and resulting in the highest ductility and formability. Additionally, the precipitates in the alloy affect the dispersion of the geometrically necessary dislocation (GND) density. This study sheds light on the fundamental understanding of how precipitates influence the trade-off between the enhancement of strength and the simultaneous reduction of both ductility and formability. • Both dimple and intergranular fractures are present in artificially aged alloy, while only dimple fractures are observed in fully annealed alloy • Precipitates in artificially aged alloy hinder slip within grain interiors, causing localised deformation near grain boundaries and intergranular fractures • Compared to fully annealed alloy, artificially aged alloy exhibits a reduction of 49.9% in ductility and 41.5% in formability • As precipitate strengthening increases, the dispersion of geometrically necessary dislocation density rises throughout lattice

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