Solidification Rate as Key Factor in Strengthening Mechanisms, Tensile Properties, and Phase Features in Cast Al-Mg-Sc Alloys

TL;DR

This study explores how solidification rate affects the microstructure and mechanical properties of Al-Mg-Sc alloys, showing that faster cooling improves strength and elongation.

Contribution

The novel contribution is the detailed characterization of solidification rate's impact on phase formation and mechanical behavior in Al-Mg-Sc alloys.

Findings

Nanometric Al3Sc precipitates form rod-like structures during solid-state cooling.

Higher solidification rates retain Sc in solid solution, affecting mechanical properties.

Al-5Mg-0.4Sc with 11.2 °C/s cooling achieved 306 MPa strength and 22.6% elongation.

Abstract

Scandium (Sc), when added together with magnesium (Mg), forms a highly effective synergistic pair in aluminum (Al) alloys, enhancing their performance in various applications. While the thermomechanical processing and heat treatment of such Al-Mg-Sc alloys have been well investigated, the behavior and features of their as-cast state remain less understood. In particular, the evolution of cellular/dendritic microstructures and the formation of phases at submicrometric and nanometric scales, especially those developing during solid-state cooling, require further elucidation. The present study employs a combination of conventional and advanced characterization techniques in the Al-5 wt.%Mg-0.4 wt.% Sc alloy, including CALPHAD, optical microscopy, scanning electron microscopy (SEM), transmission and scanning transmission electron microscopy (TEM/STEM) with energy-dispersive spectroscopy (EDS), x-ray diffractometry (XRD), tensile testing, and fractographic analysis. Al-rich dendrites surrounded by Al3Sc, AlFe, and β-Al3Mg2 phases and the formation of primary submicrometric clusters containing AlFe and Al3Sc have been identified, revealing important microstructural features that depend strongly on the solidification conditions. Moreover, nanometric Al3Sc precipitates mainly in the form of rod-like structures with sizes in the order of 50–200 nm have been observed within the α-Al matrix during solid-state cooling stage. At higher solidification rates, such as 15.3 °C/s, these precipitates remain predominantly in solid solution, indicating strong solidification rate dependence in the precipitation behavior. Comparisons between alloys containing 0.1 Sc and 0.4 Sc have demonstrated that the morphology, size, and distribution of Sc-rich phases significantly affect the stress–strain tensile response and underlying strengthening mechanisms. Distinct Portevin–Le Chatelier (PLC) effects have been observed, corresponding to very different serration activities in the stress–strain curves comparing both Al-5%Mg-0.4%Sc and Al-5%Mg-0.1%Sc alloy samples. Among the compositions and conditions studied, the Al–5Mg–0.4Sc alloy samples solidified under the fast-cooling condition (11.2 °C/s) exhibited the most improved mechanical performance, attaining a strength of 306 MPa and an elongation of 22.6%, underscoring the pivotal role of Sc content and solidification rate in achieving optimized mechanical properties.

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