Rational design of a lean magnesium-based alloy with high age-hardening response

Abstract

Magnesium-based alloys that allow fast processing, easy formability and subsequent age hardening to their final strength are highly desirable for lightweight structural applications. In this study, a lean age-hardenable wrought Mg–Al–Ca–Mn alloy, which combines precipitation hardening and grain refinement by secondary-phase pinning, was designed via thermodynamic calculation. The resulting alloy, AXM100, with a nominal composition Mg-Al0.6-Ca0.28-Mn0.25 (in wt.%), shows a remarkable improvement in tensile yield strength of 70 and 100 MPa upon artificial aging from the as-extruded state (T5) and the solution-heat-treated state (T6), respectively, reaching 253 MPa for the latter. A multi-scale microstructural analysis, combining light microscopy, transmission electron microscopy and atom probe tomography, was performed. It revealed a fine dispersion of Al–Mn precipitates with a β-Mn structure and Al–Ca-rich Guinier–Preston (G.P.) zones, which have an Al-to-Ca ratio of about 2. The former are responsible for impeding grain growth and the latter for age hardening. In addition, a fine dispersion of nanometric Ca-rich clusters preceding the G.P.-zone formation were identified which may contribute to strength. While the microstructural analysis, in terms of volume fraction and composition of the phases, reveals the limitation of the calculations, the latter successfully predict the elements contained in the various phases that play a key role in the mechanical properties, thereby proving them to be an invaluable tool for alloy design. In fact, the alloy designed in this study shows, despite its leanness, an age-hardening potential of 87 MPa and 118 MPa per 1 at.% total alloying content for the T5 and T6 condition, respectively, which is the highest among the compositions known for this type of alloys.