The Improved Age Hardening Response of Mg-11Al Alloys with Ag, Sc Addition and Corresponding Microstructure Characterization

The 1.0 wt.% Ag addition or 1.0 wt.% Ag and 0.4 wt.% Sc addition on the microstructure and age hardening response of Mg-11Al alloys have been systematically investigated via OM, XRD, SEM, TEM and micro-hardness test. The microstructure of as-cast Mg-11Al alloy is composed of α-Mg matrix and Mg₁₇Al₁₂ phase distributed along grain boundaries. With Ag and Sc addition, the stable Al₃Sc phase formed and grains were refined from 92.1 to 71.2 μm. After solid solution treatment, most of Al was dissolved into α-Mg matrix but Al₃Sc phase was still exist. Lath-shaped Mg₁₇Al₁₂ precipitates were observed in peak-aged Mg-11Al alloys. Ag and Sc addition improves the age hardening response by refining the size of Mg₁₇Al₁₂ precipitates and increasing the number density of precipitates. Ag and stable Al₃Sc phase probably act as the heterogeneous nucleation sites for precipitates and inhibit their growth. The micro-hardness value of peak-aged Mg-11Al-1Ag-0.4Sc alloy reached to 78.1 HV, increasing by 21.3% comparing with that of solid solutioned Mg-11Al alloy (64.4 HV). The strength improvement is attributed to multiple mechanisms, including grain refinement, new second phase strengthening (Al₃Sc), solid solution strengthening (Ag and Sc atoms) and precipitation strengthening (Mg₁₇Al₁₂). The novel Mg-11Al alloys with Ag, Sc addition obtain a noticeable age hardening response and corresponding mechanism was systematically discussed, which provides insights and foundations for the future research on Mg alloys containing a high content of Al.