The inhibition mechanism of abnormal grain growth in dilute Mg–Al–Ca–Mn alloy through trace Gd addition

Abnormal grain growth (AGG), a prevalent phenomenon in dilute magnesium (Mg) alloys during elevated-temperature processing, significantly compromises mechanical performance through microstructural degradation. This study investigates AGG evolution in a heat-treatable Mg–1Al–0.3Ca–0.5Mn (wt.%) alloy, revealing its fundamental mechanism through phase interaction analysis. The AGG initiation is predominantly driven by Zener pinning force attenuation around abnormally coarsened Al₈Mn₅ precipitates. Mechanistically, this heterogeneous coarsening stems from preferential Al₈Mn₅ phase growth kinetics adjacent to Al₂Ca phase during homogenization treatment, creating localized pinning force discontinuities. However, addition of 0.2 wt.% Gd facilitates phase transformation from Al₈Mn₅ to thermally stable Al₈Mn₄Gd with lower Gibbs free energy, thereby promoting a more uniform and refined precipitate distribution. Consequently, the Gd-containing alloy exhibits enhanced grain thermal stability, maintaining a refined microstructure with average grain size of ∼7.7 µm even after T4 treatment at 500 °C for 1 h, which simultaneously improves strength and ductility compared to the Gd-free alloy.