Effect of Sc and Zr additions on the microstructure and age hardening of an AlMg3MnCr alloy: Structure and age hardening of AlMgMnCrScZr

The effect of small additions of Sc and Zr on phase transformations, microstructure and hardening during the ageing of AA5754 alloy was studied. Electrical resistometry and hardness (HV10) measurements were used together with the microstructural study by transmission electron microscopy. A fine dispersion of spherical Al3(ScxZr1−x) particles precipitated after the conventional thermal and mechanical treatment (homogenization 450 °C/4 h, hot working at 420 °C and cold working). The decomposition sequence of Al-Mg-Si system was observed, namely, metastable β″→metastable β′→ stable β phase, after the solutionizing treatment 585 °C/15 h and following isochronal annealing. Simultaneously, a stable Al3(ScxZr1−x) spherical phase precipitated in cube-to-cube orientation. The pronounced age hardening at 240 °C is due to the metastable β″ phase precipitation. A combination of Al3(ScxZr1−x) spherical particles and cuboids of stable β phase lead to another age-hardening peak at 360 °C.