The effect of manganese on the microstructure and tensile response of an Al-Mg-Si alloy

The use of Al-Mg-Si-Mn alloys is increasing in the transportation sector. This study examines the role of Mn additions (alloys with 0.02 and 0.5 wt.% Mn) on the microstructure and mechanical behaviour for two different cooling paths after solution treatment, i.e. quenching in ice water and then artificial ageing at 180 degrees C to the peak strength or directly quenching into an oil bath at 180 degrees C. The grain structure and crystallographic texture were characterized by EBSD while the precipitate state was quantified by TEM, both within in the grain and at grain boundaries. The mechanical response was examined by uniaxial tensile tests where the stress strain response was measured up to the fracture point by measuring the fracture area. It is was found that the addition of Mn increased quench sensitivity, primarily due to precipitation of Mg2Si on the Mn rich dispersoids, but the strain to fracture was approximately doubled. A precipitation hardening model which accounted for the strength loss due to precipitation on grain boundaries and dispersoids was developed and found to give good agreement with the experimental yield stress results. The role of Mn rich dispersoids on the distribution of slip and the resulting final failure mechanism was examined (i.e. transgranular vs. Intergranular). It was observed that Mn dispersoids homogenize the slip process and promote a transgranular fracture mode.