Effect of Metastable Mg₂Si and Dislocations on α-Al(MnFe)Si Dispersoid Formation in Al-Mn-Mg 3xxx Alloys

The effect of metastable Mg₂Si and dislocations on the formation of α-Al(MnFe)Si dispersoids in Al-Mn-Mg 3xxx alloys were studied by a close examination of the dispersoid precipitation process using the quench technique and TEM observation. Special attention was paid to the nucleation mechanisms. Mg plays an important role in promoting the formation of α-Al(MnFe)Si dispersoids. The number density and volume fraction of the dispersoids in the Mg-containing alloy are much higher than those in the control alloy without Mg, resulting in a strong dispersoid strengthening effect. During the heating process in the Mg-containing alloy, metastable Mg₂Si precipitated and dissolved, leaving local Si-rich areas on pervious metastable Mg₂Si, which provide favorable nucleation sites for α-Al(MnFe)Si dispersoids. It was found that β′-Mg₂Si precipitates were more effective at the promotion of the dispersoid nucleation than β″-Mg₂Si. In the deformed sample, the dislocations become the preferable sites for the α-Al(MnFe)Si dispersoid nucleation. By reducing dispersoid-free zones, the dispersoid distribution became more uniform compared to the non-deformed sample. The dispersoid nucleation mechanisms based on both metastable Mg₂Si and dislocations are proposed and discussed.