Intergranular Fracture of Al–5%Mg Alloys Containing a High Amount of Sodium and Its Suppression by Bismuth or Indium Additions

The effect of high amounts of sodium on intergranular fracture of Al–5% Mg alloys without and with additional bismuth or indium was studied under different heat treatment conditions. A set of experiments made it evident that the intergranular fracture occurred even at room temperature when the amount of sodium in the Al–5%Mg alloy exceeded 23 ppm under homogenization treatment. On the contrary, Al–5%Mg alloy with large amounts of sodium (23 and 200 ppm) did not show intergranular fracture at 300°C under homogenization treatment. In addition, the Al–5%Mg alloy containing 4 ppm of sodium showed obvious intergranular fracture when the alloy was pre-deformed and subjected to the solution heat treatment. This suggests that the effect of sodium on the intergranular fracture of the Al–Mg alloys varies according to the sodium concentration, heat treatment, and testing temperature. We have also clarified that the room-temperature intergranular fracture caused by 200 ppm of sodium in the Al–5%Mg alloys was suppressed by 0.1%-ordered additional elements such as bismuth or indium. It is presumed that the suppression of the intergranular fracture by the addition of bismuth or indium was caused by the formation of sodium-bearing compounds in the Al–5%Mg alloys, leading to scavenging of sodium from GBs at room temperature.