Effects of sodium and bismuth in aluminum-magnesium alloys

Abstract

Aluminum-magnesium alloys are brittle at hot-working temperatures if contaminated by small quantities of sodium. Experimental use of a hydrogen absorption technique shows that the solubility of sodium in the alloys is about 0.0005 wt.% and that the excess is in elemental form. The onset of embrittlement is associated with 0.0005 wt.% Na and the hot elongation is reduced to less than 10% if the sodium content is >0.002 wt.%, equivalent to an intergranular layer about five atoms thick. Alloying with 0.01–0.02 wt.% Bi restores ductility. The constitutional form of the bismuth is uncertain but the fraction of the sodium content deactivated is consistent with formation of the compound NaBi. Microanalysis indicates that the bismuth is not intergranular but is distributed discretely.