EFFECTS OF SODIUM AND BISMUTH IN ALUMINUM-MAGNESIUM ALLOYS

Aluminum-magnesium alloys are brittle at hot-working temperatures if c ontaminated by small quantities of sodium. Experimental use of a hydro gen absorption technique shows that the solubility of sodium in the al loys is about 0.0005 wt.% and that the excess is in elemental form. Th e 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 > 0.0 02 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 conte nt deactivated is consistent with formation of the compound NaBi. Micr oanalysis indicates that the bismuth is not intergranular but is distr ibuted discretely.