AGE HARDENING IN A MG-9 WT PERCENT A1 ALLOY

The precipitation process and age hardening mechanism in a Mg-9 wt.% Al alloy were studied principally by transmission electron microscopy. This alloy age hardens by the general precipitation of plates of the equilibrium Mg17Al12 phase on the basal plane of the magnesium solid solution matrix. No evidence for the formation of either G.P. zones or a transition lattice of the Mg17Al12 precipitate was obtained. Cellular precipitation occurs competitively with the general precipitation at nearly all age hardening temperatures. In the solution treated condition, basal slip and {101̄2} twins are the principal deformation modes. Precipitation induces cross slip on the prism planes and tends to suppress {101̄2} twin formation. The alloys are hardened by complex dislocation tangles formed by the interaction of slip dislocations on the prism and basal planes. Neither slip dislocations nor {101̄2} twins shear the Mg17Al12 plates. © 1968.