Abstract
The effects of addition of Zn on the discontinuous precipitation (DP) behavior have been metallographically examined in AM90 (AM) and AZ91Mg (AZ) alloys aged at 423, 448 and 473 K. Prior to the occurrence of a significant age-hardening within grains, DP cells in the alloys nucleate at grain boundaries and grow into the grains ahead of reaction fronts. The boundaries migrate at a constant rate at the early stage of aging. For both alloys, the cell growth rate and average inter-lamellar spacing increase with aging temperature. The cell growth rates for the AM alloy are slightly faster than those for the AZ alloy. The incubation periods to initiate DP are identical for the alloys. Kinetics analyses of DP using the model of Turnbull, and that of Petermann and Hornbogen have yielded grain-boundary diffusion data. The activation energies of boundary diffusion are 76 to 98 kJ/mol, depending on the models and the alloys. The area fraction of DP cells in the AZ alloy at the over-aging stage is smaller than that in the AM alloy at the same stage. The smaller area fraction in the AZ alloy is ascribed to the higher number density of continuous β-Mg17Al12 precipitates which successively suppress the boundary migration.
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