Hardness/aging time curves have been obtained and systematic electron metallographic studies have been made on an Al-4%Cu alloy single crystal aged at 30°, 130°, and 250° for various periods. On the two-stage age-hardening curve at 130°, the initial rise in hardness is due to the formation of G.P.  and the second rise takes place by the growth of G.P. ; but softening occurs at the same time by the formation of new precipitate particles (θ'). The single-stage age-hardening curve at 250° is, however, associated with the growth of θ' precipitate particles. Therefore the mechanism of age-hardening of 130° and 250° aging may be completely different. From a view point of the dislocation theory of age-hardening, age-hardening at 130° is explained by Mott and Nabarro's critical dispersion theory. At the peak hardness obtained by aging at 130° for 20 days, the average distance between the nearest precipitate particles was estimated as 230 Å.
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