The precipitation hardening process in Cu-4.5, 6.5 and 7.4 wt%Ag alloys has been investigated principally by optical microscope observations and the hardness test. The age hardening of these alloys can be attributed to the general precipitation and the grain boundary reaction type precipitation. The age hardening in coarse grain specimens is almost controlled by the general precipitation, but in fine grain specimens it is controlled by the grain boundary reaction type precipitation. The softening in these alloys originates from the coalescening of general precipitates or from the second grain boundary reaction which produces nodules of very coarse interlamellar spacing. The maximum hardness obtained by ageing is higher in coarse grain specimens than in fine grain specimens. The interlamellar spacing in nodules of the first grain boundary reaction with the degree of undercooling can be expressed by the equation: l=2.1×10−3⁄(TE−T) cm. The 0.2% yield strength of the alloy wholly covered with nodules is inverse proportional to the interlamellar spacings.
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Kainuma et al. (1969) studied this question.