Abstract
The relation between the discontinuous precipitation and the continuous precipitation, and the kinetics of the discontinuous precipitation in Cu-Mg alloys containing 2.0, 2.6 and 3.2 mass%Mg have been investigated by metallographic observations and micro-hardness measurements. Variations in cell width and volume fraction of discontinuous precipitation cells with ageing time have been determined in the specimens aged at temperatures from 573 K to 873 K.
Results obtained are as follows: (1) The decomposition of Cu2Mg from supersaturated solid solutions of magnesium in copper during ageing is initiated by a discontinuous precipitation and followed by a continuous precipitation. (2) The cell growth is associated with the migration of a grain boundary. (3) The cell growth rate decreases with ageing time progressively after the linear growth rate of cells. This decrease will be attributed to the influence of the continuous precipitation on the cell growth. (4) The volume fraction of the discontinuous precipitation cells, f, can be represented by Johnson-Mehl type equation: f=1−exp(−btn), (where, t is the ageing time and b and n are constants). The value of the parameter n is 2 and independent of ageing temperature and alloy composition in the range of ageing time in which cell growth rates are constant.
