Discontinuous Precipitation in Cu-2.1 wt%Be Alloy

Abstract

Discontinuous precipitation in Cu-2.1 wt%Be alloy has been investigated by means of metallographic observations and microhardness measurements. The variation in volume fraction of discontinuous precipitation cells and cell radius with ageing time, and number of cells per unit volume have been determined by quantitative metallographic measurements in the specimens aged at temperatures from 473 to 773 K.
Upon ageing above 623 K, the growth rate (G) of cells decreases and the interlamellar spacing (l) in cells increases with ageing time progressively after the linear growth rate of cells. These phenomena may be attributed to the influence of continuous precipitates (plate-like γ′ precipitates), which take place in grain center regions where cells have not proceeded, on the cell growth. Below 573 K, continuous precipitates (fine γ′ precipitates) form prior to the discontinuous precipitation but the variation in G and l with ageing time is not observed.
Volume fraction of discontinuous precipitation cells, f, can be represented by Johnson-Mehl’s equation: f=1−exp(−btⁿ), where t is the ageing time and b and n are constants, and 2.2 above 623 K and 1.4 below 573 K are obtained as n’s values in the range of ageing times that G’s are linear. Above 623 K, the nucleation sites of cells are considered to be grain boundary edges according to Cahn’s theory. The number of cells is larger by several times in the n=1.4 regions than the 2.2 regions. These phenomena may be attributed to the influence of the elastic strain resulting from the formation of the G.P. zones and fine γ′ precipitates prior to the discontinuous precipitation upon cell nucleation.
From the dependence of the linear growth rate of cells on the ageing temperature, 115 kJ/mol is obtained as the apparent activation energy for cell growth.