Calorimetric Study on Precipitation in Cu–Be Alloys

The specific heat (Cp) versus temperature (T) curves of aged Cu-7.8 and 9.3 at% Be alloys are measured in conjunction with transmission electron microscopy in order to study the precipitation process from a viewpoint of thermal stability of metastable phases. Heat absorptions due to the re-dissolution of G.P. zones, γ″, γ′ and γ phases are observed on the Cp-T curves in both alloys. Solvus temperatures for the G.P. zones and the γ″ phase increase in the prolonged aging conditions. By means of high resolution electron microscopy, it is shown that the G.P. zones have a mono-layer structure and the γ″ phase has initially a two-layer structure separated by a matrix layer. The number of layers in the γ″ phase is considered to increase with prolonged aging. It is suggested, therefore, that the increase in the solvus temperature for the G.P. zones is attributed to the increase in size of the zones, while the increase in the size and in the number of layers contributes to the increase in the solvus temperature of the γ″ phase. The thermal stability of the γ″ phase seems to approach that of the γ′ phase with aging. This indicates the continuous change in the crystal structure of the γ″ phase into that of the γ′ phase. Therefore, a consecutive reaction is expected from γ″ to γ′. Although the solvus temperatures for the G. P. zones and the γ″ phase change depending on the aging conditions as mentioned above, the maximum values for the two phases are close to the reversion temperatures reported by other workers.