CALORIMETRIC STUDY ON PRECIPITATION IN CU-BE ALLOYS

The specific heat (C(p)) versus temperature (T) curves of aged Cu-7.8 and 9.3 at% Be alloys are measured in conjunction with transmission el ectron microscopy in order to study the precipitation process from a v iewpoint of thermal stability of metastable phases. Heat absorptions d ue to the re-dissolution of G.P. zones, gamma'', gamma' and gamma phas es are observed on the C(p)-T curves in both alloys. Solvus temperatur es for the G.P. zones and the gamma'' 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 gamma '' phase has initially a two-layer structure separated by a matrix lay er. The number of layers in the gamma'' phase is considered to increas e with prolonged aging. It is suggested, therefore, that the increase in the solvus temperature for the G.P. zones is attributed to the incr ease in size of the zones, white the increase in the size and in the n umber of layers contributes to the increase in the solvus temperature of the gamma'' phase. The thermal stability of the gamma'' phase seems to approach that of the gamma' phase with aging. This indicates the c ontinuous change in the crystal structure of the gamma'' phase into th at of the gamma' phase. Therefore, a consecutive reaction is expected from gamma'' to gamma'. Although the solvus temperatures for the G. P. zones and the gamma'' 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.