COOLING AND HEATING RATE DEPENDENCE OF PRECIPITATION IN AN AL-CU ALLOY

Differential scanning calorimetry and X-ray diffraction were used to study the cooling and heating rate dependence of precipitation in an Al-1.66 at.% Cu alloy. After homogenizing, cooling at a rate of 22 K min-1 (SC22) is sufficient to retain all copper in solid solution. GP-zone formation during subsequent heat treatment is hindered; this is ascribed to an insufficient number of (excess) vacancies. After a water quench (WQ) a large number of GP zones are formed during subsequent storage at room temperature for 1 h. The heat content of the GP-zone dissolution effect can quantitatively be described in terms of the heat of precipitation of GP 1 zones and the solid solubilities as derived from the GP 1 zone solvus. The heat content of the combined theta'-/theta-phase precipitation effect appeared to be proportional to the number of copper atoms precipitated, yielding an average value for the heat of copper precipitation of 36 kJ mol-1 copper. The activation energy for theta'-phase formation is 0.75 eV for SC22 specimens and 1.10 +/- 0.10 eV for WQ specimens. The differences in reported activation energies for theta'-phase formation are discussed in terms of mobility of dissolved atoms (related to the vacancy concentration), interfacial energy and direction of growth (normal or perpendicular to the theta'-phase plate).