Investigations on the growth kinetics of GP zones and a ′ R ‐precipitates in Al‐Zn alloys

X‐ray small‐angle scattering (XSAS) and resistivity ( R ) measurements were particularly done with an Al‐Zn (15 at·,%) alloy in rather wide ranges of both the ageing time t a and ageing temperature T a , in order to obtain information on the dependence of the growth exponent m of the l = β 0 t m growth law and the activation energy E act on t a and T a . The XSAS‐measurements yielded that within the range of the GUINIER radius r G between 1 nm and 2 nm the growth is essentially retarded ( m < 0.1) and for r G > 2 nm m depends on T a ranging from 0.15 to 0.23 with a maximum at 175°C. Reasons for these effects are discussed. The differences between the m ‐values obtained by means of XSAS‐and TEM‐measurements are explained by the distinctions of the two methods applied. The E act taken from XSAS‐ and R ‐measurements show a remarkable increase with t a . At the beginning of the decomposition E act = (0.49 ± 0.05) eV holds well explainable by the migration of quenched‐in VZn pairs, but at the end E act = (1.05 ± 0.07) eV was found. This value was also obtained from TEM‐investigations (growth of the length). It fits well the E act of ZnV pairs in thermal equilibrium at T a .