AbstractThe formation ofθin A l–3Cu–0·1Be alloy has been investigated using a resistivity technique and transmission and scanning electron microscopy, and the results are compared with those reported in an earlier investigation of an AI–3Cu alloy. The formation ofθobeys the Avrami equationξ=1−exp(−ktn), whereξis the fraction of excess solute precipitated at time t, and k and n are parameters dependent on nuclei density and growth mechanism, respectively. The rate ofθformation in Al–3Cu alloy is significantly increased by the addition of beryllium, and this is manifested in the earlierθ′→θtransition and in the faster overall transformation rate. However, the values for the growth constant (n=0·72±0·07) and activation energy (108·5 kJ) are the same (within experimental error) for both alloys, indicating that the addition of beryllium does not significantly affect theθgrowth mechanism. A beryllium-enhanced nucleation rate is proposed to account for the higher precipitate particle counts and k values (which include nuclei density) obtained for the Al–3Cu–0·1Be alloy.MST/492