Precipitation in Al–3Cu–0·1Be

The formation of θ in 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 of θ obeys the Avrami equation ξ=1−exp(−ktⁿ), where ξ is 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 of θ formation in Al–3Cu alloy is significantly increased by the addition of beryllium, and this is manifested in the earlier θ′→θ transition 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 the θ growth 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